Vertical and lateral transport of biochar in light-textured tropical soils

Vertical and lateral transport of biochar in light-textured tropical soils

Alfred Obia

^a,b,

*, Trond Børresen

^a

, Vegard Martinsen

^a

, Gerard Cornelissen

^a,b,c

, Jan Mulder

^a

aDepartmentofEnvironmentalSciences(IMV),NorwegianUniversityofLifeSciences(NMBU),P.O.Box5003,NO-1432Ås,Norway

bNorwegianGeotechnicalInstitute(NGI),DepartmentofEnvironmentalEngineering,P.O.Box3930,UllevålStadion,NO-0806Oslo,Norway

cDepartmentofAnalyticalChemistryandEnvironmentalSciences(ACES),StockholmUniversity,10691Stockholm,Sweden

ARTICLE INFO Articlehistory:

Received25September2015 Receivedinrevisedform6July2016 Accepted22July2016

Availableonline29July2016 Keywords:

Biocharparticlesize Biochar¹³Cisotope Biochartransportinsoil

ABSTRACT

FieldexperimentswereconductedinArenosols(loamyfinesand)andAcrisols(sandyloam)inZambiato quantifyverticalandlateraltransportofbiochar(BC)usingtheBCandsoil¹³Cisotopesignaturesand totalorganiccarboncontents.TherewerethreeexperimentaltreatmentscomposingofnoBC,0.5and 0.5–1mmBCseachwiththreereplicatesarrangedincompletelyrandomizeddesign.TheappliedBCs weremadefromricehusk,except0.5–1mmBCinsandyloam,whichwasfrommaizecob.Oneyearafter mixingBChomogeneouslyinthe0–5cmsurfacelayer,soildownto20cmdepthwassampled.The downwardmigrationofBCwassignificantdownto8cmdepthinsandyloamanddownto6cminloamy finesand.Belowthesedepths,therewas nosignificant differenceinBC amountsbetweentheBC amendedandthereferenceplots.Therewasageneraltendencyforgreaterdownwardmigrationforthe 0.5mmthanfor0.5–1mmBC.TotalBCrecoveryat0–5cmdepthintheBC-treatedsoilsamountedto 45–66%ofthetotalappliedamountofBC.Asonly10–20%wasrecoveredinthedeepersoillayers,24–45%

oftheappliedBCcouldnotbeaccountedforinthesoilprofile.Although,decompositionanddownward migrationtobelow20cmdepthmaycontributetothelossofBCfromthesurfacesoil,muchcanbe attributedtolateraltransferthrougherosion.Thisisthefirststudythatexplicitlyfocusesonthethemeof BCdispersionandshowsthatinArenosolsandAcrisolsofthetropics,thedownwardmigrationofBCis limited.

ã2016ElsevierB.V.Allrightsreserved.

1.Introduction

Biochar (BC),which isa biomasspyrolysis producthasbeen reported to increase crop production with the co-benefit of sequesteringcarbon(C)(Glaseretal.,2002;Jefferyetal.,2011).

Reportedincreasesincropproductionvariedwidelydependingon soilandBCtypes,butthereareindicationsthatthiseffectofBC mightbestrongerin sandyandacidicsoils(Glaseretal.,2002;

Jefferyetal.,2011;Martinsenetal.,2014),whicharewidespreadin tropicalregions.Someofthemechanismsforthereportedincrease in crop production include increase in water holding capacity, limingeffectanddirectadditionandretentionofnutrientsbyBC (Cornelissenetal.,2013;Glaseretal.,2002).IftheseBCeffectson soilpropertiesaretobenefitthecropsforextendedperiods,then BCappliedtotopsoilsshouldremainwithinthetopsoilwhereroot

densityishigh.However,anytransportofBCwouldnotaffectitsC sequestrationpotential.

Therearefewstudies,whichindicatethatBC,onceappliedto soil,might toa certainextentbemobilewithinthesoilprofile (Foereidetal.,2011;Haefeleetal.,2011;Majoretal.,2010).Such transport ofBC withinthesoil profile couldbeexacerbated by physical disintegration of BC to nano- and micrometer sized particles, moving with infiltrating water (Spokas et al., 2014).

Haefeleetal.(2011)reportedthatasmuchas50%ofBCappliedto 15cm top soil, estimated based ontotal C changes in the soil profile, migrated to deeper soil horizons of structured humic NitisolsandgleyicAcrisolsafteroneyear.ThemigrationofBCto deepersoilwasfastinsoilswithhighwaterinfiltrationrate(Nitisol andAcrisol),whereasnomigrationwas foundinsoilswithlow water infiltration rate, as heavy paddy soil. In an experiment designedtomeasurethefateofBCfrommangopruningsappliedat 0–10cminsandyclayloamFerralsolusing

d

^{13C,Majoretal.(2010)}

reportedslowdownwardmigrationofBCat15cmdepthsatarate of<0.5%oftheBCappliedtosoilperyear.

*Corresponding author at: Department of Environmental Sciences (IMV), NorwegianUniversityofLifeSciences(NMBU),P.O.Box5003,NO-1432Ås,Norway.

E-mailaddresses:[email protected],[email protected](A.Obia).

http://dx.doi.org/10.1016/j.still.2016.07.016 0167-1987/ã2016ElsevierB.V.Allrightsreserved.

ContentslistsavailableatScienceDirect

Soil & Tillage Research

j o u r n a l h o m e p a g e : w w w . e l s ev i er . c o m / l o c a t e/ s t i l l

Anadditionalnumberofstudiesreportdownwardmigrationof blackC(Hockadayetal.,2006;Leifeldetal.,2007),whichissimilar toBC.BlackCintheenvironmentareorganicproductscommonly derivedfromincompletecombustionwithoutintentionallylimit- ingoxygen(sootandcharcoal).Indrainedpeatland,blackCfrom depositedcombustionresiduesof householdwastemigratedto deepersoillayers(Leifeldetal.,2007).Leifeldetal.(2007)found between21–69%ofblackCbelowploughdepthof30cm,50years afterthelastdepositionofblackC.ThemigrationrateofblackC wasestimatedtobe0.6–1.2cmyear¹.Similarly,Hockadayetal.

(2006)foundthatblackCcanbemobileinfire-impactedforestsoil (medium sand with poorly developed Podzol), particularly the solubleorganicconstituentsresultingfromdecomposition,which leachwithpercolatingsoilwater.

Basedonamodelingstudy,Foereidetal.(2011)suggestedthat lateraltransportofBCcouldbeaveryimportanttransportpathway ofBCinsoils,butlimitedfielddataareavailable.Intheirmodelling work, the authors predicted that erosional transport of BC decreasedwithtimeduetoincorporationofBCintosoilaggregates (Awad et al., 2013; Obia et al., 2016). Due to the scarcity of experimental data onboth verticaland lateral transport ofBC, morestudiesarewarranted.Inaddition,nostudyhasreportedthe influenceofBCparticlesizeonitslateralandverticaldispersion.

Acrisols and Arenosols, characterized by low agricultural productivity, dominate central and western regions of Zambia.

Theproductivityofthesesoils,whicharewidespreadglobally,has been demonstrated to increase through the application of BC (Cornelissenetal.,2013;Martinsenetal.,2014).Oneofthemain factorsproposedtoexplaintheBC-inducedincreaseinproductivi- tyofthesesoilsistheincreaseinwaterholdingcapacityintheroot zone(Obiaetal.,2016)leadingtobetter-developedrootsystems (Abivenetal.,2015).MigrationoflargeamountsofBCtodeeper soilhorizonswithlowdensityofrootsmighteliminateorreduce theeffectofBConsoilproductivity(Haefeleetal.,2011).

Inacontrolledfieldexperimentintwolight-texturedsoilsin Zambia,wedeterminedBCtransportratesandtheirdependence onBCparticlesize.Wehypothesizedthattherewouldbegreater downwardmigrationofBCatKaoma(loamyfinesandwithhigher saturated hydraulic conductivity 5.2cmh¹) than at Mkushi (sandyloamsoilwithsaturatedhydraulicconductivityof1.7cm h¹)(Obiaetal.,unpublisheddata)andthatthismigrationwould begreaterfor finer BCfractions. Lateraltransport ofBC is also expectedtobegreateratKaomathanatMkushiandgreaterfor finer thancoarser BCs. The objective of thepresent study was thereforetoquantifythedownwardandlateraltransportoffine (0.5mm)andslightlycoarser(0.5–1mm)BCinloamyfinesand (Arenosol)and sandyloam(Acrisol). To this end, BCswith ¹³C signalsdifferentfromthoseofthesoilswereappliedandrecovered inahigh-resolutiondepthprofileby

d

^{13CandTOCanalyses.This}

studyisoneofthefewexplicitlydedicatedtostudyingBCmobility insoilandthefirsttoconsidertheinsitumobilityofdifferentBC particlesizes.

2.Materialsandmethods 2.1.Biochars

TheBCsusedinthisstudywerepreparedfromricehuskand maize cobs after shelling the grains. Rice husk is available in western Zambia, whereas maize cobs are available throughout Zambia.Pyrolysisofthefeedstockswascarriedoutinadrumretort kilnatChisamba,Zambiaatatemperatureof350Candaretention timeofoneday.Thedrumswereloadedwithmaizecoborrice huskandsealedwithalidbeforelightingwood;thestartupfuel belowthedrums.Heatgeneratedfromtheburningwooddroveout moistureandothergasesfromthefeedstockinthedrumsthrough

anexhaustpipeinthenon-retortmode.Inretortmode,nomore woodwasadded underthedrumsbutinstead,thecombustible pyrolysisgases(e.g.methaneandcarbonmonoxide)weredirected underthedrums,catchingfiretogeneratetheenergytosustainthe pyrolysis, and reducing toxicgas emissions.Photographic illus- trationscanbefoundinSparreviketal.(2015).ThemaizecobBC wasusedinextensivefieldtrials(Martinsenetal.,2014)andin mechanisticstudies(Allingetal.,2014;Haleetal.,2013).TheBCs were sieved to particle sizes of 0.5 and 0.5–1mm before applicationtosoil.

2.2.Experimentalsetup

Fieldexperimentswereestablishedin sandyloamAcrisol at Mkushi,centralZambia(S1344.839,E2905.972)andinloamyfine sandArenosolatKaoma,westernZambia(S1450.245,E2502.150) inApril2013.TheannualrainfallinMkushiandKaomais1220and 930mmandaveragetemperatureis20.4and20.8Crespectively (Martinsenetal.,2014).Ateachsites,therewerethreetreatments, each withthreereplicatesresultinginnineplotsorganizedina completely randomized design. Plot sizes were 5050cm, separated by 20cm high hard plastic sheets, inserted approxi- mately10cmverticallyintothesoil.Layoutoftheexperimental design can be found in Supplementary information Fig. S1. In Kaomaloamyfinesand,treatmentsincluded0.5mmand0.5– 1mmricehuskBC,bothaddedatarateof3.4%w/winadditiontoa reference without BC. In Mkushi sandy loam, the treatments included0.5mm rice huskBC, 0.5–1mm maize cobBC anda reference. Here, BC addition rates were 4% w/w for both treatments.AtMkushi,thecoarser(0.5–1mm)fractionwasmaize cobBC(andnotricehuskBC),duetoshortageofcoarserricehusk BC,causedbyeasycrumblingofricehuskBCduringsievingtofiner sizes.ThesameamountofBCwasaddedperplot(625g)toboth MkushiandKaomasoils, buttheBCcontents(in%w/w)differed due to differences in soil bulk density between the two sites (Table2).

Biocharwasappliedinthetop5cmofthesoil.ToapplytheBC, weremovedthetop5cmofthesoilbyhandhoeandspadeand mixed it withBC in a bucket. The top soil was dry when the experimentwassetup,sothatmixingwithBCwaseasy.Thesoil layersbelow5cmdowntoapprox.30cmwereloosenedusinga handhoetoremoveanycompactedlayerbeforeplacingbackthe soil-BCmixtureatthesurface.Looseningthecompactedsubsoilis a common farmer practice to increase root volume as recom- mended by theconservation farming unit (CFU) of Zambia for farmerspracticingconservationfarming(Cornelissenetal.,2013;

Umaretal.,2011).Thereferenceplotsweretreatedinthesame wayastheBCamendedplots.Thesoilwaslefttonaturallysettle aftertheestablishmentoftheexperiment.

Theexperimentwassetupattheendofthegrowingperiod followedbyalongdryperiodfromApriltoOctober2013.Maize wasplantedattheonsetofrainyseasoninNovember2013inthe middleofeachplotafterapplicationofNPKfertilizer(10:20:10) at arate of140:280:140kgha¹ witha topdressingof ureaat 140kgha¹.Theplotswerehandweededwithoutanytraffic.

2.3.Soilsamplingandsamplepreparation

SoilsamplesweretakenattheendofMarch2014,oneyearafter BCapplication,todeterminetheamountofBCrecoverableinthe soilprofiledownto20cmdepth.Twosamplesweretakenfrom each of eight depth intervals per plot: 0–5cm (depth of BC application),5–6cm,6–7cm,7–8cm,8–9cm,9–10cm,10–15cm, and 15–20cm. Each sample was taken by cutting 1cm thick verticalslicesofsoilacrosstheplotthroughtheentirelayerofeach

oftheeightdepthintervals.Samplesweresealedinsamplingbags priortoanalysis.

The field-moist soil samples were dried at 40C for 3days beforepassingthrougha2mmsieve.Sub-samplesofthesieved homogenoussoilsweremilledforanalysisof

d

^{13CandTOC.Milled}

sampleswerepreparedin85mmtincapsulesandsenttoStable IsotopeFacility, UniversityofCalifornia, Davisforanalysisusing isotoperatiomassspectrometry.

Coreringsamples(100cm³)weretakentodeterminethebulk densityofthesoiltoallowcalculationofTOCstocksineachofthe depth intervals and thus establishment of BC mass balances relativetotheamountappliedtotheplotsinApril2013.Thebulk density(Table2)wasdeterminedattwodepths(0–5cmand6– 10cm)foreachplot.Thebulkdensityat6–10cmdepthwasused forcalculationofTOCstocksinalldepthsfrom6to20cm.Thisis reasonablebecausethesoilsfrom6to20cmwerehomogenized duringplotestablishment(seeabove).

2.4.Calculationofbiocharamountsandmassbalance

TheTOCstock(g)persoildepthintervalateach5050cmplot wascalculatedaccordingto:

TOCðgÞ¼5050DepthBulkdensityTOCð%Þ

100 ð1Þ

WhereDepthandBulkdensityareincmandgcm³,respectively.

The fraction f of TOC contributed by BC was calculated accordingtoEq.(2),adaptedfromKocyigit(2006):

f¼

d

^13Cmixture

d

^13Cref:soil

d

^13Cbiochar

d

^13Cref:soil

ð2Þ

Where

d

^13Cmixture=

d

^{13Cofthesoil-BCmixture,oneyearafterBC}

application,

d

^13Cbiochar=

d

^{13CoftheBCand}

d

^13Cref.soil=

d

^13Cofthe

referencesoil.Thereference

d

^13Cofthe0–5cmwasdeterminedat thetimeofexperimentalsetupinApril2013whilethe5–20cm wasdeterminedinthereferenceplotsasaveragevalueatdepthof 5–20cminApril2014.The

d

^{13Cofthereferenceplotswasconstant}

withdepthattheintervalof5–20cm(Fig.1).

TheamountofBCrecovered(g)fromeachdepthintervalineach plotwascalculatedaccordingto:

BCrecovered¼TOCf 100

%CofBC ð3Þ

Thetwomeasurementvaluesof

d

^13C(

m

^{),TOC(%)andBC(g)}

recoveredperplotforeachdepthintervalwereaveragedpriorto statisticalanalysis.

2.5.Analysisofotherbasicsoilandbiocharproperties

ThesoilandBCswerecharacterizedforTOC,totalnitrogen,total hydrogen after millingusing a CHN analyzer(CHN-1000, LECO USA).Biocharwasacidifiedtoremovecarbonatesbeforedetermi- nationofTOC.LossonignitionofBCwasdeterminedbyheatingthe milledsamplesinanoven(CarboliteBamford,Sheffield,England) at550C.pHwasmeasuredinwaterataratioof1to2.5(soilorBC towater)usingapHmeter(Orion2Star,ThermoFisherScientific, Fort Collins, CO). Soil texture was measured using the pipette method.ThesurfaceareaofBCwasmeasuredusingstandardBET method with nitrogen adsorption at 77.4K. Easily soluble constituentsoftheBCsweredeterminedbymixing2gofBCin 100mldeionizedwater(1:50)followedbyovernightshaking.The BCswerethenfilteredandovendriedat105Covernight.Lossin

Fig.1.Distributionofd^{13CandTOCinthesoil}profileoneyearafterBCwasappliedtothesurfacesoil(0–5cmdepth).A1andA2indicatethed^{13CandTOCforMkushisoil} amendedwithricehuskBC(withd¹³C=27.30.03)andmaizecobBC(withd¹³C=12.30.3),whereasB1andB2indicatethed¹³CandTOCfortheKaomasoil(onlyriceBC wasadded).Forsignificantdifferences,eachdepthateachsiteforeitherd¹³CorTOCwereconsideredseparately.Differentlettersindicatesignificantdifference,Tukey’stest atp<0.05.ErrorbarisSE.

weight of BC after drying was considered as the soluble constituents.ThesoilandBCpropertiesarepresentedinTable1.

The 0.5–1mm maize cob BC was not characterized in the laboratorybecauseitsstockwasexhaustedinthefield.Calculation ofmassbalanceof0.5–1mmmaizecobBCrecoveredinthesoil layerswasbasedonCcontentofthe0.5mmmaizecobBC.TheC contentof the0.5–1mm maize cob BCis expectedto beonly slightlyhigherthanthatofthe0.5mmparticlesizeandlessthan thatof unsorted BCanalogoustothetrendsof rice husk BCin Table1.Apparently,ahigherpercentageofashesrelativetothe unsortedBCwasstillfoundinthe0.5–1mmsizefraction(notonly inthe0.5mmone).Totalrecoveryof 0.5–1mmmaize cobBC calculatedusingEq.(3)was45.5%basedonCcontent(44.8%)of 0.5mmBCbutitwouldbe37.9%basedonCcontentofunsorted BCof53.8%resultinginadifferenceinrecoveryof7.6%.Therefore useofCcontentof0.5mmmaizecobBCtocalculaterecoveryof 0.5–1mmmaizeBCresultedinaslightoverestimationby<7.6%.

2.6.Statisticalanalysis

ThedatawereanalyzedusingthesoftwarepackageR(RCore Team,2014).InordertodisplaythedistributionofBCalongthe soilprofile,

d

^{13C and TOC(%)wereplottedalong thesoildepth}

profile.TheamountofBCrecoveredateachdepthintervalandtotal amountofBCrecoveredwereanalyzedusingtwo-wayanalysisof variance(ANOVA).Inthisanalysis,BCtreatments(noBC,0.5mm BC, and 0.5–1mm BC) and sites (Mkushi and Kaoma) were consideredas the explanatoryfactors for the differencesin BC recoveredateachdepth.Thisallowedcomparisonamongtreat- mentswithineach site and comparison oftreatments between sites.SincethenumberofreplicatesinMkushiandKaomawere thesame,onlyasinglestandarderrorwasreportedforamountof BC recovered at each depth. Such single standard error was preferred as opposed to the individual standard error of each treatmentmeanbasedonthreereplicatesthatcouldbeerroneous (Webster,2007).Differencesbetweenmeanvalueswereassessed usingTukey’stestat5%levelofsignificance.Allnumberspresented intablesaremeanvaluesstandarderrors.

3.Results

The

d

^{13CsignalandtheTOCcontentsoftheBCamendedplots}

changedalongthedepthprofileinbothMkushiandKaomasoils (Fig.1).The

d

^{13CofthericehuskBC(}27.10.04

m

^)andmaizecob

BC (12.30.1

m

^{) (Table 1), which are from C3and C4 plants}

respectively, were different from those of the two soils and thereforeallowedtracingoftheBCsinthesoils.MaizecobBC(0.5– 1mmparticlesize)wasappliedonlyinMkushisoil.Thereference valueof

d

^{13C ofMkushiandKaomasoilwere}18.10.3

m

^and

20.20.1

m

^,respectively,forthe0–5cmdepthand18.90.03

m

^and20.80.03

m

^,respectively,forthe5–20cm(Table1).One yearafter BCapplication,the surfacesoil layer (0–5cm)of the neighboringreferenceplotsreceivedBCtransportedlaterallyfrom the BCamended plots asindicated bytheir

d

^{13C values (}19.6 0.3

m

^and21.70.3

m

^{,inMkushiandKaomasoil,}respectively;

Fig.1andTable3).

In Kaomaloamy fine sand where only rice husk BC (

d

^13C=

27.1

m

^{) was applied, the}

d

^{13C in the top soil (0}–5cm) was 24.70.1

m

^{for the}0.5mmBCand 25.10.1

m

^{for the0.5}– 1mmBC,respectively.Bothvaluesweresignificantlysmallerthan thesoilreferencevalueof20.20.05

m

^{priortoBCadditionin}

2013(Fig.1B1).Withincreasingsoildepth,the

d

^{13Cincreasedto}

valuesnotsignificantlydifferentfromthesoil’sreferencevalueof 20.80.03

m

^{below7cmsoildepthforboth}0.5mmBCand0.5– Table1

Soilandbiochar(BC)properties.^a

Properties Kaomasoil Mkushisoil RicehuskBC MaizecobBC

0.5mm 0.5–1mm Unsorted 0.5mm Unsorted

Sand(%) 85.4 75.1 – – – – –

Silt(%) 10.2 15.9 – – – – –

Clay(%) 4.4 9.0 – – – – –

Textureclass Loamyfinesand Sandyloam – – – – –

TotalorganicC(%) 0.62 0.74 39.3 42.8 47.8 44.8 53.8

Totalnitrogen(%) 0.00 0.01 0.61 0.52 0.82 0.79 0.65

Totalhydrogen(%) – – 2.33 2.41 2.37 2.09 2.36

H/C(molarratio) – – 0.71 0.68 0.60 0.56 0.53

pH 5.8 5.8 8.3 8.3 8.3 9.0 8.8

Lossonignition(%) – – 48.8 54.9 – 52.1 –

Solubleconstituents(%) – – 2.1 0.6 2.0 2.6 2.4

BETsurfacearea(m²g¹) – – 2.4 2.3 – 10.5 –

CEC(cmolckg¹) 2.8 1.7 – – 14.0 – 22.2

K⁺(cmolckg¹) 0.1 0.3 – – 10.4 – 16.5

Ca²⁺(cmolckg¹) 1.2 1.1 – – 2.4 – 4.3

Mg²⁺(cmolckg¹) 0.2 0.3 – – 0.9 – 1.2

d¹³C(m^{) 20.78 18.86 27.05 27.05 27.05 12.27 12.27}

aMaizecobBCwithparticlesize0.5–1mmwasnotcharacterizedinthelabbecauseitwasexhaustedinthefield.Calculationofmassbalanceof0.5–1mmmaizecobBCin thesoillayerswasbasedonCcontentofthe0.5mmBC.Thiscouldhaveresultedinaslightoverestimation(<7.6%of284gofthetotalBCrecovered).Thed¹³Cofthesoil presentedhereistheaverageofthebulksoilat5–20cmdepthmeasuredinApril2014.

Table2

Bulkdensity(gcm³)ofthesoilfrombiocharexperimentsinZambia.^a Site BCparticlesize BCdose(%) Soildepth

0–5cm 5–10cm

Mkushi Ref.soil 0 1.260.01 1.280.02

0.5mm 4 1.170.03 1.250.02

0.5–1mm 4 1.160.05 1.340.04

Kaoma Ref.soil 0 1.400.02 1.400.04

0.5mm 3.4 1.280.01 1.380.01

0.5–1mm 3.4 1.270.02 1.460.03

aThebulkdensityof0–5cmsoildepthwaspresentedinourearlierwork(Obia etal.,2016).TheBCswerefromricehuskexcept0.5–1mmBCinMkushi,whichwas frommaizecob.Valuesaremeansstandarderror(n=3).

1mmBC.TheTOCof0–5cmsoildepthintervalwassignificantly largerforthe0.5–1mmBC(1.570.05%)thanforthe0.5mmBC (1.290.02%) amended plots (Fig. 1B2). However, for both treatments, TOC decreased with depth and was no longer significantly different from that of the reference soil (0.460.03%)at6–7cmsoildepthinterval.

InthesandyloamatMkushi,theadditionof0.5mmricehusk BCwith

d

^13Cof27.1

m

^significantlyreduced

d

^13Cofthe0–5cm soilfromreferencevalueof18.10.3

m

^{(priortoBCadditionin}

2013)to23.90.3

m

^{measuredinApril2014(p}<0.05)(Fig.1A1).

Fordeepersoillayersbelowtheapplicationdepth,the

d

^13Cofthe

soil in April 2014increased graduallywith depth reachingthe referencesoilvalueof18.90.03

m

^{atthedepthof10cm(Fig.1).}

On the other hand, the 0.5–1mm maize cob BC with

d

^{13C of}

12.3

m

^increased

d

^{13CofthesoilfromreferencevaluepriortoBC}

addition of 18.10.3

m

^to 14.90.1

m

^{in the 0}–5cm soil (Fig.1A1). Below 5cm depth,

d

^{13C decreased reachingthe soil}

referencevalueat7cm soildepth. TheTOC contentof thesoil followed a similar pattern with soil depth as

d

^{13C (Fig. 1A2).}

However,thetreatmentswith0.5mmricehuskBCand0.5–1mm maizecobBCreachedthereferencesoil’sTOClevelat7cmsoil depth,3cmdepthshortofthatestimatedusing

d

^13Cfor0.5mm ricehuskBC(Fig.1A2).TheTOCcontentsofthesoilinthe5–8cm depthintervalwerelargerfor0.5mmricehuskBCthanfor0.5– 1mmmaizecobBCamendedplots.

WerecoveredgreateramountsofBCofthe0.5mmfraction belowtheapplicationdepthcomparedtothe0.5–1mmfractionin Mkushi(19%vs10%)(Table3andS1).Likewise,therewerealso greateramountsoffineBCbelowtheapplicationdepthinKaoma.

However,thedifferencebetweenthetwoBCparticlesizeswasnot significant(p=0.41)(13%vs9%).Overall,thedownwardtransport ofBCwasgreaterinMkushisandyloamthaninKaomaloamyfine sand.TherecoveredBCinthetop0–5cmoftheKaomasoilbased oneq.3afteroneyearwas56%and67%for0.5mmand0.5–1mm BCs,respectively(Table3andS1).InMkushi,sincethetwoBCs usedhaddifferent

d

^{13Csignalsandtherewas}cross-transportation ofBCsbetweenthesurfacelayersoftheplots,noaccurateestimate oftherecoveryofBCinthe0–5cmlayer,basedon

d

^{13C(Table3and}

S1)waspossible.UseofTOCchangesalonetocalculateBCrecovery indicatedarecoveryof53%and45%for0.5mmricehuskand0.5– 1mm maize cob BCs, respectively (Table S2). The overall BC recoveryinthe0–5cmatbothsitesafteroneyearweretherefore between45–67%.Thetotalrecoveryinthesoilprofiledowntothe 20cmsoildepthwas55–76%oftheBCapplied.Belowthedepthof 8cm, less than 2% of added BC was foundwith no significant differencebetweenBC contentinamended and referenceplots basedonEq.(3) (p>0.05)(Table 3and S1).Therecovery ofBC

basedonEq.(3)andbasedonTOCaloneweresimilar,especiallyfor KaomawhereonlyricehuskBCwasused.Theonlydifferencewas belowthedepthof8cmwhere

d

^{13Csignalalloweddetectionof}

smallamountsofBC.Thetotalrecoveryof0.5–1mmmaizecobBC inMkushisoilwassmall(55%)(TableS2)comparedtootherBC treatments (69–76%) at Mkushi and Kaoma (Table 3 and S1) (p<0.05).

4.Discussion

Thechangesinboth

d

^{13CsignalandTOCcontentofthesoilwith}

depthinthe5–10cmintervalatbothMkushiandKaoma(Fig.1) showedthatBCmigratedtodeepersoilhorizons.Thedownward migrationofBConeyearaftertheapplicationwasconfinedtoless than 3cm below the application depth, i.e., the 5–8cm depth interval(Fig.1andTable3).ThedownwardmigrationoftheBCs downto5–20cmwasintherangeof9–19%oftheappliedBCand wasgenerallygreaterforfineBCof0.5mmthanforcoarserBCof 0.5–1mm(e.g. 19%vs10%inMkushi).Therewasgreaterdownward migrationofBCinsandyloam(Mkushi)thaninloamyfinesand (Kaoma)(e.g.19%vs13%for0.5mmBC).MigrationofBCinthe ZambiansandyloamAcrisolandloamyfinesandArenosoldiffered inmagnitudefromthatreportedbyHaefeleetal.(2011)whofound annualdownwardmigrationratesofupto50%oftheappliedrice huskBCinhumicNitisolandgleyicAcrisolinthePhilippinesand Thailand,respectively.Majoretal.(2010)ontheotherhandfound averylowannualdownwardmigrationrateof<1%oftheapplied BCinsandyclayloamFerralsolinColombia.

Severalfactorsmayinfluencemigration ratesofBCtolower horizonsincludingBCparticlesize,tillagepractice,soiltexture,soil structure/aggregation,hydraulicconductivityandrainfallamount.

OurresultssuggestthatthefinertheBC,thefasteritwillmigrateto deeperhorizons(TOCinFig.1andrecoveredBCinTable3).The TOCcontentofthe0–5cmdepthintervalwassmallerfor0.5mm than0.5–1mmBCamendedplotsinKaoma(TOC=1.290.02%for the0.5mmBCvs1.570.05%for0.5–1mmBCplots).Thiswas notjustbecauseofsmallerTOCcontentof0.5mmBC(Table1) but also because of greater downward migration (Fig. 1 and Table 3). Tillage practice using planting basins, as common in conservationagriculture,mayaidtheincreasedmigrationrateof BCinsoilbycreatingbigsoil-packingvoidsthatmaybefilled,due tosubsequentpreferential colloidaland particletransport with percolatingwatertodeepersoilhorizon.Inthisstudy,wesuspect thatsuchpackingvoidswerethemainfactorresponsibleforthe slightlygreaterdownwardmigrationofBCinMkushisandyloam comparedtoKaomaloamyfinesand.TheKaomasoillackspacking voidsasthesandysoil(85%sand)doesnotexhibitanysignificant

Table3

Amountofbiocharrecoveredinthe0–20cmsoildepth,oneyearafterestablishingtheexperiment,ofatotalof625gofbiocharapplied.Computationsbasedond^13CandTOC contents(Eq.(3)).

Soildepth(cm) Mkushisandyloam Kaomaloamyfinesand SE

ReferenceplotBC (g)

0.5mmricehuskBC (g)

0.5–1mmmaizecobBC (g)

ReferenceplotBC (g)

0.5mmricehuskBC (g)

0.5–1mmricehuskBC (g)

0–5 40.2a 326.8bc 223.1b 56.5a 350.3c 416.9c 34.0

5–6 0.5a 62.9b 32.6ab 0.8a 46.4b 41.4ab 12.7

6–7 0.1a 30.2a 13.8a 1.2a 17.8a 4.4a 9.3

7–8 0.5a 13.4b 2.5a 0.8a 2.0a 3.2ab 3.2

8–9 0.1a 4.4a 1.4a 0.8a 1.5a 2.0a 1.0

9–10 0.3a 2.0a 1.3a 0.2a 1.2a 1.9a 0.7

10–15 0.0a 2.5a 5.3a 2.4a 7.0a 4.4a 3.3

15–20 0.0a 1.2a 4.2a 2.3a 3.6a 1.9a 2.5

TotalBC recovered

41.6a 446.4c 284.2b 64.9a 429.8c 476.2c 35.1

AmountofBCaremeanvalues(n=3)recoveredineachdepthofsoilwithasinglestandarderror(SE)forthethreeBCtreatmentsatthetwosites.Differentlettersfollowing meansforeachsoildepthindicatesignificantdifferencebetweenBCtreatmentsandbetweensites,Tukey’stest,p<0.05.

extentofaggregation,asshowninourpreviousworkonthesame sites(Obiaetal.,2016).Theslightlygreatermigrationdepthinthe sandyloamatMkushicanalsobeexplainedbyitshigherrainfall (1220mmyr¹)comparedwithKaoma(930mmyr¹).Theimpor- tanceofwater percolationforBCmovementhasbeenreported previouslyforsandyclayloamFerralsol(Majoretal.,2010).The waterflowrateinthesoil,asemphasizedbyHaefeleetal.(2011), appeared less important in the present study in determining migrationrateofBC.Thiswasshownbythesmallermigrationrate inKaoma,whichhadhighersaturatedhydraulicconductivitythan Mkushi(5.2vs1.7cmh¹measuredusingtensiondiscinfiltrom- eters;Obiaetal.,unpublisheddata).

AlthoughthedownwardmigrationofBCwasmainlywithina fewcmbelowtheapplicationdepth,thetotalBCrecoveryforthe 0–20cmdepthintervalsampledwaslessthantheamountofBC applied.TheamountofmaizecobBCrecoveredintheapplication layer(0–5cmsoildepth)atMkushi(basedonthechangeinTOC content)wassmaller(55%)thanthatofricehuskBCtreatments (69–76%)inbothMkushiandKaoma(p<0.05)(Table3andS1).

However,theamountofmaizecobBCrecoveredinthe5–20cm depthprofile,i.e., belowtheapplicationlayer,wasofthesame orderofmagnitudeasthatoftheothertreatmentsatbothsites.

Forexample,therewassimilarpatternintherecoveryofmaize cob BC and rice husk BC of 0.5–1mm at Mkushi andKaoma, respectively in 5–20cm depth (Table 3). The TOC contents corroboratedthe similarityof thetrends in thedistributionof maizecobandricehuskBCsbelowapplicationdepthatbothsites (TableS2).MaizecobBChadhigherTOCcontentsthanricehusk BC(Table1)andthesimilarTOCcontentsofthesetwoBCsinthe applicationlayer(0–5cm)(Fig.1)indicatedthatmoremaizecob BCmusthavemovedoutoftheplots.ThetotalrecoveryofBCat 0–20cmdepthwas55–76%,leavingbetween24and45%ofthe appliedBCunaccounted.

TheunrecoveredBCofatleast24%inthetop20cmofsoilcanbe attributedto(i)lossduetodecomposition,(ii)migrationassolid BCordissolvedorganicmattertosoillayersbelow20cmdepth, whichwerenotsampled,and(iii)laterallossduetobothwaterand wind erosionof thesurface soil.The decomposition rate ofBC producedwithinthesametemperaturerangeasourBCshasbeen reportedtobesmall,withvaluesintherangeof1%yr¹within thefirstyearofapplication(Carlssonetal.,2012;Kuzyakovetal., 2009;Luoetal.,2011).Someof thestudiese.g.Kuzyakovetal.

(2009)wereconductedunderoptimalconditionoftemperature and moisture throughout the year, implying that under non- continuousoptimalconditionsinthefield,thedecompositionrate is expected tobe smaller. This might especially be trueunder Zambianconditions where hardlyany rainfall occursfor seven monthsperyear.Decompositionlossesarethereforeexpectedto be<5%.MigrationofBCtosoillayersbelow20cmdepthmainlyas dissolvedorganicCwasmostlikelysmallaswell.InTable1,the watersolubleconstituentsofBCwere2%,andinourearlierrice huskBCwashingexperiment(Obiaetal.,2015),dissolvedorganicC consistedofonly2.4%ofthetotalconstituentsofBCleachate.Inthe soildepthintervalof15–20cm,theaccumulatedBCwasaslittleas 0.2–0.7%(Table3andS1).Thus,BCtransfertobelowthe20cmsoil depthisatmostofthesameorderofmagnitude(<1.4%oftotal BC).AlsoHaefeleetal.(2011),despiteobservinghighextentsofBC leaching,foundnoindicationofrice husk BCmigrationbeyond 30cmsoildepth,fouryearsafterBCapplication.Majoretal.(2010) foundverysmallamounts(<1%)ofBCmovingwithpercolating water in intact subsoil below theapplication depth (10cm) at 15cm depth both as dissolved and particulate organicC. Thus, overallatleast20%oftheBCaddedtotheMkushiandKaomasoils wasnotaccountedforandcouldnotbeattributedtoleachingto deepersoillayersortodecomposition.

BlackC,whichissimilartoBCwithrespecttoforexampletheir lowdensityrelativetosoil,hasbeenshowntoundergopreferential watererosion(Rumpeletal.,2006).LaterallossesofBCthrough erosionbywaterandwind,couldaccountforthenon-recoveredBC inthepresentstudyaswell.Theimportanceoflateraltransportat theMkushiandKaomasitesissupportedbythechangein

d

^13Cof

thereferenceplotsadjacenttotheBC-treatedones.TheBCinthe referenceplotsispartoftheBCnotrecoveredinthe0–20cmdepth oftheamendedplots.ThisBCinthereferenceplotswasprobably brought by wind and water erosion from amended plots and indicatethat much of themissingBC (24–45%)mayhavebeen transportedoutsidetheexperiment.Transportandsomeexchange ofBCalsolikelyoccurredbetweenBCtreatments.However,the insignificantdifferencebetweentherecoveredBCfrom0.5plots andthatfrom0.5–1mmBCplotsinthetop0–5cmsoildepthofthe twositesindicatesthatnettransportbetweentreatmentplotswas notsignificant.

TherewasgreaterlateraltransportofBCinloamyfinesandat KaomathaninaggregatingsandyloamatMkushi(Table3).The opposite was true for downward transport where there was smaller downwardtransport in loamy fine sand than in sandy loam.SinceweworkwithfineBCswithsizeslessthan1mm,the lateral transport through erosion and downward migration of particulate BC observed here is most likely the upper limits.

Usuallymuchcoarser,hand-crushedBCwillbeapplied,whichis less vulnerable to downward migration and erosional lateral transport.

5.Conclusions

Inthisstudy,weshowedthatsignificantdownwardmigration offinesizefractionsofBC(0.5mmand0.5–1mm)onceapplied tosoilwasmainlylimitedto3cmbelowtheapplicationdepth.

Therewasatendencyforsomewhatgreaterdownwardmigration ofthefinerBCsizefraction.Slightlygreaterdownwardmigration ofBCintheMkushisandyloamcomparedtotheKaomaloamyfine sandwaslikelycausedbymorerainfallandthepresenceofpacking voidsthroughwhichBCcouldmovedownwardwithpercolating water.InKaomaloamyfinesand,withitssingle-grain structure devoidofaggregates,formationofpackingvoiddoesnothappen.

In this study,between45 and66% ofBC wasfoundwithinthe applicationdepthafteroneyear.Afurther10–20%movedbelow theapplicationdepth.Thismeansthatbetween24and45%ofthe BC was not recovered in the upper 20cm of the soil profile.

TransportationofBCtoadjacentreferenceplotsindicatesthata largepartoftheunrecoveredBCwastransportedlaterallythrough erosion.

Acknowledgments

Weacknowledgeourfunder,theNorwegianResearchCouncil (NFR)underFRIMUFprojectNo.204112andNMBUPhDinternal financingtothefirstauthor.Co-fundingfortheworkwasobtained from NFR FriPro project No. 217918. We are thankful to the Conservation Farming Unit, Zambia for thesupport during the experimentsandsupervisionoffieldsites.WethankJeremySelby andKebbyKasangafortakingcareoftheexperimentsinMkushi andKaomarespectivelyandStableIsotopeFacility,Universityof California, Davis for analysisof oursamples for

d

^{13C and total}

carbon.

AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.still.2016.07.016.

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