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TRANSCRIPT
ONLINLE SUPPLEMENTING MATERIAL
Microbial Respiration in Arctic Upland and Peat Soils as a Source of
Atmospheric Carbon Dioxide
Christina Biasia,*, Simo Jokinena, Maija E. Marushchaka, Kai Hämäläinenb,1, Tatiana Trubnikovaa,
Markku Oinonenb and Pertti J. Martikainena
aUniversity of Eastern Finland, Department of Environmental Science, P.O. Box 1627, 70211 Kuopio,
Finland
bFinnish Museum of Natural History, University of Helsinki, Dating Laboratory, P.O. Box 64, 00014
Helsinki, Finland
1STUK-Radiation and Nuclear Safety Authority, P.O. Box 14, 00881 Helsinki, Finland
*Correspondance: Christina Biasi, Tel.: +358 40 3553810, Fax: +358 17 163750. Email:
1
Climatic characteristics of study site Seida
Mean long-term annual temperature is -5.6 °C and the mean annual precipitation amounts
to 501 mm measured between 1977 and 2006 at the closest climate station in Vorkuta, 75
km from the research site (Komi Republican Center for Hydrometeorological and
Environmental Monitoring). Mean annual temperature was 2-3 °C higher than this long-
term mean in 2008 and 2007, respectively. The length of the thermic growing season, which
is defined as a period when the daily mean air temperature is permanently above +5°C, was
80 days in 2007 and 79 days in 2008 comparable between both years.
Vegetation characteristics of the subsites and plant biomass during the study years
The vegetated areas of peat plateau are dominated by mosses, such as Dicranum sp. and
Hepaticae sp. in dry peat plateau (DPP) and Sphagnum sp. in moist peat plateau (MPP). The
dominating vascular plants are Ledum decumbens in DPP and Rubus chamaemorus in MPP.
In both subsites, also Betula nana and Vaccinium sp. are found and in the drier areas,
lichens are abundant (for example, Cladina rangifera and C. maxima). The dominant
vascular plants of tundra heath (STH) are Betula nana and Carex globularis, and also lichens
(for example, Cladina rangifera and C. maxima) and mosses (for example, Sphagnum sp.,
Polytrichum strictum) are abundant.
We measured plant biomass and plant charactersitics by monitoring height of sedges and
length of new growth and number of leaves of dominant vasuclar plants in 2007 and 2008.
This was done periodically over the growing seasons 2007 and 2008 on specifically labelled
2
plants next to collars where respiration measurements were carried out. The results for
period DOY203-218, when this study was carried out, are shown in Figure S5 below.
Soil analysis
Soil moisture content was determined on a 10 g subsample by oven drying at 60 °C for 24 h.
Soil pH was measured in a 1:2.5 soil to H2O solution (v/v). Carbon and nitrogen (N) contents
were determined from a finely ground subsample by elemental analyzer (Thermo Finnigan
Flash EA 1112 Series, San Jose, CA, USA), and soil organic matter (SOM) content was
measured as loss on ignition. Dry bulk density of soils was determined by a core method.
A subsample of surface soil from each replicate was pooled and analyzed for radiocarbon
content by the Finnish Museum of Natural History, University of Helsinki. Additionally,
radiocarbon analyses were performed for the soil profile of peat circles (PC). The sample
pretreatment procedure for radiocarbon analyses followed the typical acid-alkali-acid
method aiming to separate alkali insoluble fraction of the sample. This procedure removes
possible carbonate contaminants and alkali-soluble humic acids. Pretreated samples were
mixed with a stoichiometric excess of CuO and packed into glass ampoules, which were
pumped into vacuum and torch-sealed. The packed samples were combusted at 520°C
overnight. The released CO2 was collected and purified with liquid-N2 and ethanol traps at -
196 and -85°C, respectively. After purifying and measuring the sample 13C value with IRMS,
the CO2 samples were converted to graphite targets in presence of zinc powder and iron
catalyst. AMS measurements on the targets were eventually performed at the Uppsala
Tandem Laboratory.
3
Incubation experiment
Field-moist soil (25 g) was weighed in 500 ml jars and placed in incubators in the laboratory.
Incubation temperature was set to approximate field conditions (± 2°C) (10°C for organic
soils of MPP and mineral soils of STH and at 15°C for the other samples). A pre-incubation
period of one month, where jars were only loosely covered and water content was adjusted,
was carried out to deplete C stemming from residual fine roots including mycorrhiza which
contribute to CO2 flux. Soil respiration rates were then determined by closing the jars and
sampling headspace air (20 ml) initially and 4 times over the following six hour incubation
time. The CO2 concentration of headspace air was determined by gas chromatography (GC)
(HP 5890 series II, Hewlett-Packard with a thermal conductivity (TC) detector for CO2). Soil
respiration was calculated from the linear increase in CO2 concentration over the incubation
period. For comparison of basal soil respiration rates between different soils, all rates were
normalized to 15°C using Q10 values derived from Arrhenius equations (see main text).
For radiocarbon dating of CO2, the jars were flushed with CO2 free air (equivalent five-times
chamber volume) to remove background ambient air from the headspace. Jars were
thereafter closed until enough CO2 had accumulated to allow sampling for 14C. Collection of
14CO2 and 13CO2 was done following the procedure used for the field and described in the
main body of the text. Carbon dioxide collected in the molecular sieves was analyzed for 14C
within one week.
4
14 CO 2 collection
After finishing the SR measurements, CO2 was collected for 14CO2 analysis by the molecular
sieve sampling technique using a protocol modified after Schuur and Trumbore (2006). Prior
to the field sampling, the molecular sieve tubes were filled and regenerated according to
the procedure described by Hämäläinen and others (2010). Particularly, the tubes
containing 20 g of molecular sieve grains were heated to 500°C and evacuated until the
pressure within the tube reached 1×10-2 mbar. The sieve tubes were then closed by valves at
both ends and taken to the field site for collections. In the field, chamber air was scrubbed
first through an external soda lime cartridge by pump (flow rate: 4 l min -1) to remove
background atmospheric CO2. To achieve that, the equivalent of five chamber-volumes was
scrubbed. Then, the pump was switched off and CO2 (consisting of SMR and RR) was
allowed to accumulate for about 10 minutes while monitoring the concentration with IRGA.
The chamber air was then passed through anhydrone to dry the air and the tubes filled with
molecular sieves (type 13X, Merck 1.05703.0250) by pumping for 20 minutes or until the
saturation of the sieve tube was observed. The CO2 samples in the sieve tubes were treated
in the laboratory as follows. The sieve tube was first evacuated to remove the unwanted
volatile material inside. CO2 was released by keeping the sieve 2h at 500°C. The released CO2
was then treated correspondingly to soil samples, grafitized and analyzed by AMS. A full
molecular sieve yielded 1.3 ± 0.5 mg of C which is sufficient for radiocarbon dating. The
advantage of CO2 collection with molecular sieves over the traditional NaOH trapping
method is that risks of isotope fractionation and contamination are largely reduced (Bauer
and others 1992; Hardie and others 2005).
5
To determine the 13C of respiration, which was needed for eventual correction of
contamination of molecular sieves with air and incomplete scrubbing of atmospheric CO2 in
the chamber system, some Keeling plots were taken at the end of the sampling following
the procedure described in Biasi and others (2008a) and simultaneously an air sample was
taken for 13C analysis. The 13CO2 values were determined by gas chromatography coupled
to an isotope ratio mass spectrometer (GC-IRMS) in the laboratories of Kuopio (Biasi and
others 2008b; Biasi and others 2005)
14 C notation
The 14C content is expressed as fraction modern (F14C) (Reimer and others 2004),
representing the proportion of 14C activity in the sample compared to the 14C standard
activity. As an isotopic fractionation can occur due to the mass difference of the C isotopes,
all samples are routinely corrected for mass-dependent fractionation using the 13C values.
The typical accuracy of the F14C values values was ± 0.004. F14C is approximately equal to 1.0
in 1950, before the nuclear weapons test. Higher values reflect the time of nuclear weapon
testing (bomb C peak) whereas lower values the pre-bomb times. To obtain the
approximate age for the corresponding C fixation by photosynthetic activity, the
radiocarbon data were converted to calendar years via atmospheric calibration data (Levin
and others 2008; Levin and Kromer 2004) and IntCal09 (Reimer and others 2009), and by
using CaliBomb (Stuiver and Reimer 1986), calib.qub.ac.uk/CALIBomb) and Oxcal 4.1
software. Often two possible values are provided for post-bomb results (upslope or down-
6
slope of the bomb curve). In some cases, we could rule out one value by assuming the
following: mineral soil is older than overlying organic soil; CO2 respired is younger than bulk
soil C; and SR is younger than SMR due to contribution of roots. If some uncertainty
remained, both age results were given. In any case, post-bomb age values are considered as
approximations, because the atmospheric 14C concentration after nuclear bomb testing was
not uniform around the globe and the calibration data available for this study were from
Central Europe.
Post-processing of measured F 14 C values of CO 2 from molecular sieves
The measured F14C values of CO2 collected in molecular sieves were first corrected for blank
values (contamination with atmosphere) using the 13C values measured and isotope mixing
models as well as mass balance approaches as described in Schuur and Trumbore (2006).
The contribution of air was on average 11.8% for field samples, and 4.3% for laboratory
samples. Samples with greater than 50% air contamination (one replicate of MPP and one
replicate of DPP, field samples) were omitted from the study.
7
Figure S1
DPP MPP PC
µg CO
2 cm-3
h-1
0
1
2
3
4
5
Peat Plateau
Tundra Heath
STH
a
a a
a
b
Fig. S1: Basal soil respiration rates from laboratory incubations with root free soil, expressed
on a volume basis. DPP, MPP, PC and STH represent dry peat plateau, moist peat plateau,
peat circle and shrub tundra heath, respectively. Patterned grey bars indicate mineral soil of
STH, patterned white bars organic soil of STH. Different letter denote significant differences
between subsites at p ≤ 0.05 (Kruskal Wallice test).
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Figure S2
Age of soil (yrs)
0 1000 2000 3000 4000
Basal soil respiration rates (
g CO
2 g-1
DW
h-1
)
0
10
20
30
40
50
60
70
80
DPPMPPPCSTH-Organic horizonSTH-Mineral horizon
Age of soil (yrs)
10 20 30 40 50 60 70B
asal soil respiration rates (g C
O2 g
-1D
W h
-1)
0
10
20
30
40
50
60
70
80
Fig. S2: Basal soil respiration rates as determined from laboratory incubations (n=3, SE) are
correlated with age of soil (SE of analysis). The upper figure includes values of peat circles
(PC). The lower figure excludes them and shows then a significant correlation (p < 0.05). This
analysis assumes that the age of MPP is 45 years, but near significant correlation (p < 0.1) is
also found if the age of MPP is assumed to be 28 years. It shows that respiration rates of PC
are very high for this ancient soil thus breaking the correlation between age and respiration
rates.
9
Figure S3
DPP MPP PC STH
mg C
O2 m
-2 h
-1
0
100
200
300
400
Peat Plateau Tundra Heath
*
Fig. S3: Soil microbial respiration rates as determined with 14C partitioning approach (white
bars) and trenching approach (black bars) in Seida study site, Arctic tundra. Both techniques
were employed in two consecutive years in peak season (between 22 July and 6 August).
The star indicates a significant difference between the methods at p < 0.05 (Kruskal-Wallis
test).
10
Figure S4
SMR (g CO2 m-2 season-1)
50 100 150 200 250 300 350
NE
E (g
CO
2 m
-2 s
easo
n-1)
-600
-400
-200
0
200
400
DPPMPPPCSTH
r2 = 0.75p < 0.1
SMR (mg CO2 m-2 h-1)
0 100 200 300
NE
E (m
g C
O 2 m-2
h-1
)
-600
-400
-200
0
200
400
DPPMPPPCSTH
r2 = 0.77p = 0.125
Fig. S4: Correlations between soil microbial respiration (SMR) and net ecosystem carbon
exchange (NEE). Upper graph: soil microbial respiration (SMR) was determined with root
trenching. Both fluxes were measured over the growing season 2008 (n=5 for SMR, n=3 for
NEE; SE). Lower Graph: Both fluxes were determined in 2007; SMR rates were calculated by
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14C partitioning approach (n=2-3 for SMR, n=3 for NEE; SE). Sunny, clear days prevailed
during this period.
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Figure S5
Empetrum
nigrum
Ledum decum
bensVaccinium
uliginosum
Betula nana
Carex sp.
Length/Height (cm
)
0
5
10
15
20 2007 2008
New Growth Entire Plant
Rubus chamaem
orus
Ledum decum
bensVaccinium
uliginosum
Betula nana
Carex sp.
Leaf number
0
5
10
15
20
25
2007 2008
New Growth Entire Plant
13
Figure S5: Plant characteristics of dominate vascular plant species. Upper graph: height of
Carex sp., and length of new growth of all other vascular plants. Lower graph: leaf number
of Carex sp., and leaf number of all other vascular plants of vegetation in 2007 and 2008
growing season (all data are means of 12 measurements for each plant taken between DOY
203-218). There was no significant difference in a single parameter between both years,
indicating that plant biomass was comparable in 2007 and 2008.
14
Table S1: Pearson's Correlation Coefficient (R2) and p-values Between Basal Soil Respiration
Rates (g CO2 g-1DW h-1) and Relevant Variables as Determined from Root-free Soil during
Laboratory Incubations (univariate analysis)
Correlations that were near significant (p< 0.10) are in italic and correlations that were
significant (p< 0.05) are in bold.
Abbreviations are as followed: BD (bulk density), SOM (soil organic matter), WC (water content); age of soil is in years.
aassuming that age of soil from MPP is ~28 yrs
bassuming that age of soil from MPP is ~45 yrs
cassuming that age of CO2 respired from DPP is ~12 yrs
dassuming that age of CO2 respired from DPP is ~49 yrs
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Table S2: Pearson's Correlation Coefficient (R2) and p-values Between Soil Microbial
Respiration (SMRTR; in g CO2 m-2 season-1) and Relevant Variables as Determined with Root
Trenching Approach in 2008, Seida Study Site (univariate analysis)
SMRTR SMRTR (excluding PC)R2 p-value R2 p-value
BD (g cm-3) 0,913 0,087 0,268 0,828pH -0,566 0,434 0,714 0,494C content (g C m-2; 0-10 cm) 0,907 0,093 0,714 0,494N content (g N m-2, 0-10cm) 0,91 0,085 -0,654 0,494C/N -0,945 0,055 -0,747 0,463Moisture (VWC) 0,953 0,047 0,77 0,44Age of soila -0,024 0,976 0,71 0,498Age of soilb -0,025 0,975 0,713 0,495Age of CO2 -0,016 0,984 0,713 0,495T (°C, 2 cm) 0,066 0,809 0,142 0,909WT (cm) -0,013 0,961 0,904 0,282AL (cm) 0,292 0,273 -0,786 0,425NEE (g CO2 m-2 season-1) 0,977 0,023 0,861 0,34
ER (g CO2 m-2 season-1) 0,384 0,142 0,623 0,572
Correlations that were near significant (p< 0.10) are in italic and correlations that were
significant (p< 0.05) are in bold.
Abbreviations are as followed: BD (bulk density), VWC (volumetric water content); WT (water table), AL (active layer), NEE (net ecosystem carbon exchange), ER (ecosystem respiration).
aassuming that age of soil from MPP is ~28 yrs
bassuming that age of soil from MPP is ~45 yrs
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