biogeosciences variability of above-ground litter inputs ... · s. xu et al.: variability of...

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Variability of above-ground litter inputs alters soil physicochemicaland biological processes a meta-analysis of litterfall-manipulationexperiments

S Xu12 L L Liu 2 and E J Sayer3

1University of Chinese Academy of Sciences Yuquan Road Beijing 100049 China2State Key Laboratory of Vegetation and Environmental Change Institute of Botany Chinese Academy of SciencesXiangshan Beijing 100093 China3Department of Environment Earth amp Ecosystems The Open University Walton Hall Milton Keynes MK7 6AA UK

Correspondence toL Liu (lingliliuibcasaccn)

Received 5 February 2013 ndash Published in Biogeosciences Discuss 14 March 2013Revised 29 September 2013 ndash Accepted 21 October 2013 ndash Published 19 November 2013

Abstract Global change has been shown to alter the amountof above-ground litter inputs to soil greatly which couldcause substantial cascading effects on below-ground biogeo-chemical cycling Despite extensive study there is uncer-tainty about how changes in above-ground litter inputs affectsoil carbon and nutrient turnover and transformation Herewe conducted a meta-analysis on 70 litter-manipulation ex-periments in order to assess how changes in above-groundlitter inputs alter soil physicochemical properties carbon dy-namics and nutrient cycles Our results demonstrated thatlitter removal decreased soil respiration by 34 microbialbiomass carbon in the mineral soil by 39 and total car-bon in the mineral soil by 10 whereas litter addition in-creased them by 31 26 and 10 respectively This suggeststhat greater litter inputs increase the soil carbon sink despitehigher rates of carbon release and transformation Total ni-trogen and extractable inorganic nitrogen in the mineral soildecreased by 17 and 30 respectively under litter removalbut were not altered by litter addition Overall litter manip-ulation had a significant impact upon soil temperature andmoisture but not soil pH litter inputs were more crucial inbuffering soil temperature and moisture fluctuations in grass-land than in forest Compared to other ecosystems tropicaland subtropical forests were more sensitive to variation inlitter inputs as altered litter inputs affected the turnover andaccumulation of soil carbon and nutrients more substantiallyover a shorter time period Our study demonstrates that al-though the magnitude of responses differed greatly among

ecosystems the direction of the responses was very similaracross different ecosystems Interactions between plant pro-ductivity and below-ground biogeochemical cycling need tobe taken into account to predict ecosystem responses to en-vironmental change

1 Introduction

Above-ground litterfall is one of the most important com-ponents of carbon and nutrient cycling and the litter layerregulates soil microclimate by forming a buffering interfacebetween the soil surface and the atmosphere (Sayer 2006)Terrestrial ecosystems are undergoing simultaneous changesin climate and biogeochemical cycles and those changescould affect plant net primary production (NPP) positivelyor negatively Changes such as elevated CO2 (King et al2005) nitrogen deposition (Xia and Wan 2008) and tem-perature increases (Raich et al 2006) were found to en-hance plant productivity whereas elevated O3 (Liu et al2005) drought (Zhao and Running 2010) and acid deposi-tion (Irving and Miller 1981) generally decreased productiv-ity These changes in primary production could alter both thequality and quantity of above-ground litter inputs to soil (Liuet al 2005) and therefore physical chemical and biologicalproperties of the litter layer In addition extreme events thefrequency of which may increase in future could also lead tosudden and dramatic changes in litter inputs such as a large

Published by Copernicus Publications on behalf of the European Geosciences Union

7424 S Xu et al Variability of above-ground litter inputs alters soil physicochemical and biological processes

increase in above-ground litterfall after hurricanes or severestorms (Ostertag et al 2003) or rapid loss of the litter layerafter wildfires (Wardle et al 2003)

As one of the most important pathways for carbon and nu-trient fluxes to the soil changes in above-ground litter in-puts could lead to cascading effects on below-ground bio-geochemical processes Although changes in above-groundlitter inputs have been observed in numerous global changemanipulation experiments in multiple ecosystems most ofthose studies found that total soil carbon (C) content was gen-erally unchanged (Baer and Blair 2008 Talhelm et al 2009but see Fornara and Tilman 2012) Several mechanisms havebeen proposed to explain why soil C showed small changeseven when above-ground litter inputs were greatly alteredOne possible explanation is that soil C may be altered by lit-ter inputs but the changes are too small to be detected in thelarge and heterogeneous soil C pool (Hungate et al 1996)A change in the strength of priming effects is another pos-sible reason high litter inputs could cause greater primingeffects and the increase in new soil C derived from the littermay be offset by the decomposition of older soil C (Sayeret al 2011) Soil carbon saturation is also a plausible the-ory to explain the lack of changes in soil C content in re-sponse to varying amounts of litter inputs The theory statesthat the capacity of soil to stabilize organic C has an upperlimit which is determined by soil physical chemical and bio-chemical characteristics (Six et al 2002a)

An emerging view suggests that the majority of organic Cin soil is derived from rhizodeposition and above-ground lit-ter inputs have a limited influence on soil C storage (Schmidtet al 2011) The lack of changes in total soil C has sparked adebate about whether alterations in above-ground litter pro-duction as a result of global change will impact upon long-term soil C storage Quantification of the contribution ofabove-ground litter to soil C sequestration and nutrient cy-cling is therefore needed to assess the importance of above-ground litter in below-ground processes

A large number of litter-manipulation studies in whichlitter is experimentally added or removed have been con-ducted since the first experiments in the 1850s (Sayer 2006)The information we draw from this rich research history oflitter-manipulation experiments could help us better under-stand how above-ground litter inputs affect below-groundprocesses and therefore the mechanisms regulating the po-tential of soil to sequester additional C Below-ground pro-cesses such as soil respiration microbial activity and soil Cformation are simultaneously influenced by litter inputs andthese processes intrinsically interact with each other (Chapinet al 2011) To assess the consequences of above-groundlitter inputs on soil physical chemical and biochemical pro-cesses quantitatively we conducted a meta-analysis of litter-manipulation experiments from multiple terrestrial ecosys-tems Changes in litter productivity may be accompanied bychanges in litter biochemistry and the two interact with eachother in biogeochemical processes (Liu et al 2009) How-

ever because of the limited data availability on changes in lit-ter biochemistry this study focused on the impact of changesin litter productivity on below-ground C and nutrient cycling

2 Methods

21 Data selection

A comprehensive literature search covering relevant peer-reviewed articles and dissertations from 1950 to 2013 wasconducted using the databases of Web of Science and Pro-Quest We also cross-checked the references of the relevantarticles to identify other potential book chapters and peer-reviewed reports Only data from in situ litter-manipulationexperiments were included in our data set Studies were ex-cluded if they were conducted in a controlled laboratory set-ting (eg Liu et al 2009) or the additional carbon was sup-plied as specific organic compounds such as glucose (egPark and Matzner 2006) or cellulose (eg Fontaine et al2004) When data from the same site and treatments werepresented in multiple publications we used the data from themost recent publications Similarly when data from multipleyears were given we only used data from the most recentyear Litter-manipulation experimental sites were from mul-tiple climatic zones including arctic boreal temperate and(sub-)tropical regions (See details in Appendix B in the Sup-plement) Here ldquo(sub-)tropicalrdquo includes both tropical andsub-tropical regions

Four categories of data related to below-ground biotic andabiotic properties were extracted from the literature of insitu litter-manipulation experiments (1) soil surface physi-cal and chemical properties including soil temperature soilmoisture and soil pH (2) microbial responses including mi-crobial biomass carbon (MBC) and microbial biomass nitro-gen (MBN) (3) C fluxes and pools including soil respira-tion total carbon (C) and dissolved organic carbon (DOC)and (4) nutrient fluxes and pools including total nitrogen(N) C N ratios (C N) dissolved organic nitrogen (DON)extractable inorganic nitrogen (EIN) and extractable phos-phorus (P) Where relevant we distinguished between lit-ter layer and mineral soil for all variables but results wereonly reported where sufficient data points were available (ien ge 4) In our data set litter layer data were all from studiesconducted in boreal forest or temperate forest and includedforest floor (Fisk and Fahey 2001) O horizon (Nadelhofferet al 2004) O layer (Froumlberg et al 2005) epihumus sub-horizon (Dzwonko and Gawronski 2002) and humus hori-zon (Dzwonko and Gawronski 2002) Mineral soil in ourdata set is defined as the soil beneath the litter layer or thetopsoil from ecosystems such as grassland and (sub-)tropicalforest The sampling depth in the mineral soil ranged from1 to 40 cm with the majority of the data collected in thetop 10 cm When measurements were conducted in differ-ent mineral soil depths we only used the data reported for

Biogeosciences 10 7423ndash7433 2013 wwwbiogeosciencesnet1074232013

S Xu et al Variability of above-ground litter inputs alters soil physicochemical and biological processes 7425

Table 1Effects of litter removal and litter addition on soil physicochemical properties of different ecosystems For each response variablesthe responses of the ecosystems denoted by different superscript letter differed significantly (P lt 005) RR the mean response ratio 95 CI 95 confident intervals n no observations ldquondashrdquo no data

Litter removal Litter addition

Response variable Soil layer Group RR 95 CIn RR 95 CI n

Soil temperature Mineral soil All 104 101ndash107 21 097 093ndash100 11(Sub-)tropical forest 099a 098ndash100 11 100a 099ndash101 4Grassland 110b 103ndash117 6 092b 086ndash097 4

Soil moisture Mineral soil All 090 084ndash095 35 104 099ndash109 33Temperate forest 085a 068ndash100 7 097a 090ndash105 10(Sub-)tropical forest 093a 088ndash097 19 101a 095ndash106 6Grassland 082a 066ndash095 6 113b 106ndash120 11Shrubland ndash ndash ndash 105ab 092ndash120 6

Soil pH Litter layer All 100 097ndash103 9 102 101ndash102 2Boreal forest 100 096ndash105 6 ndash ndash ndash

Mineral soil All 100 098ndash102 11 104 099ndash109 7(Sub-)tropical forest 102 100ndash103 8 ndash ndash ndash

the uppermost depth (eg Tian et al 2010) When data weregraphically presented we extracted the numerical values bydigitizing the figures using Engauge Digitizer (Free SoftwareFoundation Inc Boston MA USA)

22 Meta-analysis

The data were analysed using the meta-analysis de-scribed by Hedges et al (1999) The effect size of litter-manipulation treatment for each individual observation wasestimated by the natural log of the response ratio (RR) lnRR = ln

(XtXc

) whereXc is the control mean andXt is the

treatment mean The average response ratio (RR) was cal-culated using the mixed model of the meta-analytical soft-ware METAWIN (Sinauer Associates Inc Sunderland MAUSA) The variance of the mean effect size was calculatedusing resampling techniques (Adams et al 1997) If thelower bound of the 95 CI of RR was larger than 1 thenthe response was significantly positive atP lt 005 If theupper bound of the 95 CI of RR was smaller than 1 thenthe response was significantly negative atP lt 005 For eachinvestigated parameter a subgroup analysis was conductedto assess whether the magnitudes of effects differed acrossecosystem types soil depths or treatment duration Althoughthere is no accepted minimum number of studies that are re-quired for a meta-analysis we adopted the criteria for sys-tematic review by Fu et al (2011) in which each subgroupshould have a minimum of four studies We therefore presentresults for separate ecosystems only where sufficient obser-vations were available in more than one subgroup Total het-erogeneity (QT) was partitioned into within-group (QW) andbetween-group (QB) heterogeneities According to Hedgeset al (1999) a significantQB indicates that the response ra-tios differ among groups Means of the groups were consid-ered significantly different if their 95 CI did not overlap

23 Regression analysis

Most studies included in our data set reported the amountsof litter added or removed Data from both litter removal andlitter addition experiments were grouped for each parameterwith negative values representing litter removal and positivevalues representing litter addition The correlation betweenthe response ratio (RR) of a biogeochemical parameter andthe amounts of litter was determined by linear regressionAll regression analyses were conducted using SAS software(SAS Institute Inc Cary NC USA)

3 Results

In total 440 observations were extracted from 70 publica-tions including ecosystems such as boreal forest (3 publica-tions) temperate forest (20 publications) (sub-)tropical for-est (20 publications) grassland (22 publications) and shrub-land (5 publications Appendix B) Across all studies themean annual air temperature ranged fromminus42 to 27Cmean annual precipitation from 315 to 5000 mm and the ex-periments ranged in duration from half a year to 20 yr Sitecharacteristics for each study are given in Appendix B in theSupplement

31 Physical and chemical properties of the litter layerand mineral soil

Across all ecosystems litter removal increased the temper-ature of the mineral soil by 4 and litter addition de-creased it by 3 (Table 1) The response of soil temperatureto litter manipulation differed among ecosystems in grass-lands soil temperature increased with litter removal and de-creased with litter addition whereas in (sub-)tropical forest

wwwbiogeosciencesnet1074232013 Biogeosciences 10 7423ndash7433 2013


Table 2 Effects of litter removal and litter addition on soil carbon cycling parameters of different ecosystems where MBC is microbialbiomass carbon MBN is microbial biomass nitrogen and DOC is dissolved organic carbon For each response variables the responses ofthe ecosystems denoted by different superscript letter differed significantly (P lt 005)



MBC Litter layer All 077 054ndash096 3 129 100ndash168 2Mineral soil All 061 048ndash076 17 126 117ndash137 23

Temperate forest 071a 036ndash107 4 ndash ndash ndash(Sub-)tropical forest 051a 040ndash066 10 ndash ndash ndashGrassland ndash ndash ndash 134a 122ndash149 15Shrubland ndash ndash ndash 112b 105ndash117 4

MBN Mineral soil All 086 077ndash096 5 146 121ndash177 10Grassland ndash ndash ndash 182 157ndash211 6

Soil respiration All 066 059ndash073 22 131 121ndash143 22Temperate forest 067a 058ndash075 5 138a 116ndash171 4(Sub-)tropical forest 065a 055ndash075 14 120a 102ndash144 6Grassland ndash ndash ndash 135a 121ndash148 12

DOC Litter layer Temperate forest 078 067ndash087 7 147 134ndash164 9Mineral soil All 069 053ndash093 7 107 076ndash140 6

Temperate forest 081 059ndash109 5 094 063ndash139 4Total C Litter layer All 095 094ndash096 2 109 104ndash117 3

Mineral soil All 090 083ndash098 19 110 101ndash120 18Temperate forest 091a 077ndash109 7 110a 097ndash128 10(Sub-)tropical forest 080a 074ndash088 4 ndash ndash ndashGrassland 094a 086ndash106 8 103a 099ndash110 6

litter removal reduced soil temperature but litter addition hadno effect (Table 1)

Across all studies litter removal decreased soil moisturein the mineral soil by 10 (Table 1) whereas litter additionhad no significant effect (Table 1) However when the datawere analysed for each ecosystem type separately litter ad-dition increased moisture in the mineral soil in grassland byan average of 13 but had no significant effect in forests orshrubland (Table 1)

Overall litter manipulation did not affect pH in the litterlayer or the mineral soil (Table 1)

32 Microbial responses

Overall litter removal reduced MBC in the mineral soilby 39 However this decrease was only significant in(sub-)tropical forest (Table 2) Litter addition increased MBCin the mineral soil by an average of 26 with grasslandshowing a greater increase than shrubland (Table 2)

Litter removal decreased MBN in the mineral soil by anaverage of 14 and litter addition significantly increasedMBN in the mineral soil by an average of 46 across allecosystem types (Table 2)

33 C cycling

Soil respiration rates decreased with litter removal by an av-erage of 34 across all studies and increased by an aver-age of 31 with litter addition (Table 2) The response ra-tio of soil respiration rates was positively correlated withthe amounts of litter added or removed (Fig 1aR2

= 065P lt 001) and with the response ratio of MBC (see Fig A3in the SupplementR2

= 066P lt 001)Litter removal reduced DOC concentrations in the litter

layer by 22 and in the mineral soil by 31 (Table 2) Litteraddition increased DOC concentrations in the litter layer byan average of 47 but had no significant effect on DOCconcentrations in the mineral soil (Table 2)

Overall litter removal decreased total C in the mineral soilby 10 and correspondingly litter addition increased it by10 (Table 2) When the data were analysed by ecosystemtype there was a significant decrease in total C of the min-eral soil in response to litter removal in (sub-)tropical forestbut no effect in temperate forest and grassland There was noclear effect of litter addition on total C in the mineral soil ei-ther in temperate forest or grassland and insufficient data for(sub-)tropical forests (Table 2) The effect of litter manipu-lation on total C in the mineral soil was strongly influencedby soil depth When data were divided into different sam-pling depths total C content in the top 5 cm of the mineralsoil was 31 higher under litter addition but there was no



Table 3 Effects of litter removal and litter addition on soil nutrient cycling parameters of different ecosystems where DON is dissolvedorganic nitrogen and EIN is extractable inorganic nitrogen For each response variables the responses of the ecosystems denoted by differentsuperscript letter differed significantly (P lt 005)



C N ratio Litter layer All 099 094ndash103 9 124 104ndash166 4Boreal forest 098 091ndash105 6 ndash ndash ndashTemperate forest ndash ndash ndash 124 104ndash166 4

Mineral soil All 094 089ndash099 8 103 099ndash108 13Temperate forest 095 087ndash102 5 101 095ndash107 7

Total N Litter layer All 092 092ndash092 2 084 062ndash102 3Mineral soil All 083 077ndash088 12 104 095ndash113 19

Temperate forest 083a 073ndash093 7 103a 090ndash120 10(Sub-)tropical forest 080a 079ndash081 4 ndash ndash ndashShrubland ndash ndash ndash 093a 083ndash102 4

DON Litter layer Temperate forest 104 079ndash127 3 176 147ndash209 3Mineral soil All 100 078ndash125 6 106 081ndash131 11

Temperate forest 100 079ndash125 6 104 076ndash135 9EIN Litter layer Temperate forest 150 114ndash197 4 125 082ndash190 5

Mineral soil All 070 052ndash096 12 076 057ndash101 10Temperate forest 121a 072ndash177 4 070 050ndash098 8(Sub-)tropical forest 047b 038ndash059 6 ndash ndash ndash

Extractable P Mineral soil All 082 064ndash103 7 116 098ndash147 5(Sub-)tropical forest 071 054ndash094 4 ndash ndash ndash

significant increase at other soil depths (see Table A1 in theSupplement)

34 Nutrient cycling

The C N ratio of the litter layer did not change with litterremoval but increased significantly with litter addition (Ta-ble 3) Overall the C N ratio of the mineral soil decreasedby 6 following litter removal whereas litter addition hadno significant effect (Table 3)

Across all ecosystems litter removal reduced total N inthe mineral soil by 17 Litter addition had no signifi-cant impact on total N in the mineral soil (Table 3) andneither litter removal nor litter addition affected DON inthe mineral soil (Table 3) Litter removal increased EINin the litter layer by 50 and decreased EIN in the min-eral soil by 30 (Table 3) whereas litter addition had nosignificant effect (Table 3)

Overall neither litter removal nor litter addition affectedextractable P in the mineral soil (Table 3)

35 The effects of litter-manipulation levels onbelow-ground processes

Regression analysis showed that the RR of soil respirationMBC in the mineral soil total C in the litter layer andmineral soil total N in the mineral soil DOC in the lit-ter layer and C N ratios in the mineral soil all showed

a positive linear relationship with litter inputs (Fig 1Table A3 and Fig A2 in the Supplement)

4 Discussion

Global change has been shown to alter plant productivitysignificantly which leads to increased or decreased above-ground litter inputs to soil with presumed knock-on effectsfor soil C stocks and nutrient supply Despite this relativelyfew litter-manipulation experiments have specifically exam-ined how variation in litter inputs affects below-ground pro-cesses in the context of global environmental change (egLiu et al 2009 Sayer et al 2011) Our systematic synthe-sis of litter-manipulation experiments aimed to improve ourunderstanding of the responses of below-ground processes tovarying inputs of above-ground litter under global changeThe magnitude of responses to litter manipulation variedamong ecosystems and this was likely partly influenced bythe number of data points available Nevertheless with fewexceptions the direction of the responses was the same re-gardless of ecosystem type (Fig 1)


The litter layer acts as a protective interface between at-mosphere and soil by regulating soil physicochemical con-ditions such as soil temperature moisture and pH (Sayer



Fig 1Relationships between the amounts of litter manipulation and the response ratios (RRs) of(A) soil respiration(B) microbial biomasscarbon (MBC)(C) total carbon (C) and(D) total nitrogen (N) in the mineral soil

2006) Our analysis demonstrates that the temperature of themineral soil increased under litter removal and decreased un-der litter addition whereas the water content of the mineralsoil decreased under litter removal but did not respond tolitter addition Litter inputs were more important in buffer-ing mineral soil temperature and moisture fluctuations ingrassland than in forest (Table 1) Forest canopies can in-tercept solar heat and precipitation by the large surface areaof branches and foliage (Lowman and Schowalter 2012)whereas the vegetation in grassland has a much shorter andsimpler structure and has limited capacity to intercept so-lar radiation and precipitation compared to the forest canopyGrasslands therefore rely more on surface litter to maintain afavourable soil environment (Amatangelo et al 2008)

Litter inputs may change soil pH via changing the releaseof organic acids or the supply of exchangeable base cationsduring the processes of litter decomposition but we foundthat litter manipulation had a small impact on soil pH Previ-ous work has shown that the direction of changes in soil pHwith litter manipulation may depend on litter type and ini-tial soil pH (Sayer 2006) but at present there are insufficientdata to analyse this quantitatively


It is generally accepted that microbes are C-limited and freshlitter can increase soil microbial biomass and activity (Fiskand Fahey 2001) We found that litter addition significantlyincreased MBC in the mineral soil whereas litter removalcaused a general decrease in MBC (Fig 1 Table 2) It islikely that this result was largely shaped by the response of(sub-)tropical forests where microbes may be more reliant oncarbon supply from fresh litter than in other ecosystems Themean residence time (MRT) of surface litter in tropical forestis 025 to 1 yr compared to 4ndash16 yr MRT in temperate forest(Olson 1963) and there is little or no build-up of an organicforest floor in lowland tropical forests (Wieder and Wright1995) Litter removal could therefore induce a greater de-cline in microbial activities in (sub-)tropical forest becausemicrobes have only limited access to organic C from othersources The observed reduction in soil-extractable P underlitter removal (Table 3) may also contribute to the decreasein MBC as microbial utilization of soil organic carbon isthought to be P-limited in tropical forests (Cleveland et al2002) Thus litter removal decreased the above-ground Csupply for microbial growth and simultaneously decreasedsoil-extractable P in (sub-)tropical forests (Table 3) whichmay also limit microbial decomposition of old soil organiccarbon



Fig 2The responses of soil carbon and nutrient cycling to changes in litter inputs where MBC is microbial biomass carbon MBN microbialbiomass nitrogen DOC dissolved organic carbon DON dissolved organic nitrogen EIN extractable inorganic nitrogen and P is extractablephosphorus The relationship between the response ratio of each parameter and the quantity of litter inputs is shown in parentheses ldquo+rdquoindicates a significant positive linear correlation ldquominusrdquo indicates a significant negative linear correlation ns is non-significant ldquordquo indicatesan unknown relationship because of data limitations

Microbial biomass is the living microbial component ofthe soil and can be used as a bio-indicator for evaluating soilorganic matter turnover rates (Wardle 1992) We found thatthe RRs of MBC and soil respiration were positively corre-lated and MBC explained 66 of the variance in changes insoil respiration (Fig A3 in the Supplement) Litter manipu-lation also has the potential to affect root respiration by alter-ing root biomass however Sayer and Tanner (2010) showedthat the contribution of root respiration to total below-groundrespiration decreased with litter addition and the strong cor-relation between the Rrs of MBC and soil respiration in ouranalysis suggests that the changes in soil respiration underlitter manipulation are largely controlled by CO2 efflux fromheterotrophic respiration during litter decomposition

43 C cycling

431 DOC

We were only able to assess the responses of DOC to littermanipulation for temperate forests as there were insufficientavailable data for other ecosystems Our results suggestedthat although DOC of the litter layer responded substantiallyto litter manipulation DOC of the mineral soil was insensi-tive to litter manipulation This may be because DOC wasquickly mineralized by soil microbial communities (Kalbitz

et al 2003) or adsorbed by the soil mineral matrix associatedwith soil organic matter (Kaiser and Guggenberger 2000)Increasing fresh litter inputs could cause priming effects andlead to an increase in DOC flux from decomposing old soilC (Kalbitz et al 2007) However our analysis cannot dis-tinguish whether the increase of DOC in the litter layer wasa result of fresh litter inputs or the decomposition of old or-ganic C in forest floor due to the priming effect

432 Total C

The processes and mechanisms underlying the influence offresh litter inputs on soil C dynamics are complex involv-ing the balance between C input and output and the turnoveramong different C pools (Kuzyakov 2011) Additional in-puts of fresh litter can cause priming effects resulting in themineralization of older soil organic matter (Sulzman et al2005 Sayer et al 2007 2011) which leads to the specu-lation that soil C storage may decrease under elevated litterinputs (Kuzyakov 2010) Although there is ample evidencefor the occurrence of priming effects under controlled con-ditions the current lack of in situ studies of soil C releaseby priming precluded a quantitative analysis of the strengthof priming effects in this study However our results showedthat total C in the uppermost layers of the mineral soil gen-erally increased with litter inputs (Fig 1c) Hence greater



inputs of new organic C may compensate for the release ofolder soil organic C by priming (Talhelm et al 2009 Hof-mockel et al 2011 Kuzyakov 2011 Leff et al 2012) sug-gesting that the soil acts as a net C sink under increased litterinputs It remains to be seen if this ldquoreplacementrdquo of oldersoil C with fresh C from increased litter inputs will have con-sequences for soil carbon stability in the longer term

The response of soil C storage to litter manipulationdiffered among ecosystems (Table 2) (Sub-)tropical forestwas more responsive to changes in litter production thanother ecosystems and total C in the mineral soil showeda greater reduction under litter removal Field experimentsin (sub-)tropical forests have also demonstrated that soil Cpools responded rapidly to litter manipulation soil C con-centrations were significantly increased by 31 after only2 yr of litter addition in a tropical forest (Leff et al 2012)whereas there was often no detectable change in soil C intemperate forest or grassland even after 5 to 11 years of el-evated litter inputs (Nadelhoffer et al 2004 Baer and Blair2008 Talhelm et al 2009) There are several likely expla-nations for the notable responses of soil C in (sub-)tropicalforest to litter manipulation firstly the duration of litter-manipulation experiments in temperate or boreal biomes (2ndash8 yr Fisk and Fahey 2001 Froumlberg et al 2005 Sulzman etal 2005) was often shorter than the mean residence timeof the litter (4ndash16 yr) in those ecosystems (Olson 1963)whereas in (sub-)tropical forest most of the C from freshlitter is rapidly mineralized and respired or transferred to themineral soil Secondly the mean residence time of soil ag-gregate C in tropical forest is shorter than that in temperateforest (Six et al 2002b) and new C inputs can be integratedinto soil aggregates more rapidly especially as tropical soilsoften have a high clay content and C from leachate can bebound easily into soil aggregates (Six et al 2002a) Soil Cconcentration was therefore more likely to respond to alteredlitter inputs during the experimental period in (sub-)tropicalforest (Leff et al 2012)

In contrast to forest ecosystems soil C in grasslandsshowed no significant responses to litter-manipulation treat-ments Root-derived C is thought to be the main sourceof soil organic C in grasslands and below-ground litter isthought to be more important for C sequestration (eg Stein-beiss et al 2008) Further a growing number of studies haveshown that solar radiation especially ultraviolet radiationcan be the dominant driver of litter decomposition in aridand semiarid regions (Austin and Vivanco 2006 Brandt etal 2007) For example in a semi-arid steppe Austin and Vi-vanco (2006) found that around 60 of C lost from above-ground litter was due to photochemical mineralization re-sulting in a large proportion of litter-derived C being lost di-rectly to the atmosphere without entering the soil

We expected changes in total C in the mineral soil to be-come more pronounced with experimental duration How-ever there was no significant effect of experimental durationon the responses of soil C pools to litter manipulation (Ta-

ble A2 in the Supplement) This is possibly due to the highvariation in mean residence times of C and litter decomposi-tion rates among biomes (as discussed above) which makesit difficult to detect an effect of experimental duration whencomparing studies across ecosystems

44 Nutrient cycling

Litter is an important pathway for the transfer of nutrientsfrom plants to soil and the litter layer is critical for nutri-ent retention in some ecosystems Our results suggest thattotal N and C N ratios but not EIN in the mineral soil in-creased with litter inputs The lack of a clear overall responseof EIN to litter manipulation was probably due to the con-trasting responses of EIN in (sub-)tropical forest and temper-ate forest EIN in the mineral soil declined significantly in(sub-)tropical forest under litter removal but was unaffectedin temperate forest (Table 3) Litter inputs were shown tobe a significant source of EIN inputs to the mineral soil in(sub-)tropical forest (Sayer and Tanner 2010) where rapiddecomposition (Zhang et al 2008) and high rates of leach-ing from the soil result in rapid turnover of mineral N Con-sequently when litter was removed EIN in the mineral soildeclined significantly In contrast the lower concentrationsof EIN in temperate forests following litter addition may bea result of slower litter decomposition (Zhang et al 2008)and greater immobilization of N in the mineral soil

Plant growth in tropical forest is generally thought to beP-limited and litter is the dominant P source to soil (Cleve-land et al 2011) It is therefore not surprising to find that lit-ter removal reduced soil-extractable P in the mineral soil in(sub-)tropical forest (Table 3) Despite this soil-extractableP of the mineral layer did not increase under litter addition in(sub-)tropical forest For example Sayer and Tanner (2010)found that after a 5-year litter-manipulation study litter-derived P inputs significantly doubled under litter additionbut extractable P in the mineral soil did not change rela-tive to the controls This is probably due to direct uptake oflitter-derived P by plant roots before it enters the mineral soil(Stark and Jordan 1978 Herrera et al 1978 Attiwill andAdams 1993) in which case soil-extractable P would notincrease under litter addition (Sayer and Tanner 2010)

5 Conclusions

Global change alters not only the amount of above-groundlitter inputs to soil but also other factors regulating soil Ccycling It is very difficult to separate the contribution ofabove-ground litter inputs to soil C and nutrient cycling fromother drivers because the processes interact in complex waysLitter-manipulation experiments therefore provide valuableinformation to estimate the effects of litter production on soilorganic C formation (Sayer et al 2011)



With few exceptions the direction of the responses wassimilar among terrestrial ecosystems for the investigated pa-rameters even though the magnitude of the responses var-ied (Fig 1) Our meta-analysis of litter-manipulation exper-iments demonstrates that changes in total soil C and soilrespiration are positively related to above-ground litter in-puts suggesting that the soil acts as a C sink even thoughC turnover is accelerated under greater litter inputs (Fig 2)Compared to temperate forests (sub-)tropical forests weregenerally more sensitive to litter-manipulation treatmentsincreases or decreases in litter inputs could therefore sub-stantially alter the turnover and accumulation of soil C andnutrients in (sub-)tropical forests over a shorter time period(Leff et al 2012) The critical role of tropical forests in reg-ulating atmospheric CO2 has been widely recognized How-ever much attention has been focused on C stored in the veg-etation rather than tropical forest soils A number of studieshave demonstrated that global change has stimulated above-ground net primary production (ANPP) in tropical forestsover recent decades (Lewis et al 2009 Pan et al 2011) Al-though soil C accumulation may be lower than expected be-cause of C release by priming effects (Sayer et al 2011) wepredict that tropical forest soils will act as a C sink alongsidethe increased ANPP under global change (Table 2) Howeverthe current lack of long-term monitoring of tropical soil Cdynamics makes it difficult to determine the C sequestrationcapacity of tropical forest soils

Litter manipulation not only directly alters C and nutrientcycling by providing substrates to soil microbes it also in-directly changes C cycling by modifying soil physicochem-ical conditions (Sayer 2006) which is especially importantin grasslands (Wang et al 2011) Although litter additionhad little effect on soil C storage in grasslands above-groundlitter inputs appear to play a critical role in maintainingfavourable soil conditions by buffering soil temperature andmoisture

Altered litter production caused by environmental changecould lead to cascading effects on soil physicochemical prop-erties C dynamics and nutrient cycling (Fig 2) Overall ourstudy suggests that increases in litter inputs generally accel-erated the rates of below-ground biogeochemical reactionsand transformations whereas decreased litter inputs reducedthem The ensuring feedbacks between altered above-groundproductivity and changes in below-ground biogeochemicalprocesses need to be considered for predicting ecosystem re-sponses to global change

Supplementary material related to this article isavailable online athttpwwwbiogeosciencesnet1074232013bg-10-7423-2013-supplementzip

AcknowledgementsThis study was supported financially by theNational Natural Science Foundation of China (13263A1001)Chinese National Key Development Program for Basic Research(2013CB956304) and National 1000 Young Talents Program

Edited by R Conant

References

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Amatangelo K L Dukes J S and Field C B Responses ofa California annual grassland to litter manipulation J VegetatSci 19 605ndash612 doi1031702008-8-18415 2008

Attiwill P M and Adams M A Nutrient Cycling in Forests NewPhytologist 124 561ndash582 1993

Austin A T and Vivanco L Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation Nature 442555ndash558 doi101038nature05038 2006

Baer S G and Blair J M Grassland Establishment under VaryingResource Availability A Test of Positive and Negative FeedbackEcology 89 1859ndash1871 doi10189007-04171 2008

Brandt L A King J Y and Milchunas D G Effects of ultravio-let radiation on litter decomposition depend on precipitation andlitter chemistry in a shortgrass steppe ecosystem Glob ChangeBiol 13 2193ndash2205 doi101111j1365-2486200701428x2007

Chapin F S Matson P A and Vitousek P M Principles of Ter-restrial Ecosystem Ecology Springer doi101007978-1-4419-9504-9 2011

Cleveland C C Townsend A R and Schmidt S K PhosphorusLimitation of Microbial Processes in Moist Tropical Forests Ev-idence from Short-Term Laboratory Incubations and Field Stud-ies Ecosystems 5 680ndash691 doi101007s10021-002-0202-92002

Cleveland C C A R Townsend P Taylor S Alvarez-Clare MM C Bustamante G Chuyong S Z Dobrowski P GriersonK E Harms B Z Houlton A Marklein W Parton S PorderS C Reed C A Sierra W L Silver E V J Tanner and W RWieder Relationships among net primary productivity nutrientsand climate in tropical rain forest a pan-tropical analysis Ecol-ogy Lett 14 939ndash947 doi101111j1461-0248201101711x2011

Dzwonko Z and Gawronski S Effect of litter removal on speciesrichness and acidification of a mixed oak-pine woodland BiolConserv 106 389ndash398 doi101016S0006-3207(01)00266-X2002

Fisk M C and Fahey T Microbial biomass and nitrogen cy-cling responses to fertilization and litter removal in youngnorthern hardwood forests Biogeochemistry 53 201ndash223doi101023A1010693614196 2001

Fontaine S Bardoux G Abbadie L and Mariotti A Carboninput to soil may decrease soil carbon content Ecology Letters7 314ndash320 doi101111j1461-0248200400579x 2004

Fornara D A and Tilman D Soil carbon sequestration in prairiegrasslands increased by chronic nitrogen addition Ecology 932030ndash2036 doi10189012-02921 2012



Froumlberg M Kleja D B Bergkvist B Tipping E and MulderJ Dissolved Organic Carbon Leaching from a Coniferous ForestFloor ndash A Field Manipulation Experiment Biogeochemistry 75271-287 doi101007s10533-004-7585-y 2005

Fu R Gartlehner G Grant M Shamliyan T Sedrakyan AWilt T J Griffith L Oremus M Raina P Ismaila A San-taguida P Lau J and Trikalinos T A Conducting quantitativesynthesis when comparing medical interventions AHRQ and theEffective Health Care Program Journal of clinical epidemiology64 1187ndash1197 doi101016jjclinepi201008010 2011

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Hungate B A Jackson R B Field C B and III F S C De-tecting changes in soil carbon in CO2 enrichment experimentsPlant Soil 187 135ndash145 doi101007BF00017086 1996

Irving P M and Miller J E Productivity of field-grown soy-beans exposed to acid rain and sulfur dioxide alone and incombination Journal Name J Environ Qual 104 473ndash478doi102134jeq198100472425001000040010x 1981

Kaiser K and Guggenberger G The role of DOM sorptionto mineral surfaces in the preservation of organic matter insoils Organic Geochemistry 31 711ndash725 doi101016S0146-6380(00)00046-2 2000

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Kalbitz K Meyer A Yang R and Gerstberger P Response ofdissolved organic matter in the forest floor to long-term manipu-lation of litter and throughfall inputs Biogeochemistry 86 301ndash318 doi101007s10533-007-9161-8 2007

King J S Kubiske M E Pregitzer K S Hendrey G R Mc-Donald E P Giardina C P Quinn V S and Karnosky D FTropospheric O3 compromises net primary production in youngstands of trembling aspen paper birch and sugar maple in re-sponse to elevated atmospheric CO2 The New phytologist 168623ndash636 doi101111j1469-8137200501557x 2005

Kuzyakov Y Priming effects Interactions between living and deadorganic matter Soil Biology and Biochemistry 42 1363ndash1371doi101016jsoilbio201004003 2010

Kuzyakov Y Ecology Prime time for microbes Nature ClimateChange 1 295ndash297 doi101038nclimate1194 2011

Leff J W Wieder W R Taylor P G Townsend A R Ne-mergut D R Grandy A S and Cleveland C C Experimen-tal litterfall manipulation drives large and rapid changes in soilcarbon cycling in a wet tropical forest Glob Change Biol 182969ndash2979 doi101111j1365-2486201202749x 2012

Lewis S L Lloyd J Sitch S Mitchard E T A and LauranceW F Changing Ecology of Tropical Forests Evidence andDrivers Annual Review of Ecology Evolution and Systematics

40 529ndash549 doi101146annurevecolsys391107071733452009

Liu L King J S and Giardina C P Effects of elevatedconcentrations of atmospheric CO2 and tropospheric O3 onleaf litter production and chemistry in trembling aspen andpaper birch communities Tree Physiology 25 1511ndash1522doi101093treephys25121511 2005

Liu L King J S Booker F L Giardina C P Lee Allen Hand Hu S Enhanced litter input rather than changes in litterchemistry drive soil carbon and nitrogen cycles under elevatedCO2 a microcosm study Global Change Biology 15 441ndash453doi101111j13652486200801747x 2009

Lowman M D and Schowalter T D Plant science in forestcanopiesndashthe first 30 years of advances and challenges (1980ndash2010) The New phytologist 194 12ndash27 doi101111j1469-8137201204076x 2012

Nadelhoffer K J Boone R D Bowden R D Canary J DKaye J Micks P Ricca A Aitkenhead J A Lajtha Kand McDowell W H Litter and Root Influences on Forest SoilOrganic Matter Stocks and Function Oxford University PressChapter 15 2004

Olson J S Energy Storage and the Balance of Producers andDecomposers in Ecological Systems Ecology 44 322ndash331doi1023071932179 1963

Ostertag R Scatena F N and Silver W L Forest Floor Decom-position Following Hurricane Litter Inputs in Several Puerto Ri-can Forests Ecosystems 6 261ndash273 doi101007s10021-002-0203-8 2003

Pan Y Birdsey R A Fang J Houghton R Kauppi P E KurzW A Phillips O L Shvidenko A Lewis S L CanadellJ G Ciais P Jackson R B Pacala S W McGuire A DPiao S Rautiainen A Sitch S and Hayes D A large andpersistent carbon sink in the worldrsquos forests Science 333 988ndash993 doi101126science1201609 2011

Park J-H and Matzner E Detrital control on the releaseof dissolved organic nitrogen (DON) and dissolved inor-ganic nitrogen (DIN) from the forest floor under chronicN deposition Environmental Pollution 143 178ndash185doi101016jenvpol200510045 2006

Raich J W Russell A Kitayama K Parton W J and Vi-tousek P M Temperature influences carbon accumulation inmoist tropical forests Ecology 87 76ndash87 doi10189005-00232006

Sayer E J Using experimental manipulation to assess the roles ofleaf litter in the functioning of forest ecosystems Biol Rev 811ndash31 doi101017s1464793105006846 2006

Sayer E J and Tanner E V J A new approach to trenching ex-periments for measuring root-rhizosphere respiration in a low-land tropical forestSoil Biology and Biochemistry 42 347ndash3522010

Sayer E J and Tanner E V J Experimental investigation of theimportance of litterfall in lowland semi-evergreen tropical for-est nutrient cycling J Ecol 98 1052ndash1062 doi101111j1365-2745201001680x 2010

Sayer E J Powers J S and Tanner E V J Increased Litterfallin Tropical Forests Boosts the Transfer of Soil CO2 to the Atmo-sphere PLoS ONE 2 doi101371journalpone0001299g0022007



Sayer E J Heard M S Grant H K Marthews TR and Tanner E V J Soil carbon release enhancedby increased tropical forest litterfall Nature 1 304ndash307doi101038nclimate1190101038NCLIMATE1190 2011

Schmidt M W I Torn M S Abiven S Dittmar T Guggen-berger G Janssens I A Kleber M Kogel-Knabner ILehmann J Manning D A C Nannipieri P Rasse DP Weiner S and Trumbore S E Persistence of soil or-ganic matter as an ecosystem property Nature 478 49ndash56doi101038nature10386 2011

Six J Conant R T Paul E A and Paustian K Sta-bilization mechanisms of soil organic matter Implicationsfor C-saturation of soils Plant and Soil 241 155ndash176doi101023A1016125726789 2002a

Six J Feller C Denef K Ogle S M de Moraes J C andAlbrecht A Soil organic matter biota and aggregation in tem-perate and tropical soils - Effects of no-tillage Agronomie 22755ndash775 doi101051agro2002043 2002b

Stark N M and C F Jordan Nutrient retention by rootmat of an Amazonian rain-forest Ecology 59 434ndash437doi1023071936571 1978

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Sulzman E W Brant J B Bowden R D and Lajtha K Contri-bution of aboveground litter belowground litter and rhizosphererespiration to total soil CO2 efflux in an old growth coniferousforest Biogeochemistry 73 231ndash256 doi101007s10533-004-7314-6 2005

Talhelm A F Pregitzer K S and Zak D R Species-specific re-sponses to atmospheric carbon dioxide and tropospheric ozonemediate changes in soil carbon Ecol Lett 12 1219ndash1228doi101111j1461-0248200901380x 2009

Tian D L Peng Y Y Yan W D Fang X Kang W XWang G J and Chen X Y Effects of Thinning and LitterFall Removal on Fine Root Production and Soil Organic CarbonContent in Masson Pine Plantations Pedosphere 20 486ndash493doi101016S1002-0160(10)60038-0 2010

Wang J Zhao M Willms W D Han G Wang Z and BaiY Can plant litter affect net primary production of a typi-cal steppe in Inner Mongolia J Vegetat Sci 22 367ndash376doi101111j1654-1103201101257x 2011

Wardle D A A comparative assessment of factors which influencemicrobial biomass carbon and nitrogen levels in soil Biol Rev67 321ndash358 doi101111j1469-185X1992tb00728x 1992

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Wieder R K and Wright S J Tropical Forest Litter Dynamicsand Dry Season Irrigation on Barro Colorado Island PanamaEcology 76 1971ndash1979 doi1023071940727 1995

Xia J and Wan S Global response patterns of terrestrial plantspecies to nitrogen addition New Phytologist 179 428ndash439doi101111j1469-8137200802488x 2008

Zhang D Hui D Luo Y and Zhou G Rates of litter decom-position in terrestrial ecosystems global patterns and controllingfactors J Plant Ecol 1 85ndash93 doi101093jpertn002 2008

Zhao M and Running S W Drought-induced reduction in globalterrestrial net primary production from 2000 through 2009 Sci-ence 329 940ndash943 doi101126science1192666 2010



increase in above-ground litterfall after hurricanes or severestorms (Ostertag et al 2003) or rapid loss of the litter layerafter wildfires (Wardle et al 2003)

As one of the most important pathways for carbon and nu-trient fluxes to the soil changes in above-ground litter in-puts could lead to cascading effects on below-ground bio-geochemical processes Although changes in above-groundlitter inputs have been observed in numerous global changemanipulation experiments in multiple ecosystems most ofthose studies found that total soil carbon (C) content was gen-erally unchanged (Baer and Blair 2008 Talhelm et al 2009but see Fornara and Tilman 2012) Several mechanisms havebeen proposed to explain why soil C showed small changeseven when above-ground litter inputs were greatly alteredOne possible explanation is that soil C may be altered by lit-ter inputs but the changes are too small to be detected in thelarge and heterogeneous soil C pool (Hungate et al 1996)A change in the strength of priming effects is another pos-sible reason high litter inputs could cause greater primingeffects and the increase in new soil C derived from the littermay be offset by the decomposition of older soil C (Sayeret al 2011) Soil carbon saturation is also a plausible the-ory to explain the lack of changes in soil C content in re-sponse to varying amounts of litter inputs The theory statesthat the capacity of soil to stabilize organic C has an upperlimit which is determined by soil physical chemical and bio-chemical characteristics (Six et al 2002a)

An emerging view suggests that the majority of organic Cin soil is derived from rhizodeposition and above-ground lit-ter inputs have a limited influence on soil C storage (Schmidtet al 2011) The lack of changes in total soil C has sparked adebate about whether alterations in above-ground litter pro-duction as a result of global change will impact upon long-term soil C storage Quantification of the contribution ofabove-ground litter to soil C sequestration and nutrient cy-cling is therefore needed to assess the importance of above-ground litter in below-ground processes

A large number of litter-manipulation studies in whichlitter is experimentally added or removed have been con-ducted since the first experiments in the 1850s (Sayer 2006)The information we draw from this rich research history oflitter-manipulation experiments could help us better under-stand how above-ground litter inputs affect below-groundprocesses and therefore the mechanisms regulating the po-tential of soil to sequester additional C Below-ground pro-cesses such as soil respiration microbial activity and soil Cformation are simultaneously influenced by litter inputs andthese processes intrinsically interact with each other (Chapinet al 2011) To assess the consequences of above-groundlitter inputs on soil physical chemical and biochemical pro-cesses quantitatively we conducted a meta-analysis of litter-manipulation experiments from multiple terrestrial ecosys-tems Changes in litter productivity may be accompanied bychanges in litter biochemistry and the two interact with eachother in biogeochemical processes (Liu et al 2009) How-

ever because of the limited data availability on changes in lit-ter biochemistry this study focused on the impact of changesin litter productivity on below-ground C and nutrient cycling

2 Methods

21 Data selection

A comprehensive literature search covering relevant peer-reviewed articles and dissertations from 1950 to 2013 wasconducted using the databases of Web of Science and Pro-Quest We also cross-checked the references of the relevantarticles to identify other potential book chapters and peer-reviewed reports Only data from in situ litter-manipulationexperiments were included in our data set Studies were ex-cluded if they were conducted in a controlled laboratory set-ting (eg Liu et al 2009) or the additional carbon was sup-plied as specific organic compounds such as glucose (egPark and Matzner 2006) or cellulose (eg Fontaine et al2004) When data from the same site and treatments werepresented in multiple publications we used the data from themost recent publications Similarly when data from multipleyears were given we only used data from the most recentyear Litter-manipulation experimental sites were from mul-tiple climatic zones including arctic boreal temperate and(sub-)tropical regions (See details in Appendix B in the Sup-plement) Here ldquo(sub-)tropicalrdquo includes both tropical andsub-tropical regions

Four categories of data related to below-ground biotic andabiotic properties were extracted from the literature of insitu litter-manipulation experiments (1) soil surface physi-cal and chemical properties including soil temperature soilmoisture and soil pH (2) microbial responses including mi-crobial biomass carbon (MBC) and microbial biomass nitro-gen (MBN) (3) C fluxes and pools including soil respira-tion total carbon (C) and dissolved organic carbon (DOC)and (4) nutrient fluxes and pools including total nitrogen(N) C N ratios (C N) dissolved organic nitrogen (DON)extractable inorganic nitrogen (EIN) and extractable phos-phorus (P) Where relevant we distinguished between lit-ter layer and mineral soil for all variables but results wereonly reported where sufficient data points were available (ien ge 4) In our data set litter layer data were all from studiesconducted in boreal forest or temperate forest and includedforest floor (Fisk and Fahey 2001) O horizon (Nadelhofferet al 2004) O layer (Froumlberg et al 2005) epihumus sub-horizon (Dzwonko and Gawronski 2002) and humus hori-zon (Dzwonko and Gawronski 2002) Mineral soil in ourdata set is defined as the soil beneath the litter layer or thetopsoil from ecosystems such as grassland and (sub-)tropicalforest The sampling depth in the mineral soil ranged from1 to 40 cm with the majority of the data collected in thetop 10 cm When measurements were conducted in differ-ent mineral soil depths we only used the data reported for











22 Meta-analysis


(XtXc





3 Results






















33 C cycling




















34 Nutrient cycling







4 Discussion















43 C cycling

431 DOC



432 Total C









44 Nutrient cycling



5 Conclusions









Edited by R Conant

References









































































22 Meta-analysis


(XtXc





3 Results






















33 C cycling




















34 Nutrient cycling







4 Discussion















43 C cycling

431 DOC



432 Total C









44 Nutrient cycling



5 Conclusions









Edited by R Conant

References

















































































33 C cycling




















34 Nutrient cycling







4 Discussion















43 C cycling

431 DOC



432 Total C









44 Nutrient cycling



5 Conclusions









Edited by R Conant

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34 Nutrient cycling







4 Discussion















43 C cycling

431 DOC



432 Total C









44 Nutrient cycling



5 Conclusions









Edited by R Conant

References










































































43 C cycling

431 DOC



432 Total C









44 Nutrient cycling



5 Conclusions









Edited by R Conant

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44 Nutrient cycling



5 Conclusions









Edited by R Conant

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Edited by R Conant

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