Contribution of roots and amendments to (1)

Contribution

of

roots

and

amendments

to

soil

carbon

accumulation

within

the

soil

pro

_fi

le

in

a

long-term

_fi

eld

experiment

in

Sweden

Lorenzo

Menichetti

a,

_*

_,

_Alf

_Ekblad

b

_,

_Thomas

_Kätterer

c

a_{SwedishUniversityofAgriculturalSciences,DepartmentofSoilandEnvironment,P.O.Box7014,Uppsala75007,Sweden} b_{SchoolofScienceandTechnology,ÖrebroUniversity,Örebro70182,Sweden}

c_{SwedishUniversityofAgriculturalSciences,DepartmentofEcology,P.O.Box7044,Uppsala75007,Sweden}

ARTICLE INFO

Articlehistory:

Received5December2013

Receivedinrevisedform30October2014 Accepted3November2014

Availableonlinexxx

Keywords: d13_C

Roothumification SOC

Subsoil Topsoil

ABSTRACT

ThecontributionofdifferentCinputstoorganiccarbonaccumulationwithinthesoilprofileintheUltuna

long-termcontinuoussoilorganicmatterexperiment,establishedin1956,wasdetermined.Until1999,

C3-cropsweregrownatthesite,butsincethenmaize(C4)hasbeentheonlycrop.Theeffectofatotalof

10differentinorganicnitrogenandorganicamendmenttreatments(4MgCha 1_yr 1_{)onSOCintopsoil}

andsubsoilafter53yearswasevaluatedandthecontributionfrommaizerootstoSOCafter10yearsof

cultivationwasestimated.

Soilorganiccarbon(SOC)andd13Csignatureweremeasureddownto50cmdepth.TheCcontentinthe

topsoil(0–20cmdepth)was1.5%atthestartoftheexperiment.After53yearsoftreatments,theaverage

topsoilCcontentvariedbetween0.9and3.8%ofsoildryweight,withtheopenfallowhavingthelowest

andthepeatamendedthehighestvalue.NitrogenseemedtopromoteCaccumulationinthetopsoil

treatmenteffectsweresmallerbelow20cmdepthandonlytwooftheamendments(peatandsewage

sludge)significantlyaffectedSOCcontentdownto35cmdepth.Despitethis,penetrometer

measure-mentsshowedsignificanttreatmentdifferencesofcompactionbelow41cmdepth,andalthoughwe

couldnotexplainthesedifferencesthispresentedsomeevidenceofaninitialtreatment-inducedsubsoil

differentiation.Tenyearsofmaizegrowthaffectedthed13_{CofSOCdownto22.5cmdepth,whereitvaried}

between 25.16and 26.33(m), andanisotopic massbalance calculation suggestedthat maizeC

accountedfor4–8%oftotalSOCinthetopsoil.Untillessthan2500yearsagothesitewasapost-glacialsea

floorandthe14_{CdatasuggestthatmarinesedimentCstilldominatestheSOCindeepersoillayers.}

Overall,theresultssuggestthat53yearsoftreatmentshascauseddramaticchangesonthestoredCinthe

topsoilinseveralofthetreatments,whilethechangesinthesubsoilismuchlessdramaticandasmallC

accumulationintheuppersubsoilwasfoundintwoofthetreatments.

The contribution from roots to SOC accumulation was generally equal to or greater than the

contributionfromamendments.Theretentioncoefficientofroot-derivedCinthetopsoilwasonaverage

0.300.09,whichishigher thanusuallyreportedintheliteratureforplantresiduesbutconfirms

previousfindingsforthesameexperimentusinganotherapproach.Thisstrengthenstheconclusionthat

root-derivedSOCcontributedmoretoSOCthanabove-groundcropresidues.

ã 2014ElsevierB.V.Allrightsreserved.

1.Introduction

The globalcarbon (C) sinkis expected toincrease with the increasein primaryproductivity drivenbyhighertemperatures andCO2concentrations(Kirschbaum,2000).However,a

simulta-neousincreaseinemissionsduetolandusechangeoranincrease insoilrespirationafteranincreaseintemperaturemaycounteract almostallthispositiveeffect(Eglinetal.,2010).Morethan

one-third(37%)ofgloballandisusedinagricultureand10%ofglobal landisunderannualcrops(FAOStatisticalDatabase,2013), and thereforeexposedtoquickchangesinmanagement.On agricul-turalland,therangeofpossibleinterventionsforclimatechange mitigation is constrained by the global requirements for food production (Powell and Lenton, 2012), but C sequestration in agriculturalsoilscanbeincreasedthrough changesin manage-mentpractices(Lal,2004;Kättereretal.,2013;Stockmannetal., 2013).

MostpreviousstudiesonCsequestrationhavefocusedonthe topsoil(e.g.,LorenzandLal,2005).Althoughthisis understand-able, since the concentrationsand turnoverof SOC are usually

* Correspondingauthor.Tel.:+46768549268;fax:+4618673156.

E-mailaddress:Lorenzo.Menichetti@slu.se(L.Menichetti).

http://dx.doi.org/10.1016/j.agee.2014.11.003

0167-8809/ã 2014ElsevierB.V.Allrightsreserved.

–

ContentslistsavailableatScienceDirect

Agriculture,

Ecosystems

and

Environment

higherin topsoilsthanin subsoils,globallysubsoilsstoremore than half of total SOC (Jobbagy and Jackson, 2000) and are thereforepotentiallyimportantforC sequestrationstrategies.A fewstudieshavebeenconducted(e.g.,Pauletal.,1997;Jobbagy andJackson,2000;BirdandKracht,2003;JenkinsonandColeman, 2008;Jenkinsonetal.,2008;Kirchmannetal.,2013),butthereis stillrelativelylargeuncertaintyregardingCindeepersoillayers (LorenzandLal,2005).Areasofuncertaintyincludetheresponse tochangesinmanagementofalargepartoftheCstoredinsoil (Poeplauet al., 2011) and itsstabilizationmechanisms (Chabbi etal.,2009),makingprecisequantitativepredictionsdif_ficult.Itis therefore important to obtain accurate information on the reactivity of different C pools to management changes and on howdifferentCinputscontributetotheformationofSOCoverthe wholesoilpro_file.Inparticular,thecontributionofroot-derived materialcanbeparticularlyrelevantforSOCaccumulationbecause of thepossible associatedprotection mechanisms(Rasseet al., 2005), and might so far be underestimated in the literature (Kättereretal.,2011).

During recent decades, long-term experiments have been shownto produceinformation on theformation of SOC stocks fromdifferentorganicCinputs(Kättereretal.,2011)andonhow theolderSOCdecays(Barréetal.,2010).Thereforeinthisstudywe useddatafromtheUltunalong-term_fieldexperiment,established inSwedenin1956andmanagedsincethenbyadditionsofafixed amountofCintheformofdifferentamendments,incombination withorwithoutnitrogen(N)fertilization.

Theaimofthestudywastoexaminethefollowingquestions: (1)HowhaveNfertilizationandadditionoforganicamendments affectedSOCaccumulation intopsoil and subsoilin theUltuna long-term_fieldexperiment?and(2)HowhaveCinputsfromroots contributedtoSOCformationandaccumulation?

To determine how management practices affect soil C accumulationor release in agricultural topsoil and subsoil, we analyzed soil cores from 0 to 50cm depth in 10 different experimentaltreatments.Thisallowed ustoevaluatetheeffect ofdifferentCinputsonSOCstocksonthewholesoilpro_filesince thestartoftheexperimentin1956.

Toconsidertheinputsfromroots,webasedourinvestigationon thefactthatthecropscultivatedintheexperimentshiftedfromC3

to C4 crops in the year 2000. Several authors have tried to

determinethe decayof SOCof differentages byexploitingthe natural difference in 13_{C content of plants with C}

3 and C4

photosynthetic cycles (Wynn and Bird, 2007; Blagodatskaya etal.,2011).Theanalysisof

d

13Csignaturesdownthewholesoil pro_fileallowedustoquantifythecontributiontoSOCfrommaize rhizodepositiontoadepthof50cmover10yearsinthedifferent experimental treatments. We then determined the retention coefficient ofthenewly formedmaterialfor each experimental treatment,investigatingthein_fluenceofrhizodepositiononSOC accumulationanddecayatdifferentdepths.

2.Materialandmethods

2.1.Studysiteandtreatments

Thelong-term_fieldexperimentislocatedinUltuna,closeto Uppsala (59.82_{N, 17.65E), in a Dfb climate (warm summer}

hemiboreal) accordingto theKöppen classification (Peel et al., 2007), with mean annual precipitation of 570mm and mean annualairtemperatureof+5.4_{C.Thetopsoil(0–20cm)isaclay}

loamwith36.5% clay,41% silt(0.002–0.06mm)and22.5% sand

(0.06–2mm)andisclassifiedasaEutricCambisol(IUSSWorking Group,2007).The parentmaterialconsistsofpost-glacial sedi-mentsandilliteisthemainclaymineral(Gerzabeketal.,1997).In 1956,thetopsoilhadanorganicCcontentof1.5%,anNcontentof 0.17%andapHof6.6.Thesitehasbeeninagriculturaluseforat least300years.Sincethestartoftheexperiment,theplotshave beencultivatedmanually.

From1956to1999,annualC3cropssuchasoats,springbarley,

sugar beet, oilseedrape, turnip rape and white mustard were cultivated.Prior to2000, cultivatedplants had anaverage

d

13_C signatureof 28.00.1

m

(Menichetti etal., 2013).In 2000, C₃

crops were replaced with forage maize, a plant with a C4

photosyntheticcycleandanaverage

d

13_{Csignatureof 12.30.1}

m

(Menichettietal.,2013).

Theexperimentaldesignconsistedof15treatmentswithfour replicateplotsinarandomizedblockdesign.Eachplotis2m2m,

separated by 40cm high steel frames extending toa depth of 30cm.ApproximatelythesameamountofC(4Mgha 1)isadded

in10ofthetreatmentsinautumneverysecondyearasdifferent organic amendments (Table 1). Inorganic N fertilizer is added annually during spring at a rate of 80kgNha 1_yr 1 _{in the}

N-fertilizedtreatments.Theexperimentalsoincludesatreatment thatreceivesneitherNfertilizernororganicamendmentsanda barefallowtreatmentthatiskeptfreefromvegetationbyregular weeding.Allplotsarefertilizedannuallywith22kgPand35–38kg

Kha 1_{.Above-ground biomassis harvested bycutting thecrop}

closetothesoilsurface,andbothgrainandabove-groundyields arerecordedeachyear.Asamplearchive,managedtogetherwith theexperiment,storessamplesfromtopsoil,plantmaterialsand amendments takeneverysecondyearsince1983 and intermit-tentlybetween1956and1982.Fromthe15treatments,thesubset of10selectedforthisstudycoveredthewholerangeofSOCquality intheexperiment(Table1).

2.2.Samplingandanalysis

SoilsamplingwascarriedoutaftercropharvestinSeptember 2009. Samples were taken with an auger at increasing depth intervals of: 0–15,15–17.5,17.5–20, 20–22.5, 22.5–25, 25–27.5,

27.5–30,30–35,35–40and40–50cm.Thesesamplingdepthshave

beenchosentoincreasetheresolutionintheintervalswithmore

Table1

Topsoil(0–20cm)characteristics(meanswithstandarderrors;n=4)oftreatmentsA–OintheUltunalong-termexperimentstudiedhere.

Treatment Crop Fertilisera

expected variation. Since the soil is ploughed to 20cm, we expectedthatvariationwouldbethehighestinthetransitionzone betweentop-and subsoil,between15and30cm depth.Dueto limitationsintheamountofsubsoilthatcanbetakenfromthe smallplotsinthisexperiment,onlyonecore(diameter2cm)per plot was taken below 20cm depth. This was extractedwithin 40cmfromtwobordersofeachplot.Thetopsoil(0–20cm)was

sampledaccordingtostandardproceduresat_fivelocationswithin eachplot.Thesampleswerebroughttoacoolroomwithin2hof samplingandstoredat5_{Cforaperiodthatvariedbetweentwo}

weeksandonemonthuntilfurthertreatment. Fromeachdepth interval, a sample of 10–15g was taken by aggregating five subsamples(2–3geach)takenatapproximately1-cmintervals.

Thebulksampleswerethendriedat105_{Cfor12h,homogenized}

and milled toa powder withan agate mortarand pestle, and subsequentlyanalyzed.

TopsoilC and Nwere analyzedusing anelemental analyzer (Leco CN-2000, St. Joseph, Michigan) and

d

13C signature was determined with an elemental analyzer (model EuroEA3024; Eurovector, Milan, Italy) coupled online to a continuous flow Isoprime isotope-ratio mass spectrometer (GV Instruments; Manchester,UK)atÖrebroIsotopeLaboratory.Soilsamplesfrom 1956and1999andsamplesoftheamendmentsusedfrom1975to 2009weretakenfromthehistoricalarchiveandalsoanalyzedfor totalCand

d

13C.Theresulting

d

13Cvalueswereexpressedinparts perthousand(

m

)relativetotheinternationalstandardofVienna

PeeDee Belemnite(V-PDB),where

d

13C=1000(Rsample R stan-dard)/Rstandard

m

andRistheratioof13C_–12C.The

d

13Cvalueofthe

amendmentswasmeasuredbyaveragingdirectmeasurementsof samplesfrom1975,1979,1989,1993,1995and2005.Theresulting

d

13_{Cvaluesforfarmyardmanure,greenmanureandsewagesludge} were 27.91.2, 27.40.9and 25.70.1

m

,respectively.The

d

13Cvalueforpeat( 25.60.4

m

)wastakenfromGerzabeketal. (1997).

Thephysicalconditionsinthesoilpro_filewerecharacterizedby measuring penetration resistance in September 2009 down to

45cm depthwith an Eijkelkamppenetrologger(model P1.52,

Eijkelkamp, Arnhem, Netherlands). Each penetration pro_file representedtheaverageoffourplots,with7repeated measure-mentswithineachplotfora totalof28 measurementsforeach penetrationpro_file.

Soilcoresfromaprevioussamplingperformedin1997inthe unamended,fertilizedtreatmentbyBergkvistetal.(2003) were analyzed for 14_{C content. The analyses were performed after}

bulkingthefourreplicatesintotwoseparatesamples.Theresults of 14_{C analyses were expressed as per cent of modern carbon}

(pMC),wherepMC=Asn/Aon(1/8267(Y 1950))% and Asn

rep-resents the particle count per minutes of the sample, Aon

represents the particle count per minutes of the international standardandYreferstotheyearofsampling.Thevalue8267refers tothe average life of 14_{C relative tothis particular convention}

(Stuiver and Polach,1977). Conventional radiocarbon age was calculatedaccordingtotheformula CRA= 8033ln(Asn/Aon)and

expressedinyearsbeforepresent(BP).Thevalue8033referstothe averagelifeof14_{Crelativetothisparticularconvention(Stuiverand} Polach,1977).

2.3.Datatreatmentandanalysis

DataanalysiswasperformedwithintheRstatistical environ-ment(RDevelopmentCoreTeam,2011).Basedonthe penetrome-terdata, three depthzoneswereidentified: (a)the topsoil(0–

20cm),wheretillageandfertilizationtakesplace,(b)theinterface between topsoil and subsoil (20–25cm) just below ploughing

depthand(c)theuppersubsoil(25–50cm).

TreatmenteffectsweretestedwithANOVAindependentlyfor eachdepthintervalandalsoaggregated(byweightedaverage)by depth zones, followed then by a Tukey’s Honestly Significant Difference (HSD)test. Correlationswereexpressed asPearson’s

correlationcoef_ficient.

Thetopsoilinthestrawtreatment(F)hasbeenroughlyinC equilibriumsincethestartoftheexperiment,havinggainedonlya 0.03% C or 0.26gCcm 3 _{over the last 56 years. We therefore}

considereditasareferenceandassumedthedifferencebetween the SOC profile in the straw treatment and that in the other treatmentstorepresenttheCgainsorlossesduetothedifferent treatments.

Allthepast profile data,takensometimesduringthewhole historyoftheexperimentatdifferentdepthintervalsfordifferent analyses, werereconciledbylinear interpolationwithavertical resolutionof1cminordertohavecompatiblemeasurementsfor subsequentcalculations.Thebulkdensityprofilesfor2009were estimatedbylinearinterpolationbetweenthetopsoilbulkdensity in2009(Kättereretal.,2011)andthebulkdensityvaluesdownto 100cmdepthtakenin1956.Changesinbulkdensityinthesubsoil sincethestartoftheexperimentwereassumedtobenegligible. ThecalculationsofmaizecontributionstoSOCwereperformed accordingtotheconceptofequivalentsoildepth(Kättereretal., 2011),inordertoaccountfortherelativechangeinsoildensityand theconsequentchangeinthetilledpartofthesoil.Thischangewas assumedtoinfluencethepreferentialrootingzonesoapplication oftheequivalentsoildepth,ascalculatedforthesameexperiment byKättereretal.(2011),wasthereforeparticularlyrelevantinthis study. Fromtherelativecontributionof maizetoSOC,we then estimatedtheamountofmaterialaddedbymaizecultivationover 10 years and its retention coef_ficient, which is de_fined as the amountofCfrommaize-derivedinputsthatis retainedasSOC. Sinceallabove-groundcropresiduesareremovedfromthefieldin ourexperiment,theretentioncoef_ficientreferstocarbonderived from below-groundbiomass which was estimated from above-ground yields usinglinear allometric functions(Bolinderet al., 2007).

The contribution from maize to total SOC was estimated according to Bayesian principles. We utilized a Monte Carlo MarkovChain(MCMC)searchalgorithmrunningthemodelinR through the JAGS sampler (Plummer, 2003), which utilizes a formallog-likelihoodcostfunction.Wechoseforthetargetvaluea prior distribution generated as uniform distributionin a range between0and0.3forthemaizeproportion,andforthemeasured valuesanormaldistributioncentredonthemeanofthevalueand withstandarddeviationcorrespondingtothemeasurederrorof

d

13Csignatures.Themodelwascalibratedbasedonfourchainsof 350,000runseach.TheapplicationofBayesianprinciplesallowed us to calibrate the desired value propagating the known uncertainty involved in the calculation, and to estimate a probability distributionfor each value.Thistechnique becomes particularlyusefulwhenworkingwithsmallisotopeintervalsfor which errors need to be quanti_fied precisely to determine significant differences, since it gives much more detailed information on the probability distribution of the results and permitsmore rigorouserrorpropagationcomparedwith deter-ministictechniques(Parnelletal.,2010).

Aprincipalcomponentanalysis(PCA)hasbeenrun(Venables andRipley,2002)onourdatasetincombinationwithdatafrom

Börjessonetal.(2011),inordertoassessthepossiblecorrelations betweenSOCmineralizationandmicrobialecology.

2.3.1.EstimationofmaizerootcontributiontoSOC

The contributions of C3 and C4 material to total SOC were

calculatedaccordingtothefollowingmass-balanceequation:

p_C4¼

d

13_C

2009

d

13C1999

d

13Cmaize

d

13C1999

(1)

wherep_C4 represents theproportionfromC4 toC,

d

13C2009the

measured

d

13Csignaturein2009,

d

13C1999thesignatureofSOCin

1999and

d

13Cmaizetheaverageisotopicsignatureof maize.The

d

13C signature change due to addition of amendments was consideredbycalculatingtheannual

d

13C changeinSOCdue to amendments for each treatment from 1956 to 1999, and extrapolatingthelineartrendtotheperiod2000–2009toestimate

thetheoretical

d

13C signatureof SOC in 2009 thatwould have resultedifonlyC3plantshadbeengrown.Thisvaluewasthenused

tocalculatetheproportionduetomaizeinordertoaccountforthe effectofamendments.

3.Results

3.1.SOC,Nand14_{Cintopsoilandsubsoil}

**ThehighestCstocks(Table3)werefoundinthepeat(M)and sludge(O)treatments,whichcontained87.0and74.7MgCha 1_,

respectively.ThisismorethanthreetimesthetotalCofthebare fallow(A),whichamountedto23.6MgCha 1_{andabouttwicethat}

of the straw (F)treatment, which amounted to38.7MgCha 1_.

Nitrogenfertilizationpositivelyin_fluenced SOCaccumulationin thetopsoil,aneffectwhichwasparticularlyclearwhencomparing thetreatmentsthatdifferedonlyinNfertilization.TheN+straw treatment(G)hadmoreSOCthanthestraw(F)treatmentandthe +N treatment (C) had more SOC than the control (B). Butthe accumulation of C4-derived SOC didnot showany statistically

significantdifferencebetweenthe+Nandthe+strawtreatment (Table2).

Significant increases between 25 and 35cm were generally recorded in the richer amended treatments (peat and sludge), whichdisplayedanincreaserelativetothestrawtreatmentofon average0.27gCcm 3_{(andrangedbetween0.12and0.31gCcm} 3_),

whiletheCcontentofthenon-amendedtreatments(A,BandC)

was notsigni_ficantlydifferentfromthestrawtreatmentatthis depth.TreatmentdifferencesinCcontentweresignificantdownto 35cmdepthwhentestedbycomparingvaluesaggregatedbylayer withanANOVA(TableA1).ConsideringCvariationinthesubsoil (depthbelow25cm)asalinearfunctionofdepth,treatmentsM (peat)andO(sewagesludge)showedsignificantly(C.I.95%)higher C contents below this depth than in the straw treatment. Interestingly,whentestedwithANOVAthepenetrometervalues showed significanttreatmentdifferences onlyabove 13cm and below41cm(Fig.2).

TocalculatetheSOCchangesinducedbythetreatmentsover thewholesoilprofile,weconsideredthestrawtreatment,whichis roughlyinCequilibrium,aszero(Table3).Relativetothestraw treatment, alltheamendmenttreatmentsgained SOC(between 0.26 and 2.27gCcm 3 _{in the 0}

–15cm range), while the

non-amendedtreatmentslostSOC(between0.42and0.85gCcm 3_in

the0–15cmrange).

TheNcontentofthesoilprofilesfollowedtheSOCcontent.The

d

15N signature in the topsoil varied according to the different fertilizationregimes,butwithnoapparentrelationshipwithtotal SOC and probably depending mainly on the signature of the fertilizer and amendments added. Treatment differences were signi_ficant in the topsoil but no signi_ficant differences were detectedinthesubsoil.

Theanalysisof14_{Ccontentwithinthepro}

fileoftheunamended fertilizedplots(treatmentC)in1997revealedasteepdeclinein moderncarbonthroughouttheprofile,whichfollowedanonlinear decreasefromaround100%inthetopsoiltoaround40%at70cm depth(Fig.3).TheaverageSOCagebetween40and60cmdepth was 1759296 years BP, while below 60cm depth it was

7747100yearsBP.

3.2.ContributionfrommaizetoSOC

Signi_ficanteffectsoftreatmentsonsoilorganic

d

13Csignature were foundto 22.5cm depth accordingto analysisof variance (Fig.4).Whencomparingthe

d

13_{Csignatureofthetopsoil(after} aggregating the data by weightedaverage for the three depth

Table3

Differences(gcm3_{)insoilorganiccarbonstocksrelativetotreatmentF.FortreatmentcodesA}

–OseeTable1.

Treatment Soildepth(cm)

0–15 15–17.5 17.5–20 20–22.5 22.5–25 25–27.5 27.5–30 30–35 35–40 40–50

A 0.850.20 0.500.07 0.360.06 0.150.02 0.150.01 0.020.00. 0.050.01 0.000.00 0.030.01 0.050.02

B 0.390.13 0.320.10 0.270.09 0.100.03 0.090.02 0.020.00 0.020.00 0.040.01 0.050.01 0.180.03

C 0.420.08 0.120.01 0.160.01 0.170.02 0.110.01 0.000.00 0.050.01 0.100.02 0.210.06 0.220.08

G 0.260.05 0.20.04 0.170.03 0.100.02 0.050.01 0.130.03 0.320.09 0.150.04 0.010.01 0.110.06

H 0.280.03 0.180.02 0.210.01 0.070.01 0.050.01 0.120.02 0.050.00 0.140.03 0.300.10 0.320.12

J 0.790.10 0.580.13 0.520.16 0.420.12 0.190.05 0.160.03 0.220.05 0.260.07 0.150.03 0.080.02

M 2.270.10 1.740.33 1.160.41 0.470.09 0.120.01 0.170.01 0.280.02 0.260.03 0.320.05 0.390.04

B 0.500.09 0.000.00 0.060.01 0.020.00 0.010.00 0.030.00 0.120.02 0.050.01 0.020.01 0.060.02

O 0.970.15 0.770.16 0.580.08 0.400.06 0.310.06 0.310.05 0.210.02 0.130.02 0.100.02 0.120.03

Table2

Soilcarbonstocks,contributionsfrommaizeandmaizeretentioncoefficientsinthetopsoil(consideringequivalentsoildepthsaccordingtoKättereretal.,2011)with standarderrorsofthemean(n=4).FortreatmentcodesA–OseeTable1.

Cstocks,Mgha 1 _C

4(%) Maizestocks,Mgha 1 Maizeretentioncoefficient

A 23.584.59 NA NA NA

B 27.28.27 0.070.01 1.880.57 0.41

C 32.476.47 0.080.02 2.450.49 0.25

F 38.685.34 0.070.03 2.610.36 0.46

G 48.5512.82 0.050.02 2.410.64 0.21

H 41.568.34 0.070.03 2.940.59 0.34

J 56.128.38 0.050.03 2.910.44 0.26

M 86.9817.26 0.040.02 3.660.73 0.26

N 45.3510.29 0.050.02 2.250.51 0.21

O 74.7211.7 0.050.02 3.840.60 0.29

zones), a Tukey’s HSD test found significant differences only betweenamendedandnon-amendedtreatments(TableA2).

Thecontributionof10yearsofmaize cultivationtothe

d

13C signatureof SOCin thetopsoil, calculatedaccordingtoEq. (1), varied between 4 and 8% of total SOC. The non-amended treatmentsdisplayedless uncertainvalues,probablydue tothe factthattherewerenoamendmentscontributingtotheCstock.In treatmentswithhighCaccumulation(M,N,OandJ,seeTable1), therelativecontributionofmaizewasbetween4and5%oftotal SOC,althoughtheabsoluteCstocksofmaizeoriginwereingeneral higherthanintreatmentswithlowerCaccumulationrangingfrom 2.41Mgha 1 _{in the N+ straw treatmentto 3.88Mgha} 1 _{in the}

sludgetreatment(Table2).Asimilarinteractionofamendments

Fig.1.SoilCcontentwithdepthinsixofthe10selectedtreatments.Shadedareas representstandarderrorofthemean(n=4).Valuesabovethehorizontaldashed lineshowsignificanttreatmentdifferences(p<0.1).FortreatmentcodesA–Osee

Table 1.

Fig.2.Penetrationforcealongthesoilprofile.FortreatmentcodesA–OseeTable1.

Fig.3.Percentageofmoderncarbon(pMC)alongthesoilprofileintreatmentC(no organicamendment, fertilised with calciumnitrate) in the Ultunalong-term continuoussoilorganicmatterexperiment.

Fig.4.d13Csignaturesalongthesoilprofile.Shadedareasrepresentstandarderror ofthemean(n=4).Valuesabovethehorizontaldashedlinesshowsignificant treatmentdifferences(p<0.1).FortreatmentcodesA–OseeTable1.

Fig.5.Relationshipbetweenmaize-derivedSOCandabove-groundyieldineach treatment.FortreatmentcodesA–OseeTable1.

withC4–Chasbeenalreadynoticedinthegasfluxesofthesame experiment(Menichettietal.,2013).Particularlyinterestingisthe comparisonbetweenthestrawandN+strawtreatments,where themaizecontributiontoSOCwas2.610.36and2.410.64Mg

ha 1_{,respectively.Thiscontraststothealmostdoubledyieldinthe}

N+strawcomparedtostrawtreatment.

There was a statistically significant positive relationship (R2=0.54; p<0.02) between above-ground biomass yield and the amount of maize-derived SOC (Fig. 5), indicating the importanceof roots fromthelast 10years totheformation of soilC stocks. Mosttreatments showeda ratiobetween maize-derivedsoilCstocksandmaizeabove-grounddrymatteryieldof between0.29and0.45,whilethestraw(F)treatment showeda ratioof0.62.Theaverageretentioncoefficient,whichisbasedon theratiobetweenmaizeCstocksandrootinputs(withthelatter linearly derived from yield), was 0.300.09 (where the error

represents the standard deviation for all treatments taken together,and hasbeencalculatedfromtheprobability distribu-tionsshowedinFig.6).Thehighestretentioncoef_ficient forthe maizerootswasfoundinthestraw(F)andcontrol(B)treatments andthelowestintheN+straw(G)andsawdust(N)treatments.

4.Discussion

The 53 years of various treatments have caused signi_ficant changesintheSOCcontentdownto35cmdepthbutnotbelowthis level(Fig.1).Acorrespondinginfluenceoffertilizationtreatments ontheupper(inthiscase30–40cm)subsoilwasreportedrecently byKirchmann etal. (2013) intwo otherlong-term fertilisation experimentsinSwedenoverasimilartimeframe.Inthatcasethe fertilizationtreatmentaffectedinamoreevidentwayothersoil properties and produced two markedly different soil pro_file descriptions.

The analysis of 14_{C contentoverthe pro}

file ofthe nitrogen-fertilised treatment revealed a distribution of modern carbon (Fig.3)compatible withthe rootdistributionfunction usedby

Kättereretal.(2011).Thisparticulartreatmentonlyshowsroot inputssincethestartoftheexperiment,andthereforerootshavea greatinfluenceonSOCformation.Accordingtothedistributionof

14_C,thein

fluenceofCinputsoverthesoilprofiledecreasesasan exponential-likefunctionofdepth. Similarlyshaped14_Cpro

files

werefoundforagriculturalsoilscroppedwithmaizeandwheatat RotthalmünsterandHalleinGermany(Rethemeyeretal.,2005).

The soil at the Ultuna site is formedfrom post-glacial marine sedimentsandispresentlysituatedat14mabovesealevel.Based on estimatedshore displacementsat Gamla Uppsala (Eriksson, 1999),anearbysite,theUltunasiteemergedfromthesealessthan 2500yearsago.Consequently,theaverageagesofthedeepsoil layerssuggeststhatthesoilCintheseisstilldominatedby pre-terrestrialCfrommarinesediments.

The penetrometer revealed some signi_ficantdifferences be-tweentreatmentsinthecompactionof deeperhorizons(below 41cmdepth),suggestinginfluencesofthetreatmentsonstructure inthesubsoil.Thesevariationscouldbeduetodirectorindirect cropactivity,forexampletodrierconditions inducedbyhigher evapotranspiration.Theorganicmaterialeluviatedfromtheupper layers might for example also differ in nature between the treatments,thus,producingdifferenteffectsontheaggregatesin the deeperlayers. Several otherexplanations might be equally possibleandamoredetailedinvestigationwouldbeneededfor obtainingapreciseanswer.

WecalculatedtheSOCbalancerelativetothestrawtreatment (Table3)asthetopsoilinthistreatmentisroughlyinCequilibrium (inputs=outputs)andisthereforeassumedtobeinsteadystate. The resulting SOC balance profile suggested again a major influenceofthetreatmentsonthetopsoil.

The effect of maize roots and rhizodeposition was not yet detectablebelow22.5cmdepthafter10years(Fig.4).Considering insteadthetime elapsedsince thestart oftheexperiment, the richertreatments(MandO)weretheonlytreatmentstoshowa signi_ficantSOCincreasealsointhesubsoil(Fig.1)whichcanhave beenduetotranslocationinducedbysoilfauna,dissolvedorganic carbonoraccumulationofrhizodeposits.Althoughdifferencesin SOCbetweenquiteextremetreatments,suchasbarefallowand compost-amended soil, were shown tobe significant to 40cm depthalreadyafter13yearsin a Swedishexperiment(Kätterer etal.,2014), theaccumulationofSOCinthesubsoil,atleastin agriculturalsoilsinNordicenvironments,canbeconsideredtobe relativelyslowrelativetotherateofchangesinthetopsoil.

WhencalculatingthecontributionofmaizetoSOC,theisotopic effect of atmospheric CO2 enrichment (Francey et al.,1999) is

contained,aserror,intheaveragemaize

d

13_{Csignatureoverthe} period of maize cultivation. The isotopic enrichment due to fractionationmentionedbyBalesdentandMariotti(1996)canbe estimatedfromthebarefallowtreatmentas0.0050.001

m

yr 1if

suchaneffectisapproximatedwithalinearrelationshipbetween

d

13Csignatureandtime(Schweizeretal., 1999).Insucharelatively shortperiod as 10years,a linear approximationshouldnot be distinguishablefromamoredetailedexponentialapproximation such as the Rayleigh equation (Balesdent and Mariotti, 1996; Menichettietal.,2014).Over10yearsitcanthereforeberegarded as quite small, given also thedifference betweenthe two end members(C3andC4signatures).In anycase,thisenrichmentis

containedinthecorrectionappliedtoeachsingletreatment,which considersthechangesin

d

13Csignaturerecordedineachtreatment from1956to1999.Thiscorrectionalsotakescareoftheeffectof thedifferentamendmentsonthe

d

13_Csignature.

Theaverageretentioncoef_ficientcalculatedformaizematerial over all treatments (0.300.09) was compatible with that

previously calculated by Kätterer et al. (2011). Those authors utilized SOC mass balance and found an average retention coefficient for root-derived C during 53 years of 0.270.09.

Althoughthesetwocoefficientswerecalculatedontwodifferent periodsusingdifferentapproaches,theyareverysimilar.

In general, the retention coefficient of root material seems higher than previously calculated (e.g., Plénet et al.,1993 and

Bolinderetal.,1999).Ourresultsareinlinewiththehypotheses proposed by Rasse et al. (2005), who suggested that besides

Fig.6. ProbabilitydensityofthecontributionofmaizetoSOCinthetopsoilinthe differenttreatmentsduring 10years, 1999–2009. Eachprobability density is calculatedfromfourMonteCarloMarkovchainsof250,000runsandexpressesthe probabilitydistributionofthecalibratedvalue.FortreatmentcodesseeTable1.

chemicalprotection, root-derivedC isalsomoreprotectedthan other forms of C inputs due to physicochemical and physical interactions.Inparticular,theCcontainedinroothairs,associated mycorrhizaeandfinerootsisconsideredtoenterthesoildirectlyat thescaleofphysicallyprotectedCinsideaggregatesor_finepores (Rasseetal.,2005;Mendez-Millanetal.,2010).Rootgrowthalso actively structures the soil and it is able to promote soil aggregationandcoatingformationaroundtheroots,whichcould contributeto subsequent SOC stabilization.The retention

coef-ficientsof roots weregenerally alsohigher than those of non-humi_fiedamendmentssuchasstraw,sawdustorgreenmanure,as foundpreviouslybyKättereretal.(2011),suggestingthatroots contributemorethantheseamendmentstoSOCformation.This

finding is in line with the above-mentioned hypotheses, as C originatingfromamendmentswouldlackthephysicalprotection thatrootChas.

Calculatingaspecificretentioncoefficientforeachtreatment allowedustodiscusseventualdifferencesbetweenthedifferent fertilisationregimesandtoexploremoreindetailsthelimitations of a uniform allometric function for every treatment. Some treatments (unfertilized, straw and green manure) which pro-ducedrelativelylow yields(Table 1)alsogavea slightlyhigher retentioncoefficientformaizerootsthantreatmentswithhigher yields(Table2).Thisobservationcouldindicatesomekindofstress factor,possiblyrelatedtonutrientdeficiencieswithinroottissues oreveninthesoilthatmighthaveinfluencedthedecomposability oftherootmaterialduringthe_firstdecade(Freschetetal.,2012).In amorelong-termperspective,however,nitrogenlimitationmay ratherleadtolowermicrobialsubstrateuseefficiencyandlowerC retention(Poeplauetal.,2015).ThisissupportedbytheeffectofN fertilization on straw C retention observed in the Ultuna experiment. During the period 1956–1999 with only C3-crops,

yieldswereinaverage91%higherinN-fertilized(treatmentCand G)compared tounfertilizedtreatments irrespectiveof whether strawwasaddedornot(BandF).Thus,itisreasonabletoassume thattheroot-derivedCcontributiontoSOCwasverysimilarinthe treatmentpairsC/BandG/F.Nevertheless,totalCstocksinGwere about10MghigherthaninFwhereasthoseinCwereonly5Mg higherthaninB(Table2).Consequently,theretentionofstrawC was higherin Gthanin F.Inabsoluteamounts,strawaddition resultedin16MgmoreSOCinGcomparedwithCbutonlyin11Mg moreinFthaninB.

Factors generating a non-linear response of yield to C accumulation caused by nutrient de_ficiencies seemed present, butonlyinextremecases.Thelogarithmicrelationshipbetween maize-derivedC and yieldresultedonly ina marginallyhigher coef_ficientofdetermination(R2_{=0.54)thanalinearrelationship}

(R2_{=0.52) (Fig. 5). This supports the validity of the linear}

relationship between roots and yields proposed by Bolinder etal.(2007) incaseoflow yields,althoughsuchcoef_ficientsof determinationsmightbeconsideredperfectlyacceptableorreally lowdependingonthepurposeofthestudy.Assuminganuniform root:shootvalueseemsaviableapproachincaseoflackofdata,but onemust bearin mindthelimitationsassociated withsuchan approach.

Wealso observed correlations betweenretention coefficient and certain phospholipid fatty acid (PLFA) classes previously

measured inthesamesitebyBörjessonet al.(2011).TwoPLFA classes indicating Gram-negative bacteria(16:1

v

7 and 16:1

v

9), whichareconsideredtobelessefficientdecomposersthan Gram-positivebacteria(ZoggandZak,1997),werefoundtobenegatively correlated with the retention coef_ficient (r= 0.2 and r= 0.4, respectively). Class 18:2, which is commonly associated with fungal cells(BååthandAnderson,2003), wasinsteadpositively correlated withtheretention coef_ficient (r=0.6).A multivariate analysis(PCA) pointed out similarcorrelations. It is difficultto drawpreciseconclusionsfromtheserelationships,sincethetwo datasetsarecomingfromtwodifferentyearsalthoughfromthe same experimentalfield.Butalsotheseresultssuggesta direct relationshipofmicrobialecologyandSOCmineralizationandits interactionwithagriculturalpractices.

5.Conclusions

ApplicationoforganicamendmentsandNfertilisationinthe Ultunalong-termexperimentalplotssubstantiallyaffectedSOCin thetopsoil,givingrisetoa4timesrangeincarbonstock.Below thisdepthasigni_ficantaccumulationofCdownto35cmdepth wasonlyfoundinrecalcitrantorprocessedamendmentssuchas peatandsewagesludge,thetwotreatmentswhichalsohadthe largest accumulation in the topsoil. In contrast, the physical conditionsin deepersoil layerswereaffected inseveralof the treatments.Maizeroots significantlycontributedtoSOCin the topsoil after 10 yearsof cultivation, and contributed approxi-mately4–8%ofthenewlyformedSOCthroughrhizodeposition.

However, SOC accumulation in most of the subsoil was not detectableafter10years.TheSOCbelow40cmdepthshowedan averageage>1500years,suggestingagainthatSOCindeepersoil layers is influenced quite slowly by changes in the topsoil. However, since organic matter stored in the subsoil may decompose slower than in the topsoil,C accumulation in the subsoilinducedfor exampleby deep-rootingvarietiesof crops remainsapromisingstrategyforCsequestration.

ThecontributionofrootstoSOCwassimilartothatreported recentlyusinganothermethodology.Rootscontributedrelatively moretoSOCthanthesamemassofcarbonderivedfrom above-groundplantmaterial.Theseresultssuggestthat root contribu-tions to SOCare oftenunderestimated and that root-derivedC should be considered one of the most effective inputs for C sequestrationinsoil.

Acknowledgements

Wearegratefultoformercolleagues,especiallyOlle Gunnars-son,HansNömmikandJanPerssonforstartingandkeepingthis experiment runningdespiteseveral threatsof closure.Pär Hill-strömhasmanagedtheexperimentoverthelast25years.Financial supportforkeepingthelong-termexperimentandforthepresent study was provided by the Faculty of Natural Resources and AgriculturalSciencesatSLU.

AppendixA

SeeTablesA1andA2

Table A2

d13C signature (m) in the experimental treatments at different depths with standard error (_n= 4). Superscript letters represent the groups identi_fied by a Tukey_’s HSD test (a= 0.1). NA = not available. For treatment codes A–O see

Table 1.

Treatment 0–15 15–17.5 17.5–20 20–22.5 22.5–25 25–27.5 27.5–30 30–35 35–40 40–50 50–60

A 25.450.31ab 25.250.53a 25.140.30a 25.250.24a 25.250.19a 25.000.29a 25.650.11a 25.740.13a 25.570.17a 25.670.14a 25.750.20a

B 25.720.09ab 25.510.21a 25.610.10ab 25.930.15ab 25.90.12a 25.830.14a 25.220.44a 25.330.15a 25.380.17a 25.530.42a NA

C 25.270.23a 25.540.10a 25.780.31ab 25.160.26a 25.270.14a 25.310.23a 25.530.10a 25.040.35a 25.240.29a 25.250.3 +a 26.430.33a

F 25.840.30ab 25.930.23a 25.970.19ab 25.730.22ab 25.240.81a 25.390.34a 25.040.49a 24.890.5a 24.610.88a 24.990.02a 25.430.21a

G 26.210.11b 26.210.35a 26.360.20b 26.060.27ab 26.10.15a 25.60.30a 25.650.31a 25.830.07a 25.570.23a 25.260.2 +a 25.220.1 +a

H 25.790.24ab 25.840.13a 25.760.26ab 25.570.09ab 26.060.11a 25.80.10a 25.610.17a 25.840.12a 25.320.13a 25.460.26a 25.740.18a

J 26.65NAb 26.040.41a 26.290.21b 26.330.28b 25.990.30a 26.080.17a 25.850.26a 25.940.12a 25.580.14a 25.480.04a 25.78NAa

M 25.80.12ab 25.760.13a 25.760.06ab 26.040.15ab 25.90.08a 25.610.32a 25.950.19a 25.560.09a 25.370.27a 25.36NAa 25.780.07a

N 25.420.07ab 25.290.03a 25.60.02ab 25.590.27ab 25.540.09a 25.210.26a 24.880.23a 25.080.38a 25.110.23a 24.590.15a 24.860.14a

O 25.470.07ab 25.610.25a 25.310.14a 25.420.18ab 25.960.33a 25.360.25a 25.520.39a 24.971.11a 25.240.02a 25.230.24a 25.090.47a

Table A1

Carbon concentrations (%) in the experimental treatments at different depths with standard error (n= 4). Superscript letters represent the groups identified by a Tukey’s HSD test (a= 0.1). NA = not available. For treatment codes A_–O seeTable 1.

Treatment 1–15 15–17.5 17.5–20 20–22.5 22.5–25 25–27.5 27.5–30 30–35 35–40 40–50 50–60 A 0.890.11e 0.880.10e 0.900.05d 0.910.08c 0.930.04b 0.780.01b 0.780.03b 0.710.07c 0.730.10a 0.620.08a 0.510.11a

B 1.020.11de 1.060.12de 0.910.13d 0.980.17c 0.970.18ab 0.840.10b 0.80.11ab 0.780.04bc 0.740.11a 0.700.09a NA

C 1.300.15cde 1.310.12cde 1.280.07cd 1.140.03bc 0.930.05b 0.870.05b 0.830.09ab 0.760.05c 0.640.08a 0.520.07a 0.370.02a

F 1.630.04cd 1.60.03bcde 1.330.12cd 1.220.10bc 1.030.13ab 0.940.05ab 0.790.03ab 0.780.09bc 0.690.09a 0.590.04a 0.480.05a

G 1.970.17c 1.890.24bcd 1.570.18bcd 1.490.14abc 0.970.09ab 0.950.08ab 0.920.12ab 1.210.19a 0.780.14a 0.690.16a 0.790.45a

H 1.750.12cd 1.790.07bcd 1.430.09cd 1.420.03abc 1.060.07ab 0.970.05ab 0.890.07ab 0.840.04abc 0.510.14a 0.430.07a 0.340.07a

J 2.31NAbc 2.020.18bcd 1.870.18bc 1.780.38ab 1.440.21ab 1.100.17ab 0.940.08ab 0.920.09abc 0.940.16a 0.750.05a 0.4NAa

M 3.790.15a 4.060.15a 3.300.35a 1.660.41abc 1.530.19a 1.090.05ab 0.990.08ab 1.140.08ab 0.990.12a 0.98NAa 0.910.07a

N 2.130.18c 2.190.04bc 1.420.05cd 1.210.12bc 1.130.09ab 0.990.08ab 0.760.12b 1.000.09abc 0.650.13a 0.560.08a 0.50.11a

O 3.010.14b 2.630.44b 2.270.25b 1.910.12a 1.520.18ab 1.320.14a 1.160.11a 1.040.07abc 0.740.01a 0.790.07a 0.70.08a

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