Department for Environment, Food and Rural AffairsResearch project final report

Project title Exploration of methodologies for accurate routine determination of soil carbon

Sub-Project iv of Defra Project SP1106: Soil carbon: studies to explore greenhouse gas emissions and mitigation

Defra project code SP1106

Contractor organisations

SKM EnvirosCranfield UniversityCentre for Ecology and HydrologyBritish Geological SurveyRothamsted Research / North Wyke

Report authors Phil Wallace (pwallace@globalskm.com), Guy Kirk, Pat Bellamy, Bridget Emmett, David Robinson, Inmaculada Robinson, Barry Rawlins, Ron Corstanje, Roland Bol.

Project start date October 2010Sub-project end date

March 2011

Exploration of methodologies for accurate routine determination of soil carbon

Sub-project iv of Defra project SP1106: Soil carbon: studies to explore greenhouse gas emissions and mitigation

Contents

Executive summary

1 Introduction2 Review of sampling and analysis for soil carbon

2.1 Identification and exploration of the different soil carbon fractions2.2 Review of sampling strategies2.3 Review of laboratory analytical techniques (UK and international)

3 Workshop discussion outcomes and SOPs (see annexes)4 Review of New and emerging technologies

Annex 1 Soil sampling Standard Operating ProcedureAnnex 2 Soil carbon determination Standard Operating Procedure

Executive Summary

There has been a number of soil monitoring exercises carried out over the last 35 years in England and Wales, as well as other areas of the UK and internationally. The methods used at each survey for soil sampling and soil carbon determination in England and Wales have not been consistent, which has led to difficulties in identifying and quantifying trends in changes in soil carbon over time. Soils contain a large stock of carbon and any changes, due to e.g. changes in land management, may be significant when compared to the UK’s annual emissions and therefore need to be accurately determined. This project was therefore tasked with standardising the methodology for future monitoring schemes through the production of standard operating procedures agreed by the UK’s soil science community. The project also explored the various soil carbon fractions, their definitions and relationships with models. In the future, soil carbon determination may be by the use of new technologies and these were reviewed.

Soil organic matter in soil may survive intact for a relatively short time period of days to a few years, an intermediate time period of years to decades, or be recalcitrant and remain in the soil for decades to centuries. Methods to measure these pools of soil carbon have been developed using physical, chemical or biological fractionation, or combinations of these methods.

Soil sampling methodologies were extensively reviewed by Black et al. (2008). Sampling should be carried out such that determinations of both soil carbon stock and change can be carried out. This requires an adequate number of sample replicates to be taken, bulked into composite samples and sub-sampled, if appropriate, from an area of land identified as suitable. Soils have to be sampled to a depth that is consistent with past measurements whilst allowing for changes in techniques in the future. Soil bulk density in the topsoil, but also by horizon for soil carbon stocks, needs to be measured. Issues were discussed and agreed at a workshop by the soil science community and a standard operating procedure proposed.

Soil carbon analysis laboratory methods were reviewed. Traditionally, wet and dry combustion methods have been used such as a modified Walkley Black or loss on ignition. Total organic carbon by combustion, measuring CO2 evolution at high temperature with an elemental analyzer after acid pre-treatment of the soil, has also been more recently utilised. These methods, as well as future technologies, were discussed at the workshop and agreement reached as to the methods to be used in future soil monitoring schemes based on the more traditional methods. A standard operating procedure was agreed.

Nine new and emerging technologies were reviewed for their potential to monitor soil carbon. Visible/near and mid infra-red diffuse reflectance spectroscopy show promise as well as thermo-gravimetry – differential scanning calorimetry and Rock-Eval pyrolysis.

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1 Introduction

Monitoring of soils in England and Wales has been carried out in two samplings of the National Soil Inventory (originally in the period 1978-1983 followed by re-sampling in phases between 1994-1995, 1995-1996 and 2003) and the samplings of the Countryside Surveys in 1990, 2000 and 2007. Other surveys in the UK have been carried out in Scotland, Northern Ireland, of forested areas and for geochemical properties, including soils, since the 1960s. Soil monitoring has been carried out in other parts of the EU and elsewhere in the world.

The sampling and analytical methods employed during the soil monitoring of England and Wales have not been standardised leading to potential differences such that changes in organic matter in soils over time are difficult to detect. In order to measure changes in soil carbon in the future in response to impacts such as climate change and management practices, standardisation of techniques is required. Soils in the UK contain about 10 billion tonnes of carbon, equivalent to over 50 times the UK’s annual greenhouse gas emissions, so small changes in soil carbon due to changes in soil management practices or land use may have a large impact on our emissions. Soil monitoring in the future should be designed to be able to detect changes in soil carbon from past surveys such that soil carbon levels can be adequately monitored.

Soil organic matter includes fresh plant and animal residues, decomposed materials in more resistant forms and elemental carbon. These fractions require definition, especially in relation to the pools in soil carbon models. Traditional methods of soil carbon measurement include the destruction of the organic matter by chemical or combustion techniques. Other, non-destructive, techniques for soil organic matter determination have been investigated, including in situ and ex situ methods.

The aims of this sub-project were:

1 To review methodologies, in UK and internationally, for determining soil carbon and its various fractions, including field sampling and laboratory analysis.

2 To define a new standard operating procedure (SOP) for use in future soil surveys, discussed and agreed through a workshop of experts from the soil science community.

3 To explore new and emerging techniques in terms of their robustness across different soil types, practicality for non-specialists, the time to develop and adopt them, and their cost-effectiveness.

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2 Review of sampling and analysis for soil carbon

2.1 Identification and exploration the different soil carbon fractions

2.1.1 Measures of SOM quality for monitoring and modelling

The decomposability or ‘quality’ of soil organic matter (SOM) as a biological substrate varies from labile material that decomposes within a few weeks or months, to recalcitrant material that can persist for centuries. Factors governing SOM quality include its chemical composition, its stabilisation by association with clay minerals and oxides and other mechanisms, and its location within the soil as this influences access of microbes, oxygen and other reactants to it (Sollins et al., 1996; Six et al., 2002; von Lützow et al., 2006). As a result, SOM quality varies more-or-less continuously across a spectrum, and discrete, operationally-defined ‘fractions’ can only describe this continuous variation approximately. Nonetheless, some form of operationally-defined fractionation is necessary for making measurements to characterize SOM for monitoring and modelling. The plethora of possibilities available for this is illustrated in Table 1.

Table 1 Soil organic matter pools defined by turnover times, and related measured fractions (after Wander, 2004)

Pools Measured fractionsLabile SOMHalf-life days to a few yearsMaterial of recent origin or living components of SOM; material of high nutrient or energy value; physical state or location does not impede access to microbes, oxygen or other reactants

Microbial biomassChloroform-labile SOM Microwave-irradiation-labile SOM Amino compounds Phospholipids Labile substratesMineralizable C or N, estimated by incubation Substrate-induced activity Soluble, extractable by hot water or dilute salts Easily oxidized by permanganate or other oxidants Residues for which chemical formula can bedescribed, inherited from living organismsLitter, vegetative fragments or residues Non-aggregate-protected SOM Polysaccharides, carbohydrates

Intermediate SOMHalf-life of a few years to decadesPhysical state or location impedes access

Partially-decomposed residues and decay productsAmino compounds, glycolproteins Aggregate-protected SOM OthersAcid/base hydrolyzable humic substancesMobile humic acids

Recalcitrant SOMHalf-life of decades to centuriesRecalcitrant because of chemical composition and/or mineral association

Refractory compounds of known originAliphatic macromolecules (lipids, cutans, algaenans,suberans) Charcoal Sporopollenins Lignins OthersHigh molecular weight, condensed SOM Humin Non-hydrolyzable SOM Fine-silt, coarse-clay associated SOM

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Most existing SOM models are based on a number of pools (usually two for plant litter and three for SOM) defined by first order decomposition rate constants, and SOM turnover is calculated from transformations between the pools (Jenkinson, 1990; Parton et al., 1993; Bruun et al., 2010). Such models have been widely and successfully used to simulate changes in total soil carbon in response to changes in soil management and land use. However, they have the practical limitation that, because the pools are hypothetical, and not directly measurable, their parameters must be fitted to data. But the calibration necessarily cannot cover all soils and land uses; notably, most current models do not cover the cold, wet, humose soils where the largest carbon stocks occur (Bellamy et al., 2005; Schultze & Freibauer, 2005). The models also have the limitation that the fitted, discrete pools are a crude representation of continuously varying SOM quality, and they are therefore poor at revealing underlying processes and mechanisms (Bruun et al., 2010). Also, the number of parameters to be fitted for a multiple pool model – including those for the dependences of the rate constants on temperature, moisture, redox and other variables – is very large and it may be difficult to fit them with much certainty.

There has been some success in developing measurement schemes that approximate to particular model pools (Hassink, 1995; Trumbore & Zheng, 1996; Magid et al., 1996; Six et al., 2000; Christensen, 2001; Sohi et al., 2001; Zimmermann et al., 2007). But the fundamental problem remains that the pools and corresponding fractions are hypothetical. This has led to calls to base models on explicitly measurable pools with variable reactivities (Hassink, 1995; Christensen, 1996; Elliott et al., 1996; Arah & Gaunt, 2001) and on continuously varying measures of SOM quality, such as particle size, particle density or resistance to oxidation (Ågren & Bosatta, 1996; Bruun et al., 2010).

2.1.2 Measurement schemes

Physical fractionation

Separation by physical fractionation procedures emphasises the role of physical protection and location in SOM turnover (Balesdent, 1996; von Lützow et al., 2007). Also these procedures are often easy and cheap in comparison with chemical or biological methods.

Particle size

Particle size reflects soil mineralogy and chemical composition, and these are important determinants of interactions between SOM and mineral matter; for example, sand-sized quartz grains are relatively inert compared with clay-sized minerals. Hence fractionation based on particle size is a reasonable basis for separating SOM of different quality. Figure 1 (from von Lützow et al., 2007) summarises typical distributions of SOM across particle size fractions in temperate soils, and the corresponding distributions of measures of SOM quality and turnover rates. It shows a near-continuously increasing proportion of SOM associated with decreasing particle size. However, the turnover times of SOM in different particle-size fractions, gauged by stable isotope fractionation, are very variable (von Lützow et al., 2007). Particle size and related interactions with SOM will vary continuously from coarse to fine particles. Better relationships between particle size and turnover time may be obtained by separating a size continuum to resolve the distribution better, possibly in combination with other quality variables (Bruun et al., 2010). Traditional methods used to separate SOM according to particle size include sieving and sedimentation, sometimes after dispersion with ultrasonic vibration or a chemical dispersant (Christensen, 1992). Bruun et al. (2010) review methods from colloid science that might be adopted to measure continuous distributions of SOM in silt and clay fractions of soils, and conclude several of the methods are promising.

So called ‘particulate organic matter’ (POM) is used as an indicator of early trends in SOM and nutrient status in managed soils (Magid et al., 1996; Gregorich & Carter, 1997; Yakovchenko et al., 1998; Carter, 2002). It is separated from coarse (> approx. 50 μm) particle size fractions by sieving and sedimentation (Cambardella & Elliott, 1993; Salas et al., 2003).

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Figure 1 Typical distributions of SOM across particle size fractions in temperate soils, and the corresponding distributions of measures of SOM quality and turnover rates (von Lützow et al., 2007)

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Density

Fractionation methods based on density generally produce two fractions – heavy and light – generally by centrifugation in a heavy liquid (Christensen, 1992). The scheme of Sohi et al. (2001) uses a sequence of separations in a concentrated sodium iodide solution, with and without ultrasonic dispersion, to obtain four fractions: a dissolved fraction; a light fraction, taken to be free organic matter; a second light fraction, taken to be intra-aggregate organic matter; and a heavy fraction, which is the residual organo-mineral matter. The fractions display consistent differences in their chemical properties, as determined by NMR spectroscopy, pyrolysis mass spectrometry and thermal analysis (Lopez-Capel et al., 2005b; Poirier et al., 2005; Sohi et al., 2001, 2005); these properties may correlate with in situ SOM quality. However, further work is needed to calibrate this indirect measure of SOM quality with the soil conditions at the time of sampling. Sequential density fractionation has been made with up to eight different densities (Baisden et al., 2002; Sollins et al., 2009), but this is a tedious procedure. An alternative may be to use density gradient centrifugation, in which a density distribution of particles is created and samples withdrawn at different depths within it (Bruun et al., 2010).

Aggregate size

Fractionation based on aggregate size takes further the role of intra-aggregate protection of SOM. A distribution of aggregate sizes can be obtained by sieving or sedimentation without applying a dispersion treatment. A difficulty is that aggregates are associated in a hierarchy, such that the smallest first order aggregates are located within larger second order aggregates, and so forth (Oades & Waters, 1991).

Others

Bruun et al. (2010) discuss possibilities for developing SOM fractionation schemes based on the role of iron oxides (Torn et al., 1997; von Lützow et al., 2007) or clay surface charge (Tipping, 2002) in binding SOM and thereby protecting it against decomposition. These are at an early stage of development.

Chemical fractionation

This includes extraction in aqueous solutions, with and without electrolytes; extraction in organic solvents; hydrolysis with water or acids; oxidation with chemical oxidizing agents; and chemical destruction of mineral phases. These are all reviewed by von Lützow et al. (2007). We also include here separation by heating, whereby organic components of different thermal stability are sequentially removed1. The main advantage of chemical fractionation is that it can isolate purely organic matter, free of mineral components. A disadvantage is that it is insensitive to SOM stabilisation by physical protection or location. Also samples are likely to be heavily altered by chemical treatments, and are therefore unsuitable for further analysis of the extracted SOM.

Chemical methods can be generalized by applying successively greater concentrations of reagents or successively longer extraction times. Bruun et al. (2010) argue that such procedures should be allowed to reach steady state before measurements are taken, not interrupted after a particular time, so that all the labile material will have been removed, even though some small part of the resistant SOM may also be removed. Under non-steady state conditions, complete removal of the labile SOM is not guaranteed, but the removal of some resistant material will occur anyway.

Hydrolysis

Hydrolysis in acid is widely used to isolate recalcitrant SOM (Martel & Paul, 1974; Laevitt et al., 1996; Paul et al., 2006). However, hydrolysable SOM appears to be relatively insensitive to soil and crop residue management (Balesdent, 1996; Plante et al., 2006). Thus its inability to reflect the changes ofsoil and crop management indicate it is not a good measures of soil organic matter quality. Bruun et al. (2010) propose that this problem may be resolved by using continuous distributions of acid concentration or treatment time.

1 Thermal analysis separates soil organic matter according to chemical bond energies.

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Oxidation

Various chemical oxidizing agents are used to separate labile SOM from more resistant material (Mikutta et al., 2005). Some studies have found that SOM remaining after oxidation has greater radiocarbon age than that removed (e.g. Helfrich et al., 2007). But, as with hydrolysis, oxidation-resistant SOM is not very sensitive to soil and residue management (Balesdent, 1996; Plante et al., 2004; Bruun et al., 2008). Again, Bruun et al. (2010) argue this may be a limitation of separation into only two fractions, and a continuously distributed measure of oxidizability may work better.

Thermal analysis

Recently thermogravimetry / differential scanning calorimetry (TG-DSC) has been used to fractionate SOM from a continuous spectrum of thermal decomposability (Manning et al., 2005; Plante et al., 2009). In TG-DSC, a soil sample is heated under a constant flow of carrier gas causing it to volatilize and, depending on the gas composition and temperature, oxidize. The mass remaining, mass evolved and sample temperature are monitored throughout, and continuous spectra of changes in mass and energy input against temperature are obtained.

In further developments a TG-DSC instrument has been connected to a stable isotope ratio mass spectrometer (Lopez-Capel et al., 2005a) and a scanning quadrupole mass spectrometer (Lopez-Capel et al., 2006) to simultaneously determine the elemental and functional group compositions of putative SOM fractions (evolved gas analysis) and the distributions of applied tracers in them (stable isotope analysis), so as to derive information on inter-fraction fluxes.

Biological fractionation

Given that the great majority of SOM transformations in soils are mediated by microbes, it makes sense to try to exploit biological processes to fractionate SOM. A distinction needs to be made between use of biological methods as a means of extracting particular types of SOM – akin to chemical fractionation – and attempts to simulate biological decomposition processes in real soils. For the latter there are various complications that need to be considered.

First, biological measurements on small portions of soil will not reproduce the full functionality of in situ soil fungal and bacterial communities; ideally an entire, intact soil pedon is required. Second, in planted soils under field conditions, the turnover of native SOM is fuelled by carbon supplied by the living plants, and measures of rates of turnover need to take account of this (Paterson, 2003). Hence the decomposition of more-recalcitrant SOM may limited by the supply of more-labile substrate, which will be continuously replenished in planted soil but may become exhausted in unplanted soil (Hartley et al., 2008). Third, the time course of soil respiration cannot be used directly to separate more and less labile SOM because part of the latter will be mobilised throughout the period of measurement. We discuss below potential ways around these complications.

Soil respiration

Measurements of the soil-atmosphere flux of carbon (J) in real, planted soils necessarily conflate the flux from the plant and recent plant debris (JP) and the flux from the SOM (JS). New non-invasive techniques are available for separating JP and JS based on 13C/12C isotope fractionation, and differences in fractionation between old and fresh forms of SOM. The natural isotope composition of CO2 derived from decomposition of SOM C differs from that derived from respiration of living plant roots and recent litter by small but detectable amounts (Hanson et al., 2000; Bowling et al., 2008; Paterson et al., 2009). Bowling et al. (2008) report a mean difference in δ13C between root and soil respiration of 4.2‰ based on 14 studies of root respiration and 38 studies of soil respiration. In principle this provides a means of separating SOM C turnover from root and litter respiration. The contribution of root respiration to the overall CO2 flux varies from anywhere between 10 and 90%. So potentially this greatly facilitates the fitting of model pools and temperature and moisture sensitivities to data.

If δA is the δ13C of CO2 from plant and litter C (normally c. -27‰ in C3 species), δB that of CO2 from SOM C (typically -23 to -25‰), and δS is the δ13C of CO2 emitted at the soil surface, which is a composite of δA and δB, then the fraction ε of SOC-derived C in the emitted CO2 is

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ε=(δS−δB )/ (δ A−δB )The end members δA and δB can, in principle, be measured separately. Therefore, this equation allows CO2 emissions to be partitioned into two component fluxes from the two fractions A and B if δA

and δB are constant between samplings. Millard et al. (2008, 2010) and Midwood et al. (2008) have successfully used this to partition autotrophic and heterotrophic soil respiration. In principle it should be possible to separate further fractions by labelling plants with 13C-depleted CO2 and measuring the time course of CO2 emission and its δ13C with sufficient resolution, and relating this to further SOM fractions.

Enzymatic digestion

Various enzymes are available to dissolve particular organic compounds in soils, and this could be the basis of a powerful SOM fractionation scheme, particularly if combined with isotope tracer techniques (Paterson et al., 2009). Enzymatic digestion could be used to estimate a quality distribution by repeated measurements of the amount of dissolved SOM (Bruun et al., 2010). The addition of enzymes results in addition of organic matter, so these techniques should be combined with some form of labelling to distinguish between added and native C.

2.1.3 Combining Measurements

Individual fractionation schemes will only give an approximate picture and are unlikely to capture the multiple mechanisms that govern SOM quality. Closer estimates may be obtained by combining multiple measurements in sequential schemes. For example, an initial fractionation based on density might be combined with thermal fractionation of the individual density separates.

Various measurements can be made on the individual separates to characterise the SOM within them, assuming the fractionation procedure has not transformed the SOM. For this, stable isotope measurements are a powerful adjunct to carbon concentration measurements, as discussed above. Also various spectroscopic methods have been used to characterise SOM in soil separates, including pyrolysis GC-MS, NMR, and NIR and MIR (near- and mid-infrared spectroscopy).

Statistical model-data fusion techniques are increasingly being used to analyse multiple data sources of this sort in parameterising soil carbon models (e.g. Raupach et al., 2005; Xu et al., 2006; Zobitz et al., 2008). In model-data fusion, data are combined with prior knowledge of a system to get an estimate of the true state of the system, which is expressed as a set of distributions of the model’s parameter values. The model performance is treated as a random variable, containing error. It is combined with initial parameter estimates to generate joint probability density functions for the model parameters; these express how the parameters co-vary based on the form of the model and its response to the measurement data. The advantage of this over other statistical fitting techniques is that it takes prior knowledge of the system into consideration, and it can provide estimates of the uncertainty of model estimates.

2.1.4 Conclusions

Many fractionation schemes are available. Agreement between fractions and independent estimates of SOM quality is often not very good. More-continuously distributed estimates across the spectrum of SOM quality are likely to better

capture underlying mechanisms. Multi-variate estimates are likely to be better still. There are several emerging techniques for continuous fractionation that look promising. Model-data fusion techniques are available to analyse the resulting datasets.

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Sollins P., Kramer M.G., Swanston C., Lajtha K., Filley T., Aufdenkampe A.K., Wagai R. & Bowden R.D. (2009) Sequential density fractionation across soils of contrasting mineralogy: evidence for both microbial- and mineral-controlled soil organic matter stabilization. Biogeochem. 96, 209231.

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2.2 Review of Sampling Strategies and how they influence organic carbon concentrations

2.2.1 Review of Sampling Schemes in UK

This review of sampling strategies used for soil carbon determination draws on the results of the Environment Agency (EA) funded project ‘Design and operation of a UK soil monitoring network’ Black et al. (2008) and the SNIFFER funded project ‘National Soil Monitoring Network: Review and Assessment Study’ Emmett et al. (2007). A number of different sampling schemes have been established across the UK and their sampling strategies and within site sampling are summarised below. All schemes included here are those where organic carbon has been measured at at least some of the sampled sites.

National Soil Inventory (NSI) England and WalesThe Inventory was made to obtain an unbiased estimate of the distribution of the soils of England and Wales and of the chemistry of the topsoil (0-15 cm depth) including organic carbon. The initial sampling was carried out between 1978 and 1883 (5600 sites), about 40% of the sites were re-sampled in 1995 (arable sites) 1996 (permanent grassland sites) and 2003 (sites with other land uses) for organic carbon and pH. There are currently no plans to re-sample the sites again.

The sites were on a 5 km grid displaced 1 km north and east from the Ordnance Survey National Grid 00 and 05 km grid lines and were visited using 1:25,000 maps to locate the sampling point. Where the point was inaccessible or there was no soil, a defined procedure was followed to find a substitute site. At each site, a soil pit was dug to 80 cm and augered to 120 cm and the site and soil described according to the Soil Survey Field Handbook (Hodgson, 1976) and recorded on a standard proforma. A bulked topsoil sample (0 - 15 cm excluding vegetation and litter) was collected with a screw auger from 4 m intervals within a 20 x 20 m square around the pit. For organic soils, this proved to provide insufficient sample and further sub-samples were collected from the same spatial intervals. (McGrath and Loveland, 1992).

Countryside SurveyThe Countryside Surveys provide a national network of sites across Great Britain, representing the main types of landscape, land cover and soil groups. The surveys aim to provide good quality data about soil chemical and biological properties for the development of national databases and to improve the understanding of links between soil biology, chemistry and the wider environment to support the development of suitable, effective strategies and policies relating to soil protection. They were sampled for soil in 1978, 1998 and 2007.

A stratified random sampling scheme stratified by land use was used to identify 256 primary sampling sites. These fixed sampling sites of 1 km square were located through GPS, photographs and OS grid coordinates. There were five fixed sampling plots within each 1 km square (X-plots), which were located as above with, in addition, a permanent marker in the soil nearby (from 1990). Four cores of varying depth were taken from a specific location relative to the centre of each X-plot of the Countryside Survey sample squares. A number of tubes of varying lengths up to 15 cm and 4 or 5 cm diameter were used for collection and hammered vertically into the soil surface. Discretion was allowed regarding relocation of sampling points if roots, stones or other obstacles prevented sampling (CEH, 2007). The soil samples taken in 1978 were taken using a trowel from top 15cm of a soil pit at the centre of the X-plots.

National Soil Inventory of Scotland (NSIS)This inventory was to provide an unbiased sample to characterise soil distribution and quantify variability in soils and properties at a broad, regional scale in Scotland and to quantify heavy metals in Scottish topsoils. The NSIS was originally sampled during the period 1978-1987 and was repeated in 2008-2010.

NSIS sample locations on a 10 km grid aligned to Ordnance Survey National Grid 00 grid lines were plotted from 1:50 000 scale Ordnance Survey maps onto air photos that had an approximate scale of 1:25 000 using a ‘sketch master’ that allowed the superimposition of both map and photo images. The location of the grid intersect was marked with a small dot (approximately 0.5 mm diameter) on the photo. The national grid reference was clearly marked on the photograph. Once at the site, a soil pit was dug, the soil and site described and relevant samples taken generally from a 10 cm band in the

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middle of each horizon. Where the site landed on an area with no soil cover, a site within 100 m from the grid intersect point was located, firstly to the north and then to the east, south or west (Soil Survey of Scotland, unpublished).

Northern Ireland National Soil Inventory (NINSI)This scheme represents the first cycle of a NI monitoring programme carried out in 2005 where the focus was on identifying and mapping the overall changes in soil fertility of Northern Ireland soils. As a result, agronomic sampling aimed at the top 75mm of soil was undertaken. However, for comparison with the AFBI 5K PITS 1995 data, the A-horizon was also sampled so that changes in soil-pH, P, K, %C and %N could be identified for the period 1995-2005. There were about 500 sites sampled in both surveys. The intention is that this will be repeated again in 10 years.

Sites on a 5 km grid using the Irish Grid were visited using 1:10,000 maps and orthophotos to locate the sampling point. Where the point was inaccessible or there was no soil, a defined procedure was followed to find a substitute site. Soil samples to 7.5 cm depth were taken at 2 m intervals along a 71 m transect chosen to be representative of the field and bulked to give a single representative soil sample for that site. A single topsoil sample (1 kg) was also taken from the entire depth of the A horizon exposed in a pit dug at the midpoint of the 71 m transect (Higgins, 2003).

BioSoilThe aims of the Biosoil scheme was to establish a background network for forest soils in Europe The initial sampling was carried out in 2006 and the intention is to re-sample every 10 years. There are 167 sites distributed across the UK.

Sites were based on a 16 km grid, where they fall within a forest. A 500 x 500 m square (divided into 25 100 x100 m grid squares) was laid out around the sampling point and each 100 x 100 m square assessed according to an identified sequence in order to find the nearest cell with >50% woodland. A 25.24 m radius circle was delineated on the map (biodiversity plot) and a soil pit location identified within this circle. The positioning of the circle could be varied in the field in response to forest conditions but its final position was permanently marked with a marker driven into the ground. The soils of the site were described, a profile, representative of the dominant soil, described according to FAO guidelines (FAO, 1990) and the soil classified according to the World Reference Base of Soil Resources (FAO et al., 1998). Soil horizons were sampled and analysed to confirm the classification. According to the sampling protocol the soil pit must be located more than 20 m in from any forest edge. Surface organic horizons were sampled using a 25 x 25 cm sampling frame and then mineral layers (mandatory or optional fixed depths of 0-10, 10 - 20, 20 - 40 and 40 - 80 cm depending on the status of the site) were taken. Bulk density samples were taken at 0 - 10 cm. (Forest Research, 2010 and Forest Soil Co-ordinating Centre, 2006)

The Geochemical Baseline Survey of the Environment (G-BASE)G-BASE is the national geochemical survey of the UK surface environment funded by the British Geological Survey (BGS) via the NERC Science Budget. The aim of the survey is to provide systematic data on the geochemistry of the GB rural surface environment based on samples of stream sediments, stream water and soils for resource management and environmental purposes. The survey is on-going, having been started in 1968, and by 2015 will cover the whole of the UK. There will be about 128000 samples but not all samples will be analysed for organic carbon.

A systematic, unaligned sampling strategy was adopted in which one sample was collected from a random location in every other 1 km square of the British  National Grid, subject to the avoidance of roads, tracks, buildings, railways, electricity pylons, and disturbed ground. One in every 100 of these sites was randomly selected and designated a duplicate sampling site at which the following sampling protocol was adopted. At each sampling site, including those selected for duplicate sampling, five incremental soil samples were collected using a Dutch auger at the corners and centre of a square with a side of length 20 m and combined to form a composite sample of approximately 0.5 kg. At each of these five points, any surface litter was removed and the soil sampled to a depth of 15 cm into the exposed soil. In the case of organic-rich soil, 5 cm of surface litter was removed and the soil samples were collected from a depth range of 5–20 cm. This composite sample was referred to as duplicate A (DUP A). At each of the duplicate sites, another composite sample was collected from one of four squares with the same support.  The second sample square (1 of a possible 4) was selected randomly, and the same sampling procedure adopted; this composite sample was referred to as

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duplicate B (DUP B). At each sampling site, the same procedure was applied to the collection of soil samples at depths of 35 and 50 cm from the soil surface using the same auger holes (B Rawlins 2011, pers. comm. 26 January).

It can be seen that these sampling schemes vary considerably in a number of areas. The actual sampling design used, that is, the spatial and temporal frequency and pattern of sampling, was determined by the objectives of the sampling exercise and the different schemes across the UK had very different objectives. For example the National Soil Inventory of England and Wales was designed as an inventory to determine the geochemical status of soil across England and Wales and it was not intended that it would be re-sampled. In contrast the Countryside Survey was designed to monitor change in the natural resources of the UK, the soil being only a small part of the survey.

In this project we have not recommended a particular sampling design as this would be determined by the specific objectives of the sampling exercise such as: are we reporting on national stocks of carbon or rates of change or do we require maps showing where the largest stocks are or where those stocks are changing. Also the differences between stratified and systematic designs have been considered extensively in Black et al. (2008) who recommended the use of a stratified design if the aims are to estimate mean rates of change at a regional scale.. However, it is important to compare the different sampling strategies that apply once a site has been selected and summarise the effect on the organic carbon determination.

2.2.2 Sampling methods and procedures

There are a number of different aspects of a schemes sampling strategy that can effect the soil organic carbon determination and these are outlined in the following section.

Sampling location

It has been shown (Lark et al. 2006) that any site sampled initially should be revisited to assess change in soil carbon over time; this is due to the correlation between soil carbon values at a site. All surveys reviewed indicated that sufficient site details should be taken to allow relocation after five to 10 years (such as OS coordinates, use of GPS and photographs). In addition, Black et al. (2008) suggested that there should be a requirement for a contingency approach to establishing the final sampling locations and numbers in any designed scheme since a proportion of sample locations will be inaccessible or lost in any single monitoring event, for example a Foot and Mouth Disease outbreak or land development. A reasonable contingency considered by Black et al. (2008) would be up to 20 per cent of the total sample number, depending on the risk of losing sample locations from contemporary risks.

Sampling support

The sampling support – the area represented by the sample of soil taken, varied considerably from survey to survey within the UK. The National Soils Inventory (NSI) of England and Wales followed the methods of the Soil Survey England and Wales handbook (Hodgson, 1976) with 25 sub-samples per sample taken by auger from a 20 m x 20 m square, the Countryside Survey involved a single core from a set location following the Countryside Survey methodology (Black et al., 2000), whereas the National Soils Inventory for Scotland (NSIS) took samples from a soil pit. The current sampling of the NSIS is comparing different methods of collecting samples which should give some indication to the variability and help to inform the requirements for sampling support. Some initial results have been published by the NSIS sampling team where they compared taking a composite sample from a 20 x 20 m plot with taking a sample from a soil pit by horizon to estimate organic carbon – this gave good agreement in mineral soils but not in organo-mineral soils (Lilly et al. 2011) (see figure 2) – demonstrating the importance of using the same sample support when re-sampling or comparing across surveys.

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Figure 2 %C as determined from individual samples taken with a soil auger at 0-15cm depth over a 400m2 area and from profile topsoil samples (taken from Lilly et al. 2011).

Saby et al. (2008) showed that the coefficient of variation of organic carbon increased with the size of the sample support based on an analysis of published literature and recommended a maximum area for the sample support of 1 ha. All the sampling schemes reviewed here follow this recommendation, the largest support being 400 m2.

Sampling depth

Within the UK, as detailed in section 2.2.1, the depth to which the soil was sampled to measure organic carbon varied from 7.5 to 30 cm. This range of depths is similarly sampled across Europe and the rest of the world.

The EU LUCAS2 project, which is similar to the UK’s Countryside Survey, samples soils from 0-10cm. In an effort to identify SOC in mineral soils across the EU a new method, named the “Area-Frame Randomized Soil Sampling” method (AFRSS) has been developed by the European Commission’s Directorate General Joint Research Centre (JRC) in Italy (Stolbovoy et al., 2005). The method considers croplands, pasture and forests:

Cropland: with regard to ploughed soils, given that the thickness of the plough horizon differs according to cultivation practices, then the AFRSS methodology proposes to keep the sampling depth in accordance with the existent thickness of the plough layer. One sample should be taken from the middle of the plough horizon (e.g., at 10-20 cm depth if plough horizon is 30 cm thick. An undisturbed soil sample with the cylinder to determine the bulk density should be taken at the same depth (Stolbovoy et al., 2007).

Pasture: follow the IPCC procedure (IPCC, 2003) which recommends sampling 30cm divided into 3 depths, 0-10, 10-20 and 20-30cm.

Forests: require that the litter layer is sampled in its entirety and then the mineral layer be divided in to 10cm depth increments, no total depth is presented, but IPCC guidelines can be followed.

In other European countries a recent survey of soils from North Belgium sampled from 0-30 cm (Meersmans et al. 2009). Whereas soil samples collected for the Irish National Soil Database (NSDB) extend from 0-10cm, split into 0-5 and 5-10 cm (Fey et al. 2007).

IPCC views a single soil layer (0-30 cm) for cropland, pastures and forests, it recommends the 30 cm be divided into 3 depths, 0-10, 10-20 and 20-30 cm (IPCC, 2003). In the USA there seems to be a range of recommendation and practice, for instance, Lal et al., (2000) recommend sampling from 0-50

2 http://www.lucas-europa.info/NewsBASE/content_eftas_lucas01/frame_deutsch.php

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cm, whilst the Enhanced US FIA monitoring framework forest soils are sampled below the organic horizons for the mineral soil between 0-20 cm (Bechtold and Patterson, 2005). The recommended sampling depth for pasture soils in mainland Australia is 0-10 cm and Tasmania and New Zealand use 0-7.5cm (Coad et al. 2010).

2.2.3 Sample preparation

Methods of sample preparation used in the UK are reviewed in Black et al., 2008, for the preparation of bulked composite topsoil samples for soil carbon analysis and are summarised below. NSI England and WalesAfter collection, samples were transferred to the laboratory and air-dried at room temperature. Half of each air-dried sample was milled in a mild-steel roller mill to pass a 2 mm sieve. All chemical analyses apart from total elemental concentrations were measured on this material without further crushing. The other half of the sample was kept in an un-ground state as an untouched sample that could be resorted to in cases of contamination at any stage. For total element analyses a 25 g sub-sample of the <2 mm soil was obtained by coning and quartering and ground to <150 µm in an all-agate planetary ball mill. (McGrath & Loveland, 1992).

Countryside SurveySoil is disaggregated and homogenised. A 10 g sub-sample is taken for pH (avoiding stones and roots). The sample is then dried at 25°C with crushing during the process (if necessary). When soils have dried sufficiently they are sieved through a 2 mm stainless steel mesh using a wooden paddle. A sub-sample is taken with further sub-samples ground (<0.5 mm) or agate-ball milled for subsequent analyses (CEH, 2007).

NSISThe field sample was spread out and dried in a warm-air room (about 30°C). The sample was then repeatedly rolled, crushed gently and shaken on a 2 mm sieve to derive a <2 mm soil fraction. Coning and quartering was used to derive a 2-3 g representative sub-sample which was subsequently finely-ground sample in an agate ball mill to approximately 100 mesh (150 µm) (Macaulay Institute, 1971).

Northern Ireland NSISoil samples were transferred to a tray. Stones were removed by hand and the sample dried in an oven not exceeding 30°C for a minimum of 40 hours and until no sign of moisture remained. The dried soil was transferred to grinding canisters containing steel pestles that processed the soil to derive a <2 mm soil fraction by sieving through a 2 mm sieve (MAFF, 1986).

BiosoilAfter removal of living material (such as mosses, roots) and objects >2 cm, collected samples are air-dried or dried at a temperature of 40°C. The sample is subsequently crushed or milled to size <2 mm (Forest Research, 2006).

Environment Agency (National Laboratory Service, NLS)The sample is air-dried for 24 hours in a drying room where the temperature is not greater that 30°C. If the sample has dried into large, hard lumps (aggregates) it may be broken up using a pestle and mortar to allow it to pass through a 10 mm sieve. The sample is then processed through a ball mill and the <2 mm fraction retained (EA, 2008).

British Standards ISO 1 1464:2006The complete sample is air dried or dried in a ventilated drying oven not exceeding 40°C; to accelerate the drying process, breaking down the larger aggregates (>15 mm) during the process. Before crushing, which is necessary if soil samples have dried into large aggregates, extraneous matter should be removed from the dried sample. This process may be facilitated by the use of a 2 mm sieve. Care is taken to minimise the amount of fine material adhering to the extraneous matter removed. If a 2 mm sieve has not been used to facilitate removal of extraneous matter, then the dried sample is sieved through a 2 mm sieve. Any large dried particles remaining on the 2 mm sieve is crushed (using suitable apparatus) to smaller than 2 mm. The apparatus used is adjusted in such a way that complete crushing of particles larger than 2 mm before drying is minimised. For the preparation of the laboratory sample, the dried sample is divided, crushed and sieved (now <2 mm)

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into representative portions of 200 g to 300 g. The laboratory sample is split into representative portions until the required sizes of sample are obtained. Milling of the material between sub sampling stages may be necessary, to ensure homogeneity as the mass of the sub-sample is decreased (British Standards Institution, 2006).

All reported schemes have broadly similar sample preparation procedures. Macaulay - NSIS, NSRI - NSI and EA - NLS are the only schemes where a sub-sample of the original pre-treated soil is retained (although it will have been air-dried first). The ISO standard described in this section covers all eventualities except possibly how to deal with leaf litter and living material and it was suggested by Black et al., (2008) that this procedure be adopted for any future UK soil monitoring programme.

2.3 Review laboratory analytical techniques (UK and international)

Carbon (C) can occur in soils in organic (materials derived from the decomposition of plants and animals) inorganic (predominantly CaCO3) or elemental (charcoal, soot, graphite, and coal) forms. The subject of this review is soil organic carbon (SOC), but it is important to consider that many soils contain inorganic carbon from parent materials. SOC is considered to be the dynamic form in UK soils, subject to pool size alteration due to climate and land-use change, whereas elemental and inorganic C is considered relatively stable, but should not be overlooked if an assessment of total carbon stock is required for soils.

Standard and widely accepted methods for measurement of SOC include either wet or dry combustion methods to determine total organic carbon (TOC) (Lal et al., 2000). However, given the current emphasis on understanding carbon cycling, in general, but in soils in particular, TOC measurements may not be the most appropriate expression for supporting soil C modelling efforts. It may be more appropriate to partition C into several pools or fractions with different levels of reactivity or biological stability (Parton et al. 1987; Jenkinson and Coleman 1994); partitioning can be conceptual or analytical and a mixture of divisions exists in the literature. In general these pools consider labile organic matter derived from plant debris, moderately to highly resistant carbon from humified organic matter, and inert or highly protected organic matter (Skjemstad et al. 1998). In the following section we review analytical methods for determination of TOC and carbon fractions, finally considering their appropriateness for modelling SOC dynamics.

2.3.1 Laboratory measurement of total organic carbon (TOC)

Recent reviews of measurement methods for TOC include, Nelson and Sommers (1996) and Chatterjee et al. (2009). These reviews generally consider methods under wet and dry combustion:

Wet combustion: this includes the Walkley and Black (1934) (WB), and subsequent modified methods (e.g. Table 2 adapted from Table 1 in Chatterjee et al., 2009). It comprises the rapid dichromate oxidation of organic matter. Issues with regard to this method include: disposal of products containing chromium (chromium containing wastes must be stabilised before landfill disposal, which adds to the costs), incomplete oxidation of organic C, and that the method is poor for digesting elemental C forms. Bisutti et al. (2004) suggest fulvic acids are most readily decomposed whereas humus, lipids and proteins are most likely to be incompletely oxidized, with elemental carbon forms being considered inert to oxidation. Walkley (1947) reported that the WB method recovered only 2–11% of SOC in carbonized materials. Walkley and Black (1934) showed that recovery of organic C using the WB procedure ranged from 60 to 86% with a mean recovery being 76% (a correction factor of 1.33 is commonly applied to the results to adjust the organic C recovery). A modified method which included extensive heating of the sample during sample digestion, not requiring a correction factor, was proposed by Tinsley (1950) and by Mebius (1960), the modified method is commonly used but requires strict temperature control as acid dichromate solution decomposes at temperatures above 150 oC.

Dry combustion: this can be divided into methods using i) loss-on-ignition (LOI) where the sample is heated in a muffle furnace between 375-800 oC, and ii) dry oxidation (for Total Carbon (TC)) using an automated analyzer that combusts the sample between 950-1150 oC. Issues with regard to the LOI method include the assumption that weight loss is due entirely to combustion of SOC, temperature and length of combustion must be carefully controlled to prevent water loss from clays, combustion of carbonates, and oxidation of Fe2+ or decomposition of hydrated salts (Schulte and Hopkins, 1996).

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Bissuti et al. (2004) suggests that humified organic material resists oxidizing below temperatures of 500oC and quotes recovery rates of 90% ±4% when heated at this temperature for 24 hrs. SOC must be determined from LOI but different soils require different conversions ranging between 0.40 and 0.58 (Nelson and Sommers, 1996; Soil Survey Staff, 1992). Many conversion equations also include clay (Bisutti et al., 2004). Broadbent (1953) proposed the use of 0.526 to convert LOI to SOC for surface soils and 0.58 for subsoils and the Countryside Survey used 0.55, (Emmett et al., 2009). Automated carbon analyzers are an alternative method to LOI or wet combustion, the sample is oxidized at high temperature with all the soil C converted to CO2, the CO2 is separated and measured (Smith and Tabatabai, 2004). This method is considered to offer higher precision than LOI or wet combustion (Chatterjee et al., 2009). Issues with regard to this method include high initial cost of equipment, the small sample sizes used, which means subsampling is critical. Temperature is also critical with regard to oxidation of carbonates, Wright and Bailey (2001) compared combustion of calcite at 1040 and 1300 oC and found less than 5% was decomposed at low temperature, but 98% was decomposed at high temperature. Total organic carbon (TOC) may therefore have to be derived from the difference between Total Carbon (TC) and Total Inorganic Carbon (TIC), determined at different temperatures; Total Nitrogen (TN) may be determined simultaneously with TC.

Comparison of methods: The literature contains a number of comparisons of the different methods, the advantages and disadvantages are summarized in Chaterjee et al., (2009) reproduced here as Table 2.

Table 2 Comparison of analytical methods (adapted from Table 1 in Chaterjee et al. 2009)

Method Principle CO2 determination

Advantages/Disadvantages

I. Wet combustionCombustion train

Sample is heated withK2Cr2O7-H2SO4-H3PO4 mixturein a CO2-free air stream to convert OC in CO2.

Gravimetric/Titrimetric

Gravimetric determination requires careful analytical techniques and titrimetric determination is less precise.

Van-Slyke-Neilapparatus

Sample is heated withK2Cr2O7-H2SO4-H3PO4 mixturein a combustion tube attached tothe apparatus to convert OC inCO2.

Manometric Expensive and easily damaged apparatus.

Walkley-Black

Sample is heated withK2Cr2O7-H2SO4-H3PO4

mixture. Excess dichromate is back titrated with ferrous ammonium sulfate.

Titrimetric Oxidation factor is needed. VariableSOC recovery. Generatehazardous byproducts such as Cr.

II. Dry combustionWeight-loss-on ignition

Sample is heated to 430◦C in a muffle furnace during 24 hours.

Gravimetric Weight losses are due to moisture and volatile organic compounds.Overestimate the organic matter content.

Automated Sample is mixed with catalysts or accelerator and heated in resistance or induction furnace in O2 stream to convert all C in CO2.

Thermal conductivity,gravimetric, IRabsorptionspectrometry

Rapid, simple, and precise but expensive. Slow release of contaminant CO2 from alkalineearth carbonates with resistance furnace.

More detailed quantitative comparisons have been presented in the literature. Soon and Abboud (1991) compared LOI (375oC) and a dry combustion procedure (1371oC), as well as three dichromate extraction procedures with titrimetric quantification (WB, the modified Tinsley method [a heated dichromate extraction with H3PO4 as part of the extraction solution], and the modified Method [a heated dichromate extraction method]), and one dichromate extraction method with

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spectrophotometric quantification. The most precise method was the spectrophotometric method and the least precise was LOI, which was especially poor for low SOM contents; the coefficient of variation among replicate samples across methods ranged from 2.7% to 5.6%. The WB method gave comparable TOC contents (correlation coefficients = 0.979 to 0.996) to the other methods tested, but required a correction factor of 1.40, instead of 1.33. Working on 383 Belgium soils, Meersmans et al. (2009) found that WB required a correction factor of 1.47, the correction factor was observed to increase with soil wetness. Grewel et al., (1991) tested 40 New Zealand soils, comparing the Dumas dry combustion method with LOI and WB. WB compared well with the reference but required a conversion factor of 1.25. Whereas LOI was less well related to the reference method, chiefly due to loss of adsorbed water. Other researchers have reported negligible affects of water from clay minerals (Soon and Abboud, 1991; Grewel et al., 1991; David, 1988), or included clay % in calibration equations (Bisutti, et al., 2004). Clearly, choice of wet or dry combustion methods will result in uncertainties in the determination of TOC. Therefore, consistency in the use of methods is important for data comparison.

Cost comparison: A comparison of soil carbon sample analysis as of 2010 has been provided by the Soil Analysis Service of the Forestry Commission Forest Research (Table 3). For comparison, a survey of carbon analysis costs for the EU indicated a range from ~£5 to 14 per sample in 2007 (Stolbovoy et al., 2007) (http://www.forestresearch.gov.uk/soilanalysis) (converted from Euros using an exchange rate of £0.87 per Euro). Modified Walkley Black is a more costly method of analysis at £17.50 per sample (R Andrews, Cranfield University 2011, pers. comm. 11 April).

Table 3 Analysis costs (Centre for Forestry and Climate Change)

Analysis < 5 samples > 5 samplesTotal Nitrogen and Total Carbon (TN, TC) (%) £5.50 £5.50Total Inorganic Carbon (TIC) (%) £6.50 £6.50Total Nitrogen, Carbon, Organic Carbon and Inorganic Carbon (TN, TC, TOC, TIC) (%)

£12.00 £12.00

LOI (Loss On Ignition) Organic matter/Organic carbon (%) £7.50 £4.50

The methods used for carbon analysis for the prices quoted in Table 3 (F Bochereau, 2010, pers. comm. December) are detailed below:

Total Carbon (TC) and Total Nitrogen (TN) on soil samples:

Soil samples are ball milled for 9 minutes for homogenisation. Around 30 mg of soil sample is weighed in tin foil and made into a capsule. The sample is then combusted in a furnace at 900 oC with an oxygen influx (combustion reached approximately 1800 oC). The furnace contains a catalyst to help the combustion process. The carbon and nitrogen species then go through another furnace packed with copper wire and the nitrogen and carbon are separated by a gas chromatography column and detected by a Thermal Conductivity detector. The instrument is a Carlo Erba Flash EA1112. Residual moisture content of the ball milled sample is measured in order to give Total Carbon and Total Nitrogen on a dry weight basis. Total Inorganic Carbon (TIC):

Soil samples undergo a pre-treatment, they are placed in a furnace a 500 oC for 2 hours and analysed as above. The organic carbon has been destroyed so only inorganic carbon remains.

Total Organic Carbon (TOC):

Total Organic Carbon is measured by difference namely TOC = TC - TIC. So to measure TOC, two runs are made as described above and so TOC, TC, TIC and TN are given. LOI for Organic Carbon and Organic Matter:

The soil samples are weighed in crucibles and placed at 450 oC for 6 hours to remove all the organic matter. The measurement of weight loss gives the amount of organic matter present in the soil and using the equation Organic Matter = 1.724 x Organic Carbon we derive the Total

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Organic Carbon content. This is a less accurate method as clay soils can lose water at higher temperatures used for moisture content and some inorganic volatile compounds can introduce some error. Also the factor 1.724 is an average factor but can vary depending on the soil type and its composition. However, this is still a good test method for organic carbon estimation.

2.3.2 The importance of soil bulk density in relation to the determination of SOC stock

SOC can be measured as a concentration (g kg-1) or as a density (g cm-3), which requires that the bulk density be known. Density is also expressed as g/m2 when Equivalent Soil Mass (ESM) is used, which is defined as the reference soil mass per unit area chosen in a layer (Lee et al., 2009). Both concentration and density are normally determined over a fixed length scale, or depth. This is adequate for a one time estimate of SOC, but leads to problems if results from subsequent samplings are to be compared to determine stock change. Inferring changes in carbon stock by determining change in SOC concentration or density is not trivial and we discuss some of the pitfalls below.

Concentration is often used as a proxy for stock (Bellamy et al. 2005; Emmett et al. 2009). In the context of change, concentration can only be used as a proxy for stock when no soil horizon phase separation has occurred (i.e. the mineral and organic components are mixed e.g. in an A horizon, and no O horizon has developed). Changes in concentration as a proxy for stock can be interpreted in a meaningful way for uniformly mixed mineral/organic horizons like A horizons, but cannot be interpreted meaningfully for phase separated organic horizons. This means that comparison of concentration change as a proxy for stock should only be used for mineral soils. Lee et al. (2009) suggested that concentration is a good surrogate for stock in mineral soils where bulk density changes a lot, both spatially and temporally.

Density can be used to estimate changes in stock for mixed, and phase separated soil materials over a fixed length scale, only, when the soil bulk density can be assumed to have not changed. If the soil mass varies, a change in bulk density can appear like an increase or decrease in SOC for a fixed depth measurement.

e.g. A field is sampled to 15 cm before and after ploughing.

• Before ploughing, a soil has a bulk density of 1.5 g cm-3 and a SOC conc. of 40 g kg-1 – so that the soil C density is 90 t C ha-1 to 15 cm.

• After ploughing, the bulk density is reduced to 1.2 g cm-3 with a SOC conc. of 40 g kg-1 – so that now the soil C density is 72 t C ha-1 to 15 cm

an apparent loss of ~20% SOC

The error introduced by changes in bulk density can be corrected in two ways, I) by knowing the change in length scale that has occurred and correcting for it, or II) by correcting to a constant mass (Ellert and Bettany, 1995). Correcting to a constant mass is termed the Equivalent Soil Mass (ESM) approach (Ellert and Bettany, 1995). Ellert and Bettany (1995) proposed that soil carbon stock should be reported as mass per unit area for a fixed mass; the fixed mass being determined by a genetic soil horizon. In practice this can be difficult to achieve so a correction can be made according to the new bulk density to obtain the equivalent mass of the original sampling; however, this assumes a negligible SOC gradient down the soil profile. Research by Lee et al. (2009) suggested the original ESM approach was the best method for determining stock change.

2.3.3 Previous methods used for organic carbon determination

Methods of soil carbon determination that have been used in previous sampling within the UK are presented in Table 4.

Table 4 from Table 5.4 Black et al. (2008)

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Survey Reference Description

NSI England and Wales

Source: McGrath, S.P. and Loveland, P.J. (1992) The Soil Geochemical Atlas of England and Wales. Blackie Academic and Professional, London.

Organic carbon was measured either by loss-on-ignition (850°C) for soils estimated to contain more than about 20% organic carbon (Avery and Bascomb, 1982), or by dichromate digest with additional heating (Kalembasa and Jenkinson, 1973).

Countryside Survey

Source:CEH (2007) Countryside Survey 2007 WP4 Soil Report Annex 2 Soil laboratory protocol. Unpublished.

Loss-on-ignition (in 1978 at 375°C and at 550°C and 375°C for CS2000 and CS2007 samples). Total soil carbon by CHN elemental analyser in CS2000 and CS2007 plus Walkley Black method (with no additional heating on a proportion of samples). Currently no allowance for carbonate, as a relatively small proportion of all samples, but may be retrospectively carried out.

NSIS Scotland

Source: Macaulay Institute for Soi l Research (1971) Laboratory Notes on Methods of Soil Analysis. Unpublished Handbook.

Total carbon was measured by CHN analyser after the sample was crushed in an agate ball mill to <150 µm. Silicon carbide milling was introduced recently. The sample was pre-treated with acid to remove any carbonates prior to analysis. Future analysis will use a CHN Analyser and loss on-ignition.

Northern Ireland NSI

Source: Agriculture Food and Environmental Science Division (2007) Total Nitrogen and total Carbon in soils - total combustion method. Unpubl ished Standard Operation Procedure.

Total carbon was measured by elemental analyser but there is no mention in the SOP of pre-treatment of calcareous soils or correction for carbonate.

BioSoil Source: Forest Research (2006). Lab Manual. Unpublished.

Direct determination of organic carbon was measured by pre-treating soils with hydrochloric acid and subsequent use of an elemental analyser. Indirect determination was carried out by determining total carbon and then correcting for carbonate, which was measured separately.

Environment Agency (National Laboratory Service)

Source: Environment Agency (2007) Determination of total organic Carbon, total Carbon, total Nitrogen and related analytes in soil, sediment and waste. Unpublished Work Instruction.

Direct determination of organic carbon was measured by pre-treating soils with hydrochloric acid and subsequent use of an elemental analyser.

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Survey Reference Description

BS 7755- 3.8:1995I S O 10694:1995

Source: British Standards Institution

The carbon present in the soil is oxidised to carbon dioxide by heating the soil to at least 900°C in a flow of oxygen-containing gas that is free from carbon dioxide. The amount of carbon dioxide released is then measured.For the determination of the organic carbon content, any carbonates present are previously removed by treating the soil with hydrochloric acid. Alternatively, if the carbonate content of the examined samples is known and corrections are made for the carbonates present then the organic carbon content is calculated.

References Sections 2.2 and 2.3

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Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, R.M., Kirk, G.J.D. (2005). Carbon losses from all soils across England and Wales 1978-2003. Nature, 437, 245-248

Bisutti I., I. Hilke, M. Raessler, (2004). Determination of total organic carbon – an overview of current methods. Trends in Analytical Chemistry 23: 716-726.

Black H., Bellamy P., Creamer R., Elston D., Emmett B., Frogbrook Z., Hudson G., Jordan C., Lark M., Lilly A., Marchant B., Plum S., Potts J., Reynolds B., Thompson R. and Booth P. (2008) Design and operation of a UK soil monitoring network EA Science report SC060073

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Broadbent, F.E. (1953). The soil organic fraction. Adv. Agron. 5:153–183.

Cambardella, C.A., and E.T. Elliott (1993). Methods for physical separation and characterization of soil organic matter fractions. Geoderma 56, 449–457.

Carter, M.R. (2002). Soil quality for sustainable land management: Organic matter and aggregation interactions that maintain soil function. Agron. J. 94, 38–47.

Chatterjee, A., R. Lal, L. Wielopolski, M.Z. Martin, and M.H. Ebinger. (2009). Evaluation of different soil carbon determination methods. Critical Reviews in Plant Sciences 28:164-178.

Christensen, B. (1996). Matching measurable soil organic matter fractions with conceptual pools in simulation models of carbon turnover: A revision of model structure. In D.S. Powlson, P. Smith, and J.U. Smith (Eds.), Evaluation of Soil Organic Models Using Long-Term Data Sets. Springer-Verlag, Berlin, pp. 143–159.

David, M. B. (1988). Use of loss-on-ignition to assess soil organic carbon in forest soils. Commun. Soil Sci. Plant Anal. 19: 1593–1599.

Ellert, B.H., Bethany, J.R., (1995). Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Canadian Journal of Soil Science 75, 529–538.

Emmett, B., Black, H.I.J., Bradley, R.I., Elston, D., Reynolds, B.,Creamer, R., Frogbrook, Z.L., Hudson, G., Jorden, C., Lilly, A., Moffat, A., Potts, J. and Vanguelova, E., 2007. National Soil Monitoring Network: Review and assessment study. Final report. SNIFFER, Project LQ09

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Emmett, B.A.; Frogbrook, Z.L.; Chamberlain, P.M.; Griffiths R.; Pickup R.; Poskitt, J.; Reynolds, B.; Rowe, E.; Spurgeon, D.; Rowland P.; Wilson J.; Wood, C.M. (2008). CS Technical Report No.3/07

Soils Manual. NERC/Centre for Ecology and Hydrology (CS Technical Report No. 3/07, CEH Project Number: C03259).

Emmett, B.A.; Reynolds, B.; Chamberlain, P.M.; Rowe, E.; Spurgeon, D.; Brittain, S.A.; Frogbrook, Z.; Hughes, S.; Lawlor, A.J.; Poskitt, J.; Potter, E.; Robinson, D.A.; Scott, A.; Wood, C.; Woods, C.. (2010) Countryside Survey: Soils Report from 2007. NERC/Centre for Ecology and Hydrology, 192pp. (CS Technical Report No. 9/07, CEH Project Number: C03259).

Forest Research 2011 http://www.forestry.gov.uk/fr/INFD-73UDF3 (accessed June 2011)

Forest Soil Co-ordinating Centre,2006 Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests Part IIIa Sampling and Analysis of Soil Research Institute for Nature and Forest, Belgium

Gregorich, E.G., and M.R. Carter (1997). Soil quality for crop production and ecosystem health. In Developments in Soil Science, Vol. 25. Elsevier, Amsterdam, pp. 399–428.

Grewal, K. S., Buchan, G. D., and Sherlock, R. R. (1991). A comparison of three methods of organic carbon determinations in some New Zealand soils. J. Soil Sci. 42: 251–257.

Hassink, J. (1995). Density fractions of soil macroorganic matter and microbial biomass as predictors of C and N mineralization. Soil Biol. Biochem. 27, 1099–1108.

Hodgson, J.M. (ed.) (1976). Soil Survey Field Handbook; describing and sampling soil profiles. Soil Survey Technical Monograph No 5, Silsoe.

Intergovernmental Panel on Climate Change (IPCC), (2003). Penman J., M. Gytarsky, T. Hiraishi, T. Krug, D. Kruger, R. Pipatti, L. Buendia, K. Miwa, T. Ngara, K. Tanabe and F. Wagner (Eds). Good Practice Guidance for Land Use, Land Use Change and Forestry. IPCC/OECD/IEA/IGES, Hayama, Japan.

Jenkinson DS, Coleman K (1994) Calculating the annual input of organic matter to soil from measurements of total organic matter and radiocarbon. European Journal of Soil Science 45, 167–174. doi: 10.1111/j.1365-2389.1994.tb00498.x

Lal R., JM. Kimble, RF. Follett, and BA. Stewart, (2000). Assessment Methods for Soil Carbon, (advances in soil science) CRC Press, Boca Raton, FL. 676p.

Lee, J., Hopmans, J.W., Rolston, D.E., Baer, S.G., Six, J., (2009). Determining soil carbon stock changes: simple bulk density corrections fail. Agriculture, Ecosystems & Environment 134, 251–256.

Magid, J., A. Gorissen, and K.E. Giller (1996). In search of the elusive “active” fraction of soil organic matter: Three size-density fractionation methods for tracing the fate of homogeneously 14C-labelled plant materials. Soil Biol. Biochem. 28, 89–99.

Meersmans, J., B. Van Wesemael & M. Van Molle. (2009). Determining soil organic carbon for agricultural soils: a comparison between the Walkley & Black and the dry combustion methods (north Belgium). Soil use and management. 25: 346-353.

Meibus, L. J. (1960). A rapid method for the determination of organic carbon in soil. Anal. Chim. Acta. 22: 120–121.

Nelson, D.W. and L.E. Sommers. (1996). Total carbon, organic carbon, and organic matter. In: Methods of Soil Analysis, Part 2, 2nd ed., A.L. Page et al., Ed. Agronomy. 9:961-1010. Am. Soc. of Agron., Inc. Madison, WI.

Parton WJ, Schimel DS, Cole CV, Ojima DS (1987) Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Science Society of America Journal 51, 1173–1179.

Rawlins B. (2011). British Geological Survey. Personal Communication 26 January

Salminen, R., Batista, M.J., Bidovec, M., Demetriades, A., De Vivo, B., De Vos, W., Duris, M., Gilucis, A., Gregorauskiene, V., Halamic, J., Heitzmann, P., Lima, A., Jordan, G., Klaver, G., Klein, P., Lis, J., Locutura, J., Marsina, K., Mazreku, A., O'Connor, P.J., Olsson, S.Ǻ., Ottesen, R.-T., Petersell, V., Plant, J.A., Reeder, S., Salpeteur, I., Sandström, H., Siewers, U., Steenfelt, A. and Tarvainen, T.,

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(2005). Geochemical Atlas of Europe. Part 1 – Background Information, Methodology and Maps. Geological Survey of Finland, Espoo.

Schulte, E. E., and Hopkins, B. G., (1996). Estimation of soil organic matter by weightloss-on-ignition. In: Soil Organic Matter: Analysis and Interpretation. SSSA Special Publication no. 46, pp. 21–31. Magdoff, F. R. Ed., Soil Science Society of America, Inc. Madison, WI.

Six, J., E.T. Elliott, K. Paustian, and J.W. Doran (1998). Aggregation and soil organic matter accumulation in cultivated and native grassland soils. Soil Sci. Soc. Am. J. 62, 1367–1377.

Skjemstad JO, Janik LJ, Taylor JA (1998) Non-living organic matter: what do we know about it? Australian Journal of Experimental Agriculture 38, 667–680. doi: 10.1071/EA97143

Smith, K. A., and Tabatabai, M. A., (2004). Automated instruments for the determination of total carbon, nitrogen, sulfur, and oxygen. In: Soil and Environmental Analysis: Modern Instrumental Techniques. pp. 235–282. Smith, K. A., and Cresser, M. S., Eds., Marcel Dekker, NY.

Soon, Y. K. and Abboud, S. (1991). A comparison of some methods for soil organic carbon determination. Commun. Soil Sci. Plant Anal. 22: 943–954.

Soil Survey Staff, (1992). National Soil Survey Laboratory Methods Manual (Soil investigations report No 42). US Government printing office, Washington, DC.

Stolbovoy, V., L. Montanarella, N. Filippi, S. Selvaradjou, P. Panagos and J. Gallego, (2005). Soil Sampling Protocol to Certify the Changes of Organic Carbon Stock in Mineral Soils of European Union. EUR 21576 EN, Office for Official Publications of the European Communities, Luxembourg. 12 pp.

Stolbovoy, V., Montanarella, L., Filippi, N., Jones, A., Gallego, J., & Grassi, G., (2007). Soil sampling protocol to certify the changes of organic carbon stock in mineral soil of the European Union. Version 2. EUR 21576 EN/2. 56 pp. Office for Official Publications of the European Communities, Luxembourg. ISBN: 978-92-79-05379-5

Tinsley, J. (1950). Determination of organic carbon in soils by dichromate mixtures. In: Trans. 4th Int. Congr. Soil Sci., Vol. 1. pp. 161–169. Hoitsemo Brothers, Gronigen, Netherlands.

Tipping, E. (2002). Cation Binding by Humic Substances. Cambridge University Press, Cambridge.

Walkley A, Black LA (1934) An examination of the Dgtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Science 37, 29–38. doi: 10.1097/00010694-193401000-00003

Walkley, A. (1947). A critical examination of a rapid method for determining organic carbon in soils: effect of variations in digestion conditions and of inorganic soil constituents. Soil Science 63:251-263.

Wander, M. (2004) Soil organic matter fractions and their relevance to soil function. In Soil Organic Matter in Sustainable Agriculture. F. Magdoff and R.R. Weil (eds). CRC Press, 398pp.

Yakovchenko, V.P., L.J. Sikora, and P.D. Millner (1998). Carbon and nitrogen mineralization of added particulate and macroorganic matter. Soil Biol. Biochem. 30, 2139–2146.

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3 Workshop discussion outcomes

Project workshop

This event was attended by 19 soil scientists from Defra, SKM Enviros, Cranfield University, CEH, BGS, ADAS, The Macaulay Institute (now the James Hutton Institute), the Environment Agency, SCRI, Rothamsted North Wyke and Forest Research.

This sub-project aimed to ‘explore the methodologies for accurate routine determination of soil carbon’ with an objective to ‘define a new standard operating procedure (SOP) for use in future soil surveys, discussed and agreed through a workshop of experts from the soil science community’ . The specific aims of the workshop were to agree the soil sampling methodology and the soil carbon determination method(s) for future soil surveys building on the previous work carried out as reported in the ‘design and operation of a UK soil monitoring network’ (Black et al., 2008).

Workshop discussion

It was agreed that the future soil sampling scheme must allow for optimal measurement and assessment of soil carbon stocks for both status and change. As well as IPCC considerations (to monitor the 0-30cm depth, in which case a 0-15cm and 15-30cm sample could be taken) it was agreed that it was important to gather data to compare with historic information from previous surveys and also to take into account technology development and application in the future. The methodology should cope with mineral, organo-mineral and organic soils. An important part of the discussion focused on sampling design, in that the uncertainties of sampling and variation in the landscape are much greater than the uncertainties for analytical carbon determination in samples.

There was considerable discussion about a future vision for moving from horizon based sampling to obtaining one soil core from the whole profile, from which an integrated stock assessment could be made. There are several reasons why a move in this direction may be desirable:

An archived repository of soils could be developed, similar to those for ice cores, so that reference materials might be available for the future. In the future, advances in technology could be exploited to understand changes that might have occurred.

A soil core would offer the potential to use equipment that could provide a profile average value of changes in bulk density, carbon, nutrients, etc. Fixed length sub-sampling of the core would then allow for past and future compatibility against other sampling, e.g. CS, RSSS and IPCC. As the rest of the world chooses different fixed length depths to sample over, an archived soil core may overcome cross compatibility issues.

An important reason for this potential transition is that there is a reducing number of soil scientists who can appropriately select soil horizons (and as such the loss of key skills, e.g. soil identification, was identified as a concern at the workshop). Selection and naming of soil horizons can be subjective.

Soils are being turned over and mixed at an ever increasing rate, and identifiable horizons are becoming difficult to decipher.

The group agreed that the current SOP for sample collection and storage from Black et al. could be refined, but there was a need to investigate future options and technologies, and back this up with some measurements and evidence for the use of technologies such as infrared for carbon and other technologies for other properties such as colour scanning, CT-scanning, XRF, and chemical sensing.

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Soil sampling

The SOP proposed for soil sampling by Black et al. was discussed and it was agreed to separate the methodology for the assessment of soil organic carbon status from change (Table 5).

Table 5 Summary of outcome discussions on SOPs.

SOP section from Black et al. Workshop outcome

1. Bulked composite sample from 20m square around the pit divided into 5m grid - sample at intersections (25 cores/composite) 0-15cm with gouge auger.

Agreed. This is required to monitor change from previous surveys.

2. Sample up to four horizons from described profile face in pit to a maximum depth of 75 cm ensuring that any A horizon and the thickest B horizon are sampled.

Amended. These samples are required for carbon status (stock) determination with soil bulk density and carbon being measured using the same samples.

Sample all visible horizons from described profile face in pit to a maximum depth of 75 cm using sampling cores of 5 - 10 cm length and known and equal volume, taking at least 3 sets of cores from each pit.

Take a representative photograph.

If horizons are thinner than bulk density sampling core lengths, combine horizons and record methodology employed.

In the laboratory1, record air dry weight and sub-sample to determine moisture content to obtain dry bulk density. Measure soil carbon on air-dried sub-sample and correct for moisture.

For peat soils, record the total depth of the peat. A sample may need to be taken deeper than 75cm to obtain the most humified material.

Take a core and store for future reference and for measurements e.g. by new technologies.

3. Bulked triplicate core samples with sampling tubes of 5 - 10 cm length and known and equal volume collected from adjacent to pit:

i Mineral soils at two depths centred on 5 and 20 cm from the soil surface (excluding vegetation and litter)

ii Peat soils at depths centred on 10, 25, 50 and 75 cm

Amended. To monitor change and linked to item 1. Five samples taken vertically from around the pit within the 20 x 20m area using sampling cores of known and equal volume to 15cm depth to be analysed for soil bulk density

1 Methodology for laboratory bulk density determination, including adjustments for stones, and calculations is provided in Countryside Survey Technical Report No.03/07 (Emmett et al., 2008)

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Suggestions for future work:

Table 5.2. Takes samples from one pit at a site. If samples are taken from the four sides of the pit rather than just the one side described (or not), how does this compare with Table 5.1 in terms of management of variability.

Table 5.3. Are five samples adequate and should the five samples be bulked at sampling or is the greater analysis of variability preferable by keeping samples separate? Defra research project SP1305 sub-project B will assess methods which are used to determine soil bulk densityand their influence on estimates of soil carbon stocks (project duration April 2011 – March 2012).

Table 5.2. Investigate ‘future proofing’ sampling method to enable e.g. scanning the whole profile in situ.

Stones and rocks – check the methodology for the compensation for stones in samples and also for variable soil depth in the landscape/rocky outcrops

Soil carbon determination

The workshop discussed methods and agreed that the larger subsample size used for loss on ignition was preferable compared with the combustion analyzer methodology that requires strict subsampling to ~0.15g. Instrumentation for combustion analyzers may improve over time with regard to sample size, whereas loss on ignition (apart from improved oven design) and wet chemistry of modified Walkley-Black are less prone to change over time and back compatible.

As soil pH was likely to be monitored in any future scheme, it was agreed that soil pH should be determined on all samples, especially if a combustion analyzer is to be used, to indicate the need to remove inorganic carbon by pre-treatment prior to organic carbon analysis. Black et al. (2008) recommended that a 1:2.5 v/v water:air dried soil samples < 2mm be used. Additional pH methods on fresh soil and using water and/or CaCl2 solutions could also be used (Emmett et al., 2008).

The workshop agreed that all samples should be analysed by both:

a) Loss on ignition at 375oC for 16 hours using 10g soil (LOI) andb) Modified Walkley-Black using up to 0.5 g soil (WB).

In addition, it was agreed that the factor of 1.724 was appropriate to convert carbon to organic matter and vice versa. This is the ‘traditional’ value with a reciprocal of 0.58; the more recent Countryside Survey used a reciprocal value of 0.55 based on many data from their earlier surveys.

It was suggested at the workshop that a proportion of samples (e.g.10%) should be analysed for total carbon by combustion analyzer to increase the amount of data available to correlate between this and the more traditional methods of analysis. Acid pre-treatment should be investigated, in relation to sample pH, to determine if it is required for all soil samples prior to carbon determination by combustion analyzer. Calcium carbonate combusts at the high temperatures in a combustion analyzer. However, it is a kinetic process, and the speed of combustion may be too fast for much carbonate vaporization to occur, as supported by evidence from Wright and Bailey (2001). They compared combustion of calcite at 1040 and 1300 oC and found less than 5% was decomposed at low temperature (1040) but 98% was decomposed at high temperature (1300). The methodology for total carbon by combustion analyzer will be as per manufacturers’ instructions for the equipment.

The reasoning for choosing LOI and WB was that analysis is cheap compared with sample collection and preparation, and that the small quantities of soil used for combustion analyzers, requiring rigorous subsampling, make combustion analyzers currently undesirable. It was also preferred to be able to compare future data with past data using the same methods as in the past whilst also not discouraging the use of new technologies in the future.

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Conclusions

It was concluded that monitoring should continue to use a combination of these methods. The essential requirements are that:

methods follow standardised protocols, documented in sufficient detail to allow them to be repeated in independent laboratories;

samples from subsequent samplings are analysed following exactly the same methods as for the earlier samplings;

a proportion of archived samples from earlier samplings is analysed at the time of subsequent samplings.

Suggestions for future work:

In addition to measuring soil carbon it is important to develop a general vision for measuring the state and change of soil stocks. We need to investigate costs and methods associated with obtaining intact soil cores, archiving them, and identifying technologies that could be applied to determine profile properties as a function of depth to produce both profile distributions and integrated averages. Mature technologies such as X-ray fluorescence are currently available for determining element concentrations. Technologies, such as infrared, for determining carbon and clays, are developing. CT-scanning can be used to determine structure and density as a function of depth, and chemical sensors using chips can measure pH and Eh without the need for sample extraction and loss.

There is a need to test the reliability of proximal sensors, such as infra-red, for carbon analysis. Potentially these offer a cheap, non-destructive, alternative to decomposition methods, and the uncertainties associated with carbon determination, may not be that much greater than those associated with LOI or WB.

If soils slowly degrade over time, root concentrations may fall. During soil preparation only a proportion of roots are removed (those visible) and bias may be introduced as should live roots be counted within the soil carbon stock?

Standard soil for inter-laboratory cross checks over long time periods would be useful. The possibilities of using other reference materials could be investigated.

A joint database and a sharing of information to enable improved comparison of soil carbon analysis methods would be an advantage.

Standard Operating Procedures

Black et al. (2008) developed a number of Standard Operating Procedures (SOPs) including:

UKSMS 001 – Establishment of a sampling point

UKSMS 002 – Site and soil profile description

UKSMS 003 – Sample collection and storage

UKSMS 004 – Archiving of soil samples

UKSMS 005 – Data management and archiving

Based on the workshop outcomes above, UKSMS 003 has been amended (see Annex 1). An additional SOP for soil analysis for both organic carbon by modified Walkley Black and organic matter by loss on ignition has been proposed (see Annex 2).

These methods and recommendations have been made based on the experience and results of numerous soil surveys that have been carried out across the UK and beyond but are not limited to use in soil surveys alone. Any soil studies where sampling of the soil is required from a farm, field or experimental plot should also follow these recommendations to achieve reproducible, comparable and robust measurements of soil carbon.

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References Section 3

Black H., Bellamy P., Creamer R., Elston D., Emmett B., Frogbrook Z., Hudson G., Jordan C., Lark M., Lilly A., Marchant B., Plum S., Potts J., Reynolds B., Thompson R. and Booth P. (2008). Design and operation of a UK soil monitoring network. The Environment Agency Science Report – SC060073

Emmett B.A., Frogbrook Z.L., Chamberlain P.M., Griffiths R., Pickup R., Poskitt J., Reynolds B., Rowe E., Spurgeon D., Rowland P., Wilson J. and Wood P.M. (2008). Countryside Survey Technical Report No. 3/07 Soils Manual. Centre for Ecology and Hydrology.

Nelson, D.W., and Sommers L.E. (1982). Total carbon, organic carbon, and organic matter. p. 539–580. In A.L. Page et al. (ed.) Methods of soil Analysis. Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI.

Wright, A. F. and Bailey, J. S. (2001). Organic carbon, total carbon, and total nitrogen determination in soils of variable calcium carbonate contents using a LECO CN-2000 dry combustion analyzer. Commun. Soil Sci. Plant Anal. 32: 3243–3258.

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4 New and emerging technologies for monitoring soil carbon

In this section we summarise new and emerging techniques that could be used for future monitoring of soil organic carbon. To date, the method for determining change in soil organic carbon (SOC) stocks has been based on the repeated inventory approach; repeated measurements of SOC stocks at the same location over a period of time. This requires a cost-effective and reliable method for determination of SOC concentrations in many samples. An alternative approach to monitor changes in SOC is to examine changes in specific fractions (Zimmermann et al., 2007b) of soil carbon over time; these might provide a first indication of long-term changes in soil carbon stocks (Rodeghiero et al., 2010). Here we focus our review on those techniques which could lead to substantial savings in terms of overall cost in estimating SOC concentrations comparison to current approaches of soil carbon monitoring. Our assessment considers nine technologies which could be deployed. We assess each in terms of a set of five criteria:

1. Whether they are field or laboratory-based techniques, or whether they can be deployed in both.

2. Whether field deployment is practical? What support3 is carbon estimated over? Larger supports imply more robust estimates of carbon quantities.

3. Whether the technique destroys the sample material (i.e. whether the material cannot be used for subsequent analysis or not).

4. How long before the methodology could be deployed in a monitoring context at the national scale?

5. How cost-effective is the technique in relation to traditional methods?

We also stipulate any specific advantages or disadvantages for each technique and provide an overall score on a scale from 1 to 10 to give an indication of future potential.

Each technology is summarised in Table 6. A brief description of each technique and specific comments relating to it are made in the accompanying text. We conclude by discussing aspects of the most promising techniques (overall scores > 5).

3 The size and shape (in three dimensions) of the volume of soil material which constitutes an individual specimen in a sample for which a single value is determined for any soil property. If referring to a composite sample in which individual samples have been combined (bulked), the support is defined by the description of the discrete samples (core shape, dimensions and depth), their number and the configuration over which they were collected (e.g. at the corners and centre of a square of specified dimensions).

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Table 6. Summary of new and emerging technologies for use in soil carbon monitoring

Method Field/Lab/Both

Non-destructive (Y/N)

Field Practicalities and size of sample support (small/large) 1

Time toimplementation(yrs) / available now

Cost effectiveness

Advantages Disadvantages References Score(1= worst; 10= best)

Visible and Near infra red (VNIR-DRS) diffuse reflectance spectroscopy

Both Y Field portable systems available and in use.

Field-based analyses less accurate than lab-based equivalent.

Small support

2-3 Large sample numbers processed for low cost (~£<2) once statistical models established

Archived materials from monitoring networks could be used to optimise SOC spectral models for different soil types.

Spectra can also be used to estimate bulk density.

Generally lower accuracy for estimation of SOC than for MIR.

(Reeves Iii, 2010)

(Viscarra Rossel et al., 2006)

6

Mid infra red (MIR)diffuse reflectance spectroscopy

Both Y Field portable systems available and in use.Questions over field-based accuracy: sample and sieve on site? Scanning dry/wet samples?

Small support

2-3 Large sample numbers processed for low cost (£<2) once statistical models established

Archived materials from monitoring networks could be used to optimise SOC spectral models for different soil types.

Generally higher accuracy of estimating soil C greater than for VNIR. Can be used to provide information on soil carbon fractions.

Spectra can also estimate bulk density.

Lab-based system more accurate if soil sample diluted in KBr powder

Accuracy of field-based measurement compromised by influence of variation in i) particle size and ii) soil moisture.

Lab-based dilution of soil sample in KBr powder increases time/cost

(Reeves Iii, 2010) (Viscarra Rossel et al., 2006)

(Zimmermann et al., 2007a)

8

31

Method Field/Lab/Both

Non-destructive (Y/N)

Field Practicalities and size of sample support (small/large) 1

Time toimplementation(yrs) / available now

Cost effectiveness

Advantages Disadvantages References Score(1= worst; 10= best)

Laser-Induced Breakdown Spectroscopy

Both Y Small support >5 years Uncertain Field and lab deployment and accuracy.

Interference from plant roots and minerals

(Cremers et al., 2001)

4

Remote sensing

Field Y Large support 3-5 yrs Cost effective

Remote sensed. Imprecise due to interferences with image gathering (cloud cover, vegetation cover); surface soil only.

Ladoni et al. 2010

2

Thermo gravimetry- differential scanning calorimetry

Lab N Small support Available now Moderate cost - £15-20 per sample

Could provide information on soil carbon pools (e.g. quantify the amount of recalcitrant organic carbon)

Slower throughput of samples than traditional soil carbon measurement techniques.

(Lopez-Capel et al., 2005)

6

Thermo-gravimetry linked with Raman spectroscopy

Lab N Small support 2/3 yrs Expensive Supplies information on the types of molecular bonds, and further info on the nature of soil C

Experimental Unpublished data from Cranfield University, (R. Corstanje, 2011. pers. comm.)

4

32

Method Field/Lab/Both

Non-destructive (Y/N)

Field Practicalities and size of sample support (small/large) 1

Time toimplementation(yrs) / available now

Cost effectiveness

Advantages Disadvantages References Score(1= worst; 10= best)

13C NMR Lab N Small support Available now Considerably more expensive than traditional C analysis methods (£100/ sample)

Lower sample throughput than traditional analyses

Provides the greatest amount of detail on the molecular characterisation of carbon in soil and could help to identify transformations in soil C fractions.

Removal of soil mineral component with HF often necessary to avoid interference effects

(Fang et al., 2010)

2

Rock-Eval pyrolysis

Lab N Small support Available now Moderate cost (£15-£20 / sample)

Provides information on both total carbon and soil carbon fractions without the requirement for pre-treatment to remove soil minerals

More costly than some of the infra red techniques

(Sebag et al., 2006)

6

Specific Ultraviolet Absorbance

Lab N Not practical Available now Cost effective

Supplies cost effective information soil C aromaticity and humification

Requires a liquid extraction

(Ohno, 2002) (Weishaar et al., 2003)

5

1 “small support” implies quantities of around 1 kilo possibly representing a composite sample across a square with side length of 20 metres. A “large support” implies measurement based on more than 100 kilos of soil from a large volume (many cubic metres).

33

Visible and near infra-red diffuse reflectance spectroscopy (VNIR-DRS)

This technique is based upon capturing the reflected spectra of light from soil samples in the visible and near infra-red range (0.35-2.5 microns) and developing statistical models between the spectra a set of reference soil samples which have been analysed for total carbon content using a traditional technique (e.g. a total carbon analyser). The statistical model can then be applied to spectra from other samples to estimate their carbon content; typically these samples are drawn from the same geographic region as the reference samples. VNIR-DRS can be deployed using field-portable devices although the accuracy of soil organic carbon estimates is likely to be lower than laboratory-based approaches (Reeves Iii, 2010). Reported root-mean square deviation for SOC based on an independent set of samples using VNIR-DRS was 0.831 across the C range 0.09 to 9.8% (Reeves et al., 2006); this study was based upon a set of 237 samples from a total of 14 sites. The statistical models upon which estimates of SOC are based typically span the full spectral range. Bias may be introduced into these models if they are applied to different soil types because their spectral response is influenced by the mineralogical composition of the soil (Minasny et al., 2009). Therefore different models would need to be established for the different soil types across England and Wales; this should be achievable with some testing and refinement. Prior to capturing the soil spectra in the laboratory soil samples are often dried and ground to improve estimation accuracy of soil properties. It has been shown that VNIR-DRS is not particularly effective in estimating soil bulk density (Moreira et al., 2009), the other property required to calculate the quantity of carbon at a site.

Mid infra-red diffuse reflectance spectroscopy (MIR-DRS)

The basic principle of MIR-DRS is the same as that for the VNIR-DRS technique, but in this case the reflected spectra of light span the mid infra-red range (2.5-25 microns). The accuracy of SOC estimation using MIR-DRS is typically greater than VNIR-DRS. For example, in their comparison study Reeves et al., (2006) reported a root mean square deviation (RMSD) for SOC by MIR-DRS of 0.334; substantially smaller than that reported by VNIR-DRS (see above). Variations in moisture content and grain-size reduce the accuracy of statistical models for soil-property estimation based on MIR spectra (Reeves Iii, 2010), as such, laboratory based spectral analysis of dried and ground samples is likely to be the preferred method for SOC monitoring. This is not a significant extra requirement assuming soil samples collected for monitoring are subject to other analyses which also require the sample to be dried and ground. A major advantage of MIR-DRS is its potential to provide accurate estimates of carbon in the different organic matter fractions (particulate, stabilised and recalcitrant; (Zimmermann et al., 2007a) which are required for modelling soil carbon turnover. Such estimates could help to explain any observed changes in SOC concentrations which cannot be inferred using the other methods. In common with VNIR-DRS, MIR-DRS does not appear to offer much potential for estimating soil bulk density (Minasny et al., 2008). Other methods using MID-IR spectroscopy include attenuated total reflectance or ATR and photo-acoustic spectroscopy. These MID methods however do rely on entire different optical mechanisms than DR spectroscopy as explained in Reeves Iii, (2010).

Remote sensing

There are two basic approaches to determine SOC remotely. The first is an approach taken by Kheir et al. (2010) in which Normalized Difference Vegetation Index and Normalized Wetness Index are derived from a Landsat image and subsequently used as part of a suite of covariates to derive empirical relationships between these and SOC. These covariates are often derivatives of a digital elevation model, and the methods used are typical to digital soil mapping (McBratney et al., 2003). Combined with NDVI and NWI, Khier et al. (2010) were able to map SOC with moderate success, although NDVI and NWI were not identified as key predictor variables. The second approach derives SOC estimates directly from a spectral image. Ladoni et al. (2010) review this approach, which is similar to the methods described earlier in VNIR-DRS, but then sensed remotely. As reported in the table below, the results are mixed. In a further study by Sullivan et al. (2005), the thermal infra-red (TIR) index dominated total variation in SOC (93 %) when compared to the VIS and NIR spectra.

Table 7 presents the coefficients of determination between reflectance values for the blue (B), green (G), red (R) and NIR bands of the spectrum from a bare soil image and laboratory values of soil OM (Ladoni et al., 2010).

34

Table 7 - Coefficients of determination (Ladoni et al. 2010)

Source image Wavelengths R 2 SPOT R–G 0.22Aerial photograph B–G–NIR 0.22–0.28AVIRIS VIS–NIR 0.72Arial photograph B–G–R 0.92Color slide B–G–R 0.93ATLAS R–NIR 0.95ATLAS G–R–NIR 0.89Landsat TM G–R–NIR 0.51IKONOS B–G–R 0.06Digital orthophotograph B–G–R 0.08–0.16Hyperspectral image VIS–IR 0.74

Thermogravimetry-differential scanning calorimetry (TG-DSC)

Thermogravimetry (TG) combined with differential scanning calorimetry (DSC) has been applied to explore the chemical changes in OM during decomposition, especially with regard to the composition of the bulk constituents of degrading plant material. TG-DSC analysis involves continuous and simultaneous measurement of weight-loss (TG) and energy change (DSC) during heating. Exothermic decomposition of aliphatic and carboxyl groups predominates at temperatures of 300–350C, with more refractory aromatic C being lost at higher temperatures of around 400 – 450C. When coupled to isotope ratio mass spectrometer (IRMS) this analytical approach allows the d13C values of volatile organic components evolved during heating to be determined during a single heating experiment, providing valuable insights into the dynamics of labile and recalcitrant components of SOM. Thermal analysis is uniquely suited to this task, as different C compounds decompose during a heating cycle at different temperatures. Thermal analysis methods (TG-DSC) have been used to characterise chemical changes in the organic matter fractions of soils, compare the proportions of active and more stable components in organic matter fractions under contrasting conditions (Dell'Abate et al., 2000; Lopez-Capel et al., 2005; Lopez-Capel et al., 2006).

13C NMR

Carbon-13 NMR (13C NMR or sometimes simply referred to as carbon NMR) is the application of nuclear magnetic resonance (NMR) spectroscopy to carbon. It is an important tool in chemical structure elucidation in organic chemistry. Carbon-13 NMR detects only the 13C isotope of carbon, whose natural abundance is only 1.1%. Quantitative, direct-polarization 13C nuclear magnetic resonance (NMR) spectroscopy has been used to characterize the types and amounts of organic C present in whole and HF-treated soil samples, their clay fractions, as well as plant and particulate organic matter. Direct-polarization NMR provides more quantitative spectra of highly aromatic soil organic matter than does the more commonly used cross-polarization (CP) method (Fang et al., 2010).

Rock-Eval Pyrolysis

In this technique, bulk dried samples are heated in an inert atmosphere and, upon pyrolysis, the main emission products (hydrocarbons, CO2, CO) are quantified by flame-ionization (FI) and infrared (IR) detection. These measurements are used to calculate several basic parameters, e.g. total organic carbon contents, thermal maturity, and the Hydrogen Index and Oxygen Index correlated to H/C and O/C values, respectively. These various parameters were defined to study the properties of mature OM from bedrock geology, but recent work showed that they could be used to characterize immature OM from recent sediments (Luniger & Schwark, 2002).

35

Laser-Induced Breakdown Spectroscopy

In the technique known as laser-induced breakdown spectroscopy (LIBS) which is based on atomic emission, the carbon content of the soil is determined by analyzing its unique spectral signature. A laser beam at a specific wavelength is focused on each sample to form a micro-plasma that emits light that is characteristic of the sample’s elemental composition. A grated-intensified photodiode array detector is used to spectrally resolve the emitted light. Intact soil cores or discrete, pressed samples are used for analysis; spectra are collected along a soil core or from each discrete sample. The spatial variability of C in soil profiles is accounted for by the ability to analyze and average multiple spots. The data from LIBS measurements were compared those from dry combustion and a large and positive linear correlation (r = 0.96) was observed (Cremers et al., 2001) for soils of similar morphology and with a precision of 4–5%.

Conclusions

Our analysis has highlighted four emerging techniques which could be given further, detailed consideration for monitoring changes in soil organic carbon (in no particular order):

1. Visible and near infra-red diffuse reflectance spectroscopy;

2. Mid infra red (MIR) diffuse reflectance spectroscopy;

3. Thermo gravimetry - differential scanning calorimetry; and

4. Rock-Eval pyrolysis.

Methods 1 and 2 of the above list require initial, primary measurements of soil carbon to develop models for predicting soil carbon concentrations, but thereafter would be very cost-effective and non-destructive in their application. In the medium-term, these techniques would be less expensive than currently deployed methods of soil carbon analysis used in surveys across England and Wales. One of their main advantages is that once the spectra are obtained and stored, these can be used again if new types of applications become available (Reeves Iii, 2010). They also provide a rapid-screening technique to identify errors in sample mis-labelling. For example, where a sample from one site may be confused with that from another, a spectral database of samples from these sites would most likely resolve any confusion. Instruments for laboratory and field-based infra red analysis are available at various research centres and university departments across the UK, with soil spectral libraries and statistical models for estimation of soil carbon under development.

Given the availability of archived soil material and associated organic carbon analyses (using methods such as loss-on-ignition, combustion or modified Walkley-Black) from previous national and regional soil surveys and monitoring networks, it would be a relatively inexpensive task to establish how accurately infra-red techniques are for estimation of SOC concentrations when applied to the range of soil types across England and Wales. In addition, the ability of these techniques to provide information on the types of relative amounts of soil carbon fractions could also be tested. For example, based on the application of established laboratory separation techniques (Zimmermann et al., 2007b), it may be possible to estimate the relative proportions of particulate and mineral-associated organic carbon using infra-red methods.

The other two techniques (3 and 4 above) are based on progressive heating of a soil sample and simultaneous measurement of either energy changes or evolved gases. These techniques would most likely be more expensive than those currently used to measure total soil carbon concentrations in samples from surveys across England and Wales. The main advantage of these techniques is they both provide total soil carbon concentrations and information which could be used to assess the quantities of organic carbon in different soil fractions. As above, the latter would only be possible if additional laboratory-based fractionation analyses could be undertaken on the same material (to quantify these soil fractions) for a subset of samples, and statistical models could then be developed to estimate carbon fractions in other samples. Analytical costs associated with the latter two techniques will be greater than the former; but it may be possible to reduce the unit cost if a larger number of analyses were required. It was beyond the scope of this study to provide precise unit costs for these two types of analyses; this would be a primary requirement of a subsequent, more detailed evaluation of these techniques.

36

References Section 4

Cremers, D. A., Ebinger, M. H., Breshears, D. D., Unkefer, P. J., Kammerdiener, S. A., Ferris, M. J., Catlett, K. M. & Brown, J. R. 2001. Measuring total soil carbon with laser-induced breakdown spectroscopy (LIBS). Journal of Environmental Quality, 30, 2202-2206.

Dell'Abate, M. T., Benedetti, A. & Sequi, P. 2000. Thermal methods of organic matter maturation monitoring during a composting process. Journal of Thermal Analysis and Calorimetry, 61, 389-396.

Fang, X. W., Chua, T., Schmidt-Rohr, K. & Thompson, M. L. 2010. Quantitative C-13 NMR of whole and fractionated Iowa Mollisols for assessment of organic matter composition. Geochimica Et Cosmochimica Acta, 74, 584-598.

Kheir, R. B., Greve, M. H., Bocher, P. K., Greve, M. B., Larsen, R. & McCloy, K. 2010. Predictive mapping of soil organic carbon in wet cultivated lands using classification-tree based models: The case study of Denmark. Journal of Environmental Management, 91, 1150-1160.

Ladoni, M., Bahrami, H. A., Alavipanah, S. K. & Norouzi, A. A. 2010. Estimating soil organic carbon from soil reflectance: a review. Precision Agriculture, 11, 82-99.

Lopez-Capel, E., Abbott, G. D., Thomas, K. M. & Manning, D. A. C. 2006. Coupling of thermal analysis with quadrupole mass spectrometry and isotope ratio mass spectrometry for simultaneous determination of evolved gases and their carbon isotopic composition. Journal of Analytical and Applied Pyrolysis, 75, 82-89.

Lopez-Capel, E., Sohi, S. P., Gaunt, J. L. & Manning, D. A. C. 2005. Use of thermogravimetry-differential scanning calorimetry to characterize modelable soil organic matter fractions (vol 69, pg 136, 2005). Soil Science Society of America Journal, 69, 930-930.

Luniger, G. & Schwark, L. 2002. Characterisation of sedimentary organic matter by bulk and molecular geochemical proxies: an example from Oligocene maar-type Lake Enspel, Germany. Sedimentary Geology, 148, 275-288.

Minasny, B., McBratney, A. B., Tranter, G. & Murphy, B. W. 2008. Using soil knowledge for the evaluation of mid-infrared diffuse reflectance spectroscopy for predicting soil physical and mechanical properties. European Journal of Soil Science, 59, 960-971.

Minasny, B., Tranter, G., McBratney, A. B., Brough, D. M. & Murphy, B. W. 2009. Regional transferability of mid-infrared diffuse reflectance spectroscopic prediction for soil chemical properties. Geoderma, 153, 155-162.

Moreira, C. S., Brunet, D., Verneyre, L., Sá, S. M. O., Galdos, M. V., Cerri, C. C. & Bernoux, M. 2009. Near infrared spectroscopy for soil bulk density assessment. European Journal of Soil Science, 60, 785-791.

Ohno, T. 2002. Fluorescence inner-filtering correction for determining the humification index of dissolved organic matter. Environmental Science & Technology, 36, 742-746.

Reeves Iii, J. B. 2010. Near- versus mid-infrared diffuse reflectance spectroscopy for soil analysis emphasizing carbon and laboratory versus on-site analysis: Where are we and what needs to be done? Geoderma, 158, 3-14.

Reeves, J. B., Follett, R. F., McCarty, G. W. & Kimble, J. M. 2006. Can Near or Mid-Infrared Diffuse Reflectance Spectroscopy Be Used to Determine Soil Carbon Pools? Communications in Soil Science and Plant Analysis, 37, 2307 - 2325.

Rodeghiero, M., Heinemeyer, A., Schrumpf, M. & Bellamy, P. 2010. Determination of soil carbon stocks and changes. In: Soil Carbon Dynamics: An integrated methodology. eds W. L. Kutsch, M. Bahn & A. Heinemeyer), Cambridge University Press, Cambridge, pp. 49-75.

Sebag, D., Disnar, J. R., Guillet, B., Di Giovanni, C., Verrecchia, E. P. & Durand, A. 2006. Monitoring organic matter dynamics in soil profiles by 'Rock-Eval pyrolysis': bulk characterization and quantification of degradation. European Journal of Soil Science, 57, 344-355.

Viscarra Rossel, R. A., Walvoort, D. J. J., McBratney, A. B., Janik, L. J. & Skjemstad, J. O. 2006. Visible, near infrared, mid infrared or combined diffuse reflectance spectroscopy for simultaneous assessment of various soil properties. Geoderma, 131, 59.

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Weishaar, J. L., Aiken, G. R., Bergamaschi, B. A., Fram, M. S., Fujii, R. & Mopper, K. 2003. Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon. Environmental Science & Technology, 37, 4702-4708.

Zimmermann, M., Leifeld, J. & Fuhrer, J. 2007a. Quantifying soil organic carbon fractions by infrared-spectroscopy. Soil Biology and Biochemistry, 39, 224-231.

Zimmermann, M., Leifeld, J., Schmidt, M. W. I., Smith, P. & Fuhrer, J. 2007b. Measured soil organic matter fractions can be related to pools in the RothC model. European Journal of Soil Science, 58, 658-667.

38

SP1106 iv

Annex 1 Soil sampling Standard Operating Procedure

UK Soil Monitoring Scheme - Sample Collection and Storage

Versions

Version No. Date Comments1 29.04.08 First draft2 08.05.08 Second draft incorporating amendments from Brian

Reynolds3 28.02.2011 Amended by SP1106 sub-project iv

Standard operating procedure SP1106 iv - 003

SP1106 iv

UK Soil Monitoring Scheme - Sample Collection and Storage

1 ScopeThe procedure describes the stages in the collection of samples from a sampling site and their subsequent storage prior to preparation for laboratory analysis.

2 Referenced documents(1) UKSMS_001: 2008 UK Soil Monitoring Scheme - Establishment of a Sampling Site

(2) UKSMS_002: 2008UK Soil Monitoring Scheme - Site and Soil Profile Description

3 OutlineThe procedure includes the collection of the following samples:

1. a bulked, composite topsoil sample from a square of land around the pit for soil carbon (for soil carbon change),

2. horizon samples from the described profile face within the pit for soil carbon and bulk density (for soil carbon status),

3. samples for the measurement of bulk density from around the pit within the square of land (for soil carbon change).

and their storage prior to transfer to a receiving laboratory.

4 Resources and equipmentThe following equipment (additional to that required for completing UKSMS 002: 2008) will be required:

1. Gouge auger/corer marked for 15 cm depth sampling,2. Hand or pocket knife,3. Metal or plastic dustpan,4. Sample bags and waterproof labels,5. Three bulk density sampling tubes of 5 - 10 cm length and known and equal volume

with plastic end caps, and6. Bulk density sampling equipment for 15cm depth.

Mobile refrigerator for bulked composite samples if Potentially Mineralisable Nitrogen is to be determined.

5 Procedure

5.1 Collection of composite topsoil sampleUsing a short-handled gouge auger, collect 25 cores at 5m intervals over a 20 x 20 m area around the pit. The samples should be from the 0-15 cm soil layer measured after vegetation and any L layer litter has been removed. A total weight of 1 - 2 kg soil should be collected from topsoils with less than approximately 5 per cent organic carbon. Proportionately more

Standard operating procedure SP1106 iv - 003

SP1106 iv

soil should be collected from topsoils that are organic or have higher organic carbon content because of their potentially high water content (<5 kg).

Place each subsample in a double plastic bag. Once sample collection has been completed, secure the inner bag with a tie and insert a waterproof label marked COMPOSITE TOPSOIL SAMPLE with the site number and grid reference between the inner and outer bags.

5.2 Collection of horizon samples for bulk density and soil carbonBegin by carefully removing any vegetation and/or L horizon litter from a side of the pit on which you have not trodden.

Identify all of the soil horizons to 75cm depth and sample each one. Where a horizon is thinner than the sampling tube length make appropriate notes and sample accordingly (combining horizons, if required).

For all soil horizons:

Cut a step down to a depth of the centre of the first horizon less half the length of the bulk density sampling tubes to be used. Push the three sampling tubes in to their full depth using the minimum force required (using a knife around the tubes during insertion may reduce compaction effects). The tubes should be sufficiently far apart to prevent any interference from the soil disturbance caused by insertion of the tubes. Carefully cut away the surrounding soil, slice off any surplus soil at either end of the tube and extract the content of each tube into a double plastic sample bag to form a single composite sample of known volume. Seal the inner bag and insert a waterproof label marked HORIZON SAMPLE with the site number, grid reference, sample depth range and aggregate volume between the inner and outer bags.

Repeat this procedure at each horizon.

For peat soils:

Assess total depth of peat if greater than the depth of the soil pit by rodding or augering. If the most humified horizon is greater than 75cm depth, take a sample in this horizon if practicable and safe to do so.

5.3 Collection of bulk density samples

From 5 positions around the soil pit, take soil cores to 15cm depth (one 15cm or 3 x 5cm). Remove surface vegetation or litter (L) layer prior to sampling and do not sample the L layer. Place each sample in a plastic bag, secure each inner bag, place in an outer bag and insert a waterproof label marked BULK DENSITY SAMPLE with the site number and grid reference between the inner and outer bags.

5.4 Sample storage and transferIf Potentially Mineralisable Nitrogen is to be determined on the COMPOSITE TOPSOIL SAMPLE, place this sample in a mobile refrigerator as soon as possible and transfer to mains refrigerated storage prior to transfer in an insulated cool box, marked SAMPLES REQUIRE REFRIGERATED STORAGE, to the receiving laboratory. If this is the case, samples should be dispatched to a receiving laboratory within 48 hours of collection in the field.

Take all other samples from the field, put them in a sturdy container and store them in a dark, cool place prior to their dispatch as soon as practicable to the receiving laboratory.

Standard operating procedure SP1106 iv - 003

SP1106 iv

Annex 2 Soil carbon determination Standard Operating Procedure

UK Soil Monitoring Scheme – Soil carbon determination

Versions

Version No. Date Comments1 28.02.2011 First draft

Standard operating procedure SP1106 iv - 006

SP1106 iv

UK Soil Monitoring Scheme – Soil carbon determination

1 ScopeThe procedure describes the laboratory analysis of soils for soil carbon by loss on ignition and modified Walkley Black methods.

If total carbon is required on a proportion of samples, this can be carried out using an elemental analyser after acid pre-treatment of the soil.

2 Referenced documents(1) BS ISO 11464:2006 Soil Quality. Pre-treatment of samples for physico-chemical

analysis.

(2) Emmett B.A., Frogbrook Z.L., Chamberlain P.M., Griffiths R., Pickup R., Poskitt J., Reynolds B., Rowe E., Spurgeon D., Rowland P., Wilson J. and Wood P.M. (2008). Countryside Survey Technical Report No. 3/07 Soils Manual. Centre for Ecology and Hydrology.

(3) Method 3.1 Test 8 of British Standard BS 1377:1975 - Soils for Civil Engineering Purposes

3 OutlineThe procedure includes:

1. The preparation of resources and equipment

2. Loss on ignition - use of an oven and scales to determine weight loss when soil is heated to 375oC for 16 hours (based on above-referenced document 2(2))

3. Wet digestion by modified Walkley Black method (based on above-referenced document 2(3))

4. Calculation of the results

4 Resources and equipment

4.1 Loss on ignitionThe following equipment will be required:

1. Electronic balance accurate to 4 decimal places2. Oven3. Muffle furnace4. Tongs for picking up crucibles5. Crucibles6. Desiccator using silica gel desiccant relative humidity 20%7. Reference soils (QC samples)

4.2 Modified Walkley Black

The following equipment will be required:

1. Electronic balance accurate to 4 decimal places2. Oven3. Fume cupboard

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SP1106 iv

4. Conical flasks 250ml5. Calibrated dispensers6. Anti bumping granules7. Reference soils (QC samples)

The following reagents will be required:

1. Orthophosphoric acid (1.7 specific gravity)2. Sulphuric acid (1.84 specific gravity)3. Potassium dichromate solution4. Ammonium iron (II) sulphate solution5. N-phenylanthranilic acid indicator solution

5 Procedure

5.1 Loss on ignitionSoil should be pre-dried at air temperatures (25oC) and sieved to 2mm according to BS ISO 11464:2006. QC soil samples to be included in e.g. every 25 soils tested.

Measurements to be taken are:

Sample IDCrucible IDWeight of dry & cool crucible W1

Weight of crucible + 10g air-dried soil sample W2

Weight of crucible + soil dried at 105oC (after sample has cooled) W3

Weight of crucible + soil dried at 375oC (after sample has cooled) W4

Method:

1. Dry crucibles (e.g. 25 comprising 23 samples, 1 repeat & 1 QC standard soil sample) in oven at 105ºC (± 5°C) for about 30-40 minutes, or in the muffle furnace at 375°C for 10 minutes.

2. Cool the crucibles to room temperature in desiccator and weigh3. Add 10 ±0.2 g air dry soil and weigh4. Place the crucibles on an aluminium tray and place in the oven set at 105ºC (±5°C)

to dry the samples overnight.5. Cool the crucible to room temperature in desiccator and weigh

6. Place the crucibles containing the oven-dried sample in muffle furnace. Do not place the aluminium tray used previously in step 4 in the furnace.

7. Space the crucibles apart and away from the wall and leave space for the inset on the inside of the door.

8. Ash the material (i.e. subject to heat in muffle furnace) for 16 hours at 375°C9. Turn the muffle furnace off and allow the muffle to cool to below 150°C.

10. Transfer to a desiccator and when cool weigh

NB. Samples that have cooled to room temperature in the muffle must be re-dried in the oven at 105ºC (±5ºC) for approximately 30 minutes before cooling in a dessicator and weighing.

11. Retain sub-samples until calculation complete, then dispose.12. Wash & dry crucibles for re-use13. Calculate the carbon content

Soil organic matter % = W4 –W3 * 100 W3-W1

Soil organic carbon % = soil organic matter / 1.724

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5.2 Modified Walkley BlackSoil should be pre-dried at air temperatures (25oC) and sieved to 2mm according to BS ISO 11464:2006. The soil should then be milled to < 0.5mm. QC soil samples to be included in e.g. every 25 soils tested.

1. Weigh not more than 0.5g (to 0.0001g) sample of air dried soil into a conical flask (m1)

2. Carry out a blank digest3. Add approximately five anti-bumping granules to each flask and, in the following

order:4. Add 20ml potassium dichromate solution5. Add 5ml of orthophosphoric acid6. Add 25ml concentrated sulphuric acid

7. Heat the mixture rapidly to boiling under reflux, and keep boiling for between 20 and 25 minutes.

8. Remove from heat and allow the flasks to cool9. Wash down the inside of the condenser with demineralised water into the flask.

10. Titrate the digested mixture with the ammonium iron (II) sulphate solution, using 4-5 drops of the prepared indicator. The end point is as the purple colour changes to green. Record the titre, which should be at least 10ml; if it is not, repeat using less soil.

11. Calculate the organic carbon content allowing for the actual amount of soil used.

Tf = Tb

20Where

Tf is the correction factor;Tb is the titre, in ml, of the blank digest.

% Organic Carbon = 1.545(Tb - Ts)/Tf

10m1

Where:

Tb is the titre, in ml, of the blank digest;Ts is the titre, in ml, of the sample digest;Tf is the correction factor; andm1 is the mass, in grams, of the sample.

Report the organic carbon to one decimal place.

Standard operating procedure SP1106 iv - 006