Conversion of forest to agriculture in A

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ContentslistsavailableatScienceDirect

Agriculture,

Ecosystems

and

Environment

j ou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / a g e e

Conversion

of

forest

to

agriculture

in

Amazonia

with

the

chop-and-mulch

method:

Does

it

improve

the

soil

carbon

stock?

Anne-Sophie

Perrin

a,∗

_,

_Kenji

_Fujisaki

a,b

_,

_Caroline

_Petitjean

c

_,

_Max

_Sarrazin

d

_,

Mathieu

Godet

a

_,

_Bernard

_Garric

a

_,

_Jean-Claude

_Horth

a,e

_,

_Luiz

_Carlos

_Balbino

f

_,

Austrelino

Silveira

Filho

g

_,

_Pedro

_Luiz

_Oliveira

_de

_Almeida

_Machado

h

_,

_Michel

_Brossard

b

a_Centre_Technique_{Interprofessionnel}_des_Oléagineux_et_du_Chanvre_(CETIOM),_{Etablissement}_Public_Local_{d’Enseignement}_et_de_Formation_{Professionnelle}

Agricole(EPLEFPA)delaGuyane,SavaneMatiti,BP53,97355Macouria,Guyanefranc¸aise,France

b_IRD_–_UMR₂₁₀_Eco&Sols_(INRA,_SupAgro,_CIRAD,_IRD),_bâtiment_12,₂_place_Viala,_F-34060_Montpellier_Cedex_02,_France c_CNRS_–_Université_des_Antilles_et_de_la_Guyane,_UMR_EcoFoG,_Campus_agronomique,₉₇₃₁₀_Kourou,_Guyane_franç_aise,_France d_IRD_–_US122,_Laboratoire_des_Moyens_Analytiques_(LAMA),_route_de_Montabo,_F-97323_Cayenne_Cedex,_Guyane_franç_aise,_France e_Chambre_{d’Agriculture}_de_Guyane,₈_avenue_du_Général_de_Gaulle,_BP_544,_F-97333_Cayenne_Cedex,_Guyane_franç_aise,_France f_EMBRAPA_Cerrados,_Cx_Postal_08223,_CEP_73310-970_Planaltina,_DF,_Brazil

g_EMBRAPA_Amazonia_Oriental,_Cx_Postal_48,_CEP₆₆₉₁_7-900_Belém,_PA,_Brazil

h_EMBRAPA_Arroz_e_Feijao,_Cx_Postal_179,_CEP_75375-000_Santo_Antonio_de_Goias,_GO,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15April2013

Receivedinrevisedform1November2013 Accepted8November2013

Keywords:

FrenchGuiana Fire-free Deforestation Annualcrops Brachiaria No-tillage

a

b

s

t

r

a

c

t

Fire-freeforestconversionwithorganicinputsasanalternativetoslash-and-burncouldimprove agro-ecosystemsustainability.Weassessedsoilcarbonmasschangesinasandy–clayeyandwell-drainedsoil inFrenchGuianaafterforestclearingbythechop-and-mulchmethodandcropestablishment.Atthe experimentalsiteofCombi,nativeforestwascutdowninOctober2008;woodybiomasswaschopped andincorporatedintothetop20cmofsoil.Afteraboutoneyearoflegumeandgrasscover,threeforms oflandmanagementwerecompared:grassland(Urochloaruziziensis),maize/soybeancroprotationwith disktillageandindirectseedingwithouttillage.Therewerefourreplicates.Wemeasured14.16kgm−2 ofcarbonin2mm-sievedsoildownto2mdepthfortheinitialforest.Forestclearingdidnotinduce significantsoilcompaction;neitherdidanyspecificagriculturalpractice.Inconvertedsoils,Cstocks weremeasuredinthe0–30cmlayeraftereachcropforthreeyears.Carbonmasschangesforsoil frac-tions<2mm(soilCstock)and>2mm(soilCpool)inthe0–5,5–10,10–20and20–30cmsoillayers wereassessedonanequivalentsoilmassbasis.Oneyearand1.5yearsafterdeforestation,higherC stocks(+0.64to1.16kgCm−2_yr−1₎_and_C_pools_(+0.52_to_0.90_kg_C_m−2_yr−1₎_were_measured_in_converted soils,comparedtothoseoftheforestintothetop30cmofsoil.However,themassesofcarboninthese convertedsoilsdeclinedlater.Thehighestratesofcarbondecreaseweremeasuredbetween1.5and2 yearsafterforestconversioninthe<2mmsoilfraction,from0.46kgCm−2_yr−1_(in_grassland_soils)_to 0.71kgCm−2_yr−1_(in_cropland_under_no_tillage)._The_carbon_pool_declined_during_the_third_year_at_rates of0.41kgCm−2_yr−1_(cropland_under_disk_tillage)_to_0.76_kg_C_m−2_yr−1_(grassland_soils)._Three_years_after forestconversion,Cmassesinthetop30cmofsoilsforgrasslandshowedsimilarvaluesthanforforest. Incomparison,thecarbonstockincroppedsoilsmanagedundernotillageindirectseeding(without mulch)wassignificantly17%and16%lowerthaninforestandgrasslandsoils,respectively.Noneofthe studiedagriculturalpracticessucceededinaccumulatingcarbonfromthechoppedforestbiomass.

1. Introduction

Despite theirimportance,thedynamics of soilorganic

mat-ter in relation to changes in land use, as well as agro-system

∗_{Corresponding}_author.

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

sustainability, are still not given sufficient consideration,

espe-cially in tropical humid and equatorial climates (e.g. Powlson

et al.,2011; Lal, 2012).Between2000 and 2007,grosstropical

deforestation is estimatedtohave resultedin CO2–Cemissions

of2.82±0.45PgCyr−1(including1.37inLatinAmerica)thathas

to be compared to the C sink due to tropical forest regrowth

of1.72±0.54PgCyr−1 (0.86inLatinAmerica)(Panetal.,2011).

Thus, emission from tropical land-use change is estimated to

1.10±0.70Pgyr−1 of equivalent CO₂–C(0.51 in LatinAmerica)

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102 184 (2014) 101–114

andrepresentsapproximatelytheemissionfromgloballand-use

change.Duringthe2000–2007period,landusechangeinthe

trop-icscontributedtoabout12%ofglobalgreenhousegasesemissions

(∼6%forLatinAmerica)(GCP,2012).

Intheseregions,slash-and-burnpracticesarewidelyusedto

convertlandfor animaland humanfoodproduction aswellas

forurbandevelopment.Toachieveasustainableagriculture,

slash-and-burnmustbecoupledwithsuitablefallowmanagement(e.g.

Denichetal.,2005).Althoughappropriateslash-and-burnpractices

cancontributetomaintainswiddenwithveryhighlevelof

bio-diversityinthetropics(e.g.Padochand Pinedo-Vasquez,2010),

much harm is attributableto current slash-and-burn practices.

Soilsinthehumidtropicsparticularlythosedevelopedonhighly

weatheredmaterialspresentlowactivityandvariable-chargeclays

suchaskaoliniteandoxy-hydroxidemineralswithverylowcation

exchangecapacity. Thus, improvementsto thecation exchange

capacityofthesesoilsarecloselyrelatedtotheirorganicmatter

content(e.g.Boyer,1982).Burningand installationofcropsand

pasturehave beenshowntodecreaselong-term biological(e.g.

Luizãoetal.,1992;Decaënsetal.,2004),chemical(e.g.Laletal., 1986;Sarrailh, 1990; Cerriet al.,1991; Desjardinsetal., 1994; McGrathetal.,2001;Palmetal.,2005;Vågenetal.,2006;Okore

etal.,2007)andphysical(e.g.GrimaldiandBoulet,1989;Chauvel

etal.,1991;Müller etal.,2004;Desjardinsetal.,1994;Koutika

etal.,1997)propertiesrelevanttoprovideadequatesoilfertility

forcropproduction.Inaddition,damagetohealthcanresultfrom

burningsuchaspulmonaryillnessduetosmokeorintoxication

bymercuryleachingintotheaquaticenvironment(e.g.Carmouze

etal.,2001;Farellaetal.,2006).Lessobviously,largescaleburning

offorestisalsothoughttoincreasethesizeoftherefractorypoolof

dissolvedorganiccarboninthedeepocean(Dittmaretal.,2012).

Furthermore,whencomparedtotheproducingarea,thedeforested

areaismuchgreaterinslash-and-burnbasedagrosystems,which

requirelandrotationandregularburning(Katoetal.,1999;Sanchez

etal.,2005;Denichetal.,2005).Consequently,whiledemographic

pressureexpands,thesepracticesleadtoincreasingprimaryforest

deforestation.

InFrenchGuiana,forestcoversmorethan96%oftheemerged

surface(FAOandITTO,2011).Demographicandeconomicgrowth

israpid(INSEE,2010;CEROM,2008)andtheareaofforest

con-vertedinto cropped areasfor subsistence or family/small scale

farminghasalmostdoubledbetween1990–2006and2006–2008

(IFN,2009a,b).

Alternativeagriculturalpracticesshouldaimtolimitsoilorganic

carbonlossesandparticulateemissionsfromburning(e.g.Brady,

1996).Forestconversionwithhighorganicinputs,asanalternative

toslash-and-burn,couldimproveagrosystemsustainability.The

effectsofaddingwoodyresiduesonsoilorganiccarbonstocksand

cropproductionsrequiremoreinvestigation,especiallyintropical

climates(e.g.Barthesetal.,2010).Slash-and-mulchor

chop-and-mulchproductionpracticesthatarespecificallyadaptedforuseon

steepslopesandinareasofmonsoonalclimatesweretraditionally

used,forexampleinCostaRica(locallynamedfrijotapado)where

therainfallissocontinuousastoprecludeburningofvegetation.

Inthiscontext,Bellowsetal.(1996)reportedthatnaturalfallows

andmulchingpracticesprovidenutrientrecycling,reducepestand

diseaseinfestationsandinhibitweedregrowth.Themulchlayer

isalsoreportedtoreduceoreliminatetheneedforcrop

mainte-nancelaborinputsandtocontrolnutrientlossesduetoerosion(e.g.

Bellowsetal.,1996).Morerecently,intheBrazilianstateofPará

intheeasternAmazonregion,inthecontextofsmall-scalefarms

andswidden-fallowagriculture,theuseofchop-and-mulch

prac-ticeshasbeenexpandedtoconservegoodcropproductionwith

shortenedfallowregenerationtime(Katoetal.,1999;Denichetal.,

2004,2005;Borneretal.,2007).Higherorganiccarboncontents

weremeasuredinthesystemwithonecroppingcycle,andtwo

successivecroppingcyclesleadtoincreasedsoilCandNof

micro-bialorigin.TreatmentswiththehighestlevelsofmicrobialCand

Nwerethosewheretheresidueswerecut,shredanddistributed

overthesoilsurface(Lopesetal.,2011).Fromanotherexperiment

inthesameregion,Comteetal.(2012)arguethatchop-and-mulch

ofenrichedfallowsduringtheconversionofsecondaryforestinto

cultivatedlandcouldcontributetotheaccumulationand

conser-vation oflargequantitiesof organicmatter and thusrepresent

animportantnutrientsupplier.CO2-equivalentemissionsrelated

toglobalwarming potential(GWP)analysiswere estimatedby

Davidsonetal.(2008)frommeasurementsofCH4,N2OandNO

emissionsfromsoiltoassessthefire-freealternativesystemused

inthisBrazilianregion.ResultsshowedthatGWPover100years

wasatleastfivetimeslowerinchop-and-mulchcomparedwith

slash-and-burntreatments,mainlyduetothelackofCH4andN2O

emissionsduringburningphase(67%and27%,respectivelyofthe

totalCO2-eqcalculatedforchop-and-mulch).Duetoanunsuitable

samplingmethod,theanalysisdidnotincludeanassessmentof

changesinthesoilorganiccarbonstock,soitmighthave

underes-timatedtheadvantageofthechop-and-mulchmethod.Thus,the

effectofthisfire-freeclearingmethodwithforestresidueinputs

onsoilcarbonstocksinAmazoniaremainsunknown.

Thepresentstudyfocusesonanexperimentwithneotropical

forestconversion.Thisexperimentwasconductedonadedicated

siteinFrench-Guianausingachop-and-mulchmethodwith

incor-porationofforestbiomassinthetop20cmofthesoil.Crops(a

maize/soybeanrotation,withandwithoutsoiltillage)and

grass-landsystemswereestablishedonsiteoneyearafterforestclearing

andwerecultivatedfor2years.

Theobjectiveofthisstudywastoassesstheimpactofforest

conversionintoagricultureusingthechop-and-mulchsystemon

short-termsoilorganiccarbonstocks.Carbonstockchangeswere

evaluatedbeforedeforestationandthenat4datesduringthefirst

three years followingforest conversion.Soil organiccarbon (C)

masschangesweremeasuredinfineandcoarsersoilfractionsand

theinfluenceofsoilmanagementpracticesonthesechangeswas

assessed.

2. Materialsandmethods

2.1. Sitedescriptionandexperimentaldesign

InFrenchGuianathewettropicalclimate(AMitype

accord-ingtoKöppen–Geigersystem,inPeeletal.,2007adaptedfrom

Köppen,1936)isdirectlyinfluencedbytheseasonalnorth/south

movementsof theInter-TropicalConvergenceZone, witha dry

seasonfrom mid-Augusttomid-November,and a rainy season

the rest of the year (usually interrupted by a short dry

sea-son in February/March). The experimental site of this study,

“Combi” (5◦₁₇′₅₅′′_N/52◦₅₅′₀₁′′_W), _is _located ₁₂_km _south _of

Sinnamary. Mean annual precipitation and mean annual air

temperatureare2771.2±628.8mmand 27.3±0.5◦_C;_minimum

rainfallisrecordedinSeptember(46.8±55.0mm)andmaximumin

May(477.7±190.1mm)(Météo-Francedata1970–2009).Monthly

rainfalldoesnotexceed110mmfromAugusttoNovemberwith

only11%ofthemeanannualprecipitationfallingduringthis

4-monthperiod.

Slopesarelessthan10%anddonotexceed7%over90%ofland

area.Thesandy–clayey,nutrient-poorsoilsareclassifiedas

“fer-rallitisolmeublekaolinitiquejaune”(AFES,2009)orHyperferralic

Ferralsol(FAO,2006).

Theclearingofthe2haofnativeforestoccurredatthe

begin-ning of October 2008 (dry season) (Figs. 1 and 2). Trees and

stemswithadiameteroflessthan15cmwerechoppedusinga

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Fig.1.ForestclearingattheCombiexperimentalsite:(A)choppingoftreesandstemsinprimaryforestwithahydraulicverticalaxismulcherequippedwithchains,mounted ona20tonwheeledexcavatorinOctober2008,(B)largewoodchipsincovercrops(April2009),(C)choppingoflargewoodchipsandcovercropswithaforestrymulcher mountedonaself-propelledwheeledmachineinOctober2009,and(D)soybeanplotsinJuly2010.

wheeledexcavator (Fig. 1A).Timber withnocommercialvalue

andresidualtrunkswerepiledevery40minwindrowsandwere

carefully removed from the site during the 2009 dry season.

Track-typetractorswereusedtominimizeimpacts(compaction

and depletion) on the surface soil layer. After soil liming and

incorporationwithdiscsto20cmdepth,covercropsweresown(a

grassesandlegumesmixture)andNPKfertiliserswereappliedon

soils(Fig.2).Onthe26thofOctober2009,thecovercropandlarge

woodchipsofforesttreeslyingonthesoilsurface(Fig.1B)were

choppedintosmallerpieces(uptoabout5–7cmlongchips)bya

forestrymulchermountedonaself-propelledwheeledmachine

andincorporatedintothe0–10cmdepthsoillayer(Fig.1C).

DISK TILLAGE

FOREST

Maize Soybean Maize Legumes cover

Maize + Uroch. Soybean Maize Legumes cover

Cover crops (Grasses & legumes)

T0

T1

T2

NO TILLAGE

Urochloa ruziziensis

Chop & mulch

T3

GRASSLAND

T1.5

(08-10-01)

Tillage 0-20cm

(09-1

1-18)

(10-04-25) _(10-10-04) ₍₁

1-1

1-07)

Fig.2.Historyoflanduseforthefourtreatmentscompared.November2008:neotropicalforestclearingfollowedbymanualspreadingofcrushedlimestone(1000kgha−1_: 50%CaO)anddolomite(450kgha−1_:_30%_CaO_and_20%_MgO)_on_soil_which_was_mixed_into_the_0–20_cm_layer_with_heavy_disk_harrow._January_2009:_manual_broadcast seedingofpaddyrice(OryzasativaL.),Urochloaruziziensiscv.ruzi(oftenreferredtotheliteratureasB.ruziziensis),StylosanthescapitataandStylosanthesmacrocephalacv.BRS CampoGrande,CalopogoniummucunoïdesDesv.wascarriedout,whichreceived40.5kgha−1_of_P2O5_(super_phosphate_granules_containing_46%_of_Ca2+₎_and_25.5_kg_ha−1_of N,P2O5andK2O.InNovember2009:settingupofcroppingsystems,fourtreatments:(1)neotropicalforest,(2)grasslandbasedonUrochloaruziziensiscv.ruzimowed2–3 timesannually,(3)conventionaltillagebasedonsurfaceplowingwithdiskstwiceayearandmaize(ZeamaysL.)/soybean(GlycinemaxL.Merr)annualcroprotation,and(4) directseedingofmaize/soybeanannualcroprotation(no-tillage).GandCTplotswerepreviouslytilledwithanoffsetdiskharrow.InNovembereachyear,CTandDSplots received450and300kgha−1_of_CaO_and_MgO,_{respectively.}_In_grassland_parcels,_the_same_doses_of_CaO_and_MgO_were_applied_beginning_of_November₂₀₁₀_only._Grassland developedfromsowingofUrochloaruziziensiscv.Ruzi.andofsmallquantityofregrowthplantsofthepreviouscoverplants.Grasslandplotsreceivedrespectively51–51–51 and50–60–60kgha−1_of_{N–P2O5–K2O}_respectively_on_December₂₀₀₉_and_June_2011._Grassland_biomass_was_cut_and_either_removed_at_the_beginning_of_September₂₀₁₀ andApril2011,andattheendofAugust2011ormowedandleftonsoilsatthebeginningofMay2010andmiddleofJanuary2011.InDecember2009:maizewassownas wellasUrochloaruziziensiscv.ruziineachinter-rowofDSplotandreceived136–90–90kgha−1_of_{N–P2O5–K2O,}_{respectively.}_In_May_and_June_2010:_soybean_was_sown_for CTandDSplots,respectively(seedswerepreviouslyinoculatedwithpeat).TheplantingofUrochloaruziziensiscv.ruziandStylosanthesguianensiscv.Campograndefailed inDS.Soybeanreceived114–120kgha−1_of_P2O5–K2O._In_December_2010,_maize_was_sown_and_received_{169–111–132}_kg_ha−1_of_{N–P2O5–K2O,}_{respectively.}_In_June_2011,

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104 184 (2014) 101–114

Threemanagementsystems(Fig.2)weresetup(outsidethe

windrow areas) with a randomized complete block design of

10m×20mplotsdistributedovertheareatakingintoaccountof

declivity(fourreplicatesforeachtreatment):

-G:GrasslandofUrochloaruziziensiscv.ruzi(syn.Brachiaria

ruz-iziensis)notgrazedbutmowed2–3timesperyear

-NT:maize(ZeamaysL.)/soybean(GlycinemaxL.Merr)rotation

undernotillageandmanagedwithdirectseeding

-DT:maize(Z.maysL.)/soybean(G.maxL.Merr)rotationunder

disktillageusingtwopassesofaheavydiskharrow.

Theadjacentforestsitewasusedasareferencesite(Fig.2).

2.2. Soilsampling,physicalandchemicalanalyses

Duringthedryseason(endofSeptember2008)andjustbefore

sitedeforestation,soilcoreswerecarefullycollectedin17pitsof

dimensions0.60m×0.60m×0.60mdistributedoverthearea.The

litterlayerwascarefullyremovedbeforesamplings.Deepersoil

sampleswerecollecteddownto2mdepthintwosoilpits.Cores

(0.05mthickness×0.10mdiameter)werecollectedcontinuously

downto0.75mdepth(coreswerecollectedonenexttotheother

toavoidsoilcompaction).

In agricultural plots, soil sample cores (0.05m

thick-ness×0.08m diameter) were collected following a regular

gridsampling methodwitha hand auger atsix points ineach

10m×20mplots(evenly distributedover theplot,atleast3m

fromtheedges,intheinter-row).Collectedsoillayerswere0–5,

5–10,10–20and20–30cm.Twocoresof0.05mthicknesswere

mixed toform the 10–20 and 20–30cm layers. These samples

weretakenafterthecropharvestsduringdryperiodinNovember

2009(T1)andthenatthebeginningofApril2010,andinOctober

2010andNovember2011.

Soilsampleswereair-driedandsieved (2mm)before

analy-ses.Forallsamples,plantresiduesandmineralsoilfractionslarger

than2mmwerecarefullyisolatedandweighed.Specialcarewas

takentoseparatethefineplantresiduesfromthefineearth;plant

residuesthatpassedthroughthe2mmsievewereisolated and

addedtoplantdebrisofthesoilfraction>2mm.Thesampleswere

weighedand theirmoisturecontentwasdeterminedona

sub-sample(ca.30g)afterovendryingat105◦_C_for₄₈_h_to_obtain_the

drymassinordertocalculatethebulkdensity.Theplantresidues

fractionwasovendried at 60◦_C _for ₇₂_h_and _weighed_prior _to

analysis.Soilbulkdensity(Db)wasdeterminedonallsoilsamples

collectedwithcylindersof392.5cm3_(forest_soils)_and_251.2_cm3

(agriculturalsoils).

Particlesizedistributionsweredeterminedforthe<2mmsoil

fractionsoneachforestsoilsampleafterhexametaphosphate

dis-persionandsedimentation(manualpipettemethod).SoilpHin

water(1:2.5M:M)and1NKClweredetermined.Theexchangeable

cationsCa2+_,_Mg2+_,_Na+ _and_K+_were_extracted_from_all

individ-ualsamplesinasolutionofammoniumacetate(1N,pH7.0)and

analyzedusingatomicabsorptionandemissionspectrometry

Var-ian AA1275 (NF-X31-108, AFNOR, 1996). P was extracted in a

sodium bicarbonate and ammonium fluoride solution (pH 8.5)

(Olsenmodified, Dabinetal.,1967)andmeasuredby

colorime-tryaftercomplexationofphosphateswithammoniummolybdate

in the presence of antimony (III) and reduction with ascorbic

acid(MurphyandRiley,1962).Cationexchangeablecapacitywas

determinedinaccordancewiththenormAFNOR(1996)

NF-X31-310 standard. Exchangeable acidity and potential acidity were

determinedafteranextractionwith1NKClandsodiumacetate

0.5N(pH7.0)solutions,respectively(EMBRAPA,1997).Al3+_and

H+_were_analyzed_by_titration_using_a _Metrohm_E₅₃₆

potentio-graphequippedwitha665DosimatMetrohm(Metrohm,Herisau,

Switzerland).TotalCandNcontentsinsoilandplantresidue

sam-plesweremeasuredbydrycombustionusingaThermoQuestNA

2100analyzeroncrushedsamples(<200␮m)(NFISO10694andNF

ISO13878).Theanalyzerwaspreviouslycalibratedwithacetanilide

C8H9NO(CEinstruments33836700).The standardrangeswere

checkedusingSoilReferenceMaterialforNandC(detn◦₃₃₈₄₀₀₂₅

lotN12A)andtheprecisionsofCandNcontentmeasurementswere

1.29%and0.5%,respectively.Inthisexperiment,soilorganic

car-boncontentwasassumedtobeequaltototalcarboncontentsince

inorganiccarboncanbeignored.

2.3. Samplingandanalysesofbiomassinputsandoutputs

Afewdaysafterforestclearing,thewoodybiomasslyingon

thesoilsurface(excludingthelitterbiomass)wasquantifiedin14

randomlydistributed quadrantsof0.65m2 _over _the

experimen-talsite.Sampleswereair-driedfor7monthsinanair-conditioned

roomat25◦_C_before_being_weighed_and_crushed_into_smaller_pieces

(<10cm).Justbeforeharvest,thefreshbiomassofcropswas

esti-matedineachplotusingsamplesof2adjacentareas(1linearmeter

foreach)i.e.3and2m2_in_a_location_chosen_randomly_in_maize

andsoybeancrops,respectively.Foreachsample,thestoverwas

separatedfromthegrain,driedand weighedtodeterminetotal

drymatterandgrainyield.Thebiomassofmulchcover(mainlyU.

ruziziensis)andweedcontributionsweremeasuredfrom3m2_and

2m2_samples._Total_grass_biomass_returned_to_soils_or_exported_was

measuredjustbeforecutting(harvestorregenerationcutting)from

two1m2_quadrants_per_plot._The_quantities_of_biomass_(crops_or

grassresidues)returnedtosoilscorrespondtothetotaldrymatter

ofthesamples(minustheexportedgrainformaizeandsoybean).

Allsub-samplesweredriedat65◦_C_until_constant_weight,_then

weighedandgroundtoformapowdersuitableforchemical

anal-yses.Cinputsfrombiomasswerequantifiedbymultiplyingtheir

drymatterbythecorrespondingmeanCcontent,measuredbydry

combustionusinga ThermoQuestNA 2100analyzeroncrushed

samples(<200␮m).Standardrangeswerecheckedusingstandard

referencematerial1573a(tomatoleaves)attestedbytheNational

InstituteofStandardsand Technology(USA)and theprecisions

were2.0%and1.08%,respectivelyforNandCanalyses.Themassof

carboncontainedinasampleisobtainedbymultiplyingtheplant

massby42.7%forforest(meancarboncontentmeasuredinforest

rootsinBréchet,2009),andby47%foragriculturalplantresidues

(thisstudy,seeTable3).

2.4. Massesofcarboninsoilfractionsandcalculatingratesof change

Foreachsoilsample,thecarbonstockinthe<2mmsoilfraction

(C<2mminkgm−2)wascalculatedfromthemeasuredsoilorganic

carboncontent(gCkg−1₎_in_the_<2_mm_soil_fraction_for_the_layer

thicknessconcerned(lindm):

C<2mm=SF_(<2mm)×C_(<2mm)×Db×1,

whereSF(<2mm)istheproportion(%,w/w)of<2mmsoilfraction

inthewholedrysoilsample,C(<2mm)istheorganiccarbon

con-centration(gCkg−1₎_of _this_fraction_and _D

b is soilbulk density

(Mgm−3_).

Themassofcarboncontainedinthe>2mmsoilfraction(C>2mm

inkgm−2₎_is_calculated_as:

C>2mm=SF_(>2mm)×C_(>2mm)×Db×1

whereSF(>2mm)istheproportion(%,w/w)ofthe>2mmsoilfraction

inthewholedrysoilsampleandC(>2mm)istheCcontent(gCkg−1)

ofthisfraction.

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ThemassesofC<2mmandC>2mmfortheagriculturaltreatments

werecalculatedonanequivalentmasstoaccountfordifferences

inbulkdensitiesasrecommendedbyEllertandBettany(1995)and

EllertandGregorich(1996).Forestsoilwasusedasareferencei.e.

carbonstocksinagriculturaltreatmentswerecorrectedtoreferto

thesamesoilmassasintheforestsoillayerconcerned.Cstocks

andpoolswerecalculatedbysubtractingthetotalCconcentration

oftheextra-weightedofsoilinthedeepestlayer(either0–5,5–10,

10–20or20–30cmforthestocksorpoolsto5,10,20and30cm,

respectively).

ChangesinC<2mmorC>2mmmassesovertimeatsuccessivetime

periodsofthe3-yearstudywerecalculatedas:

tn−tn−1C=C_n−C_n−₁

where tn−tn−1C is the rate of change of soil C mass

(kgCm−2_year−1_); _Cn _and_Cn

−1 arethesoil C masses(kgCm−2)

measuredatsamplingdatetn andprevioussamplingdatet(n−1),

respectively.

Inthis study,theCcontainedinthe<2mmfractionsis

gen-erallyreferred toasthe“carbonstock” intheliteraturesinceit

isconsideredtobethemorestablesoilcarbonfraction.C>2mmis

herereferredtoasthecarbonpool,whichactsasasupplytothe

<2mmsoilfraction.Theconsiderationofpoolsisessentialinthis

studydealingwithshort-termland-usechangewithhighinputsof

organicmatterandnon-stabilizedsoils.

2.5. Statisticalanalysis

For all variables, treatment means, sampling dates or time

periods were compared using one-way analysis of variance

(ANOVA), after verification of the normal distribution of data.

ANOVAwasfollowedbytheTukeyorDunnett(forcomparisonwith

referenceforest)posthoctestatasignificancelevelof0.05(ifnot

specified).Inthesecomparisons,weconsideredthatCmassesin

forestsoillayersdidnotchangeduringthetimeoftheexperiment.

ThesestatisticalanalyseswereconductedusingXLSTAT

soft-wareversion7.5.2(Addinsoft®_).

3. Results

3.1. Physicalandchemicalpropertiesandcarbonstockofforestsoils

Thesoilsofthesitehaveasandy–clayeytextureinthe0–0.2mlayer(clay con-tent259±28gkg−1)toaclayey–sandytextureinthe0.2–2mlayers(claycontent

362±22gkg−1).Gravelsrepresentabout78,119,91and66gkg−1inthe0–20,

20–30,30–60and>60cmoftheforestbulksoil.Coarsesandinthe<2mmsoil frac-tionrepresentmorethan70%oftotalsand(Table1).Siltrepresentsonly2.9–4.6%. Soilbulkdensitiesareincreasingfrom1.02insurface5cmto1.51onaveragebelow 45cm(Table2).

The low values measured throughout the soil profile for pH, CEC, and basesaturation(v)arecharacteristicofAmazonianferralsols.Exchangeable alu-minumconcentrationsrangebetween1.2cmol(+)kg−1_in_the_0–10_cm_layer_and 0.6cmol(+)kg−1_in_the_30–60_cm_layer._These_soils_are_very_poor_in_total_nitrogen (<2gkg−1₎_and_extractable_phosphate_(<8_mg_kg−1_)._These_low_values_are compara-bletothosepreviouslyrecordedinFrenchGuiana(e.g.Lévêque,1967).

Intheforestsoils,meanbulkdensityis1.02Mgm−3_in_the_0–5_cm_layer_and increasesprogressivelyupto1.47Mgm−3_in_the_20–30_cm_layer₍_Table₂_and_Fig.₃_). Meancarbonconcentrations(Table2)are26.80gkg−1 _in_the_first₅_cm_of_soil, <20gkg−1_from₁₀_to₂₅_cm,_<10_g_kg−1_from₂₅_to₅₅_cm_and_then_<5_g_kg−1_down to200cm.Cstocksinforestsoilsare5.50±0.68kgCm−2inthefirst30cmofsoil,

6.55±0.71kgCm−2downto40cm,9.67kgCm−2and14.16kgCm−2inthefirst

meterandinthe0–2msoilprofile,respectively.

3.2. Biomassandcarboninputstosoilsduringthechop-and-mulchexperiment

AerialbiomassinputstosoilsintheconvertedplotsareshowninTable3.A biomass(choppedvegetation)of24.05±14.28Mgofdrymatterperhectarewas

lyingonthesoilafterforestclearing(Table3).Thelargestandarddeviationonthis measurementistheresultofthespatialvariabilityofbiomassinputsduetonatural variationintheagesandspeciesoftreesintheinitialforest.However,thisvariation wasdecreasedbytheuseofaforestrymulcherinNovember2009priortocrop

establishment,whichincorporatedsmallerwoodchipsintothesurface0–10cmof Table

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106 184 (2014) 101–114

Table2

Soilbulkdensity(Mgm−3_),_organic_carbon_content_(g_kg−1₎_and_carbon_stock_(kg_C_m−2₎_in_soil_fraction_<2_mm_at_Combi_site_before_forest_clearing.

Soillayer(cm) Bulkdensity(Mgm−3₎

Mean±SE

Carbon(gkg−1_of_soil)

Mean±SE

Carbonstock(kgCm−2₎

Mean±SE

0–5 1.02±0.04 26.80±2.34 1.24±0.06

5–10 1.26±0.03 18.10±1.85 1.05±0.06

10–15 1.30±0.03 15.90±1.17 0.95±0.03

15–20 1.40±0.02 12.90±1.02 0.81±0.04

20–25 1.45±0.03 11.60±0.79 0.81±0.04

25–30 1.49±0.02 9.50±0.68 0.63±0.03

30–35 1.49±0.03 8.10±0.34 0.58±0.03

35–40 1.45±0.02 7.30±0.38 0.48±0.01

40–45 1.46±0.01 6.80±0.04 0.44±0.02

45–50 n.m. n.m. n.m.

50–55 1.52±0.00 5.10±0.42 0.36±0.02

55–60 1.48 4.70 0.32

60–65 1.50 4.60 0.32

65–70 1.61 4.40 0.33

70–75 1.48 4.00 0.27

180–200 1.47 3.00 0.82

n.m.,notmeasured.

Mean±standarderror,n=17perlayerexceptfor20–25cm,30–35cmand40–45cm(n=3),>55cm(n=2)and180–200cm(n=1).

thesoil(Fig.1).Duringforestconversiontofarmland,alloftheforestlitterandpart oftherootswerealsoincorporatedintothesoilsurface20cmatthesametimeas thewoodchipsandagriculturallimeinDecember2008.Duringthestudiedperiod, totalaerialbiomassinputstothesoilwere67.5±15.78and65.3±15.65Mgofdry

matterperhectareintreatmentsNTandDT,respectively.Inputsfromchopped vegetationresiduesandlitterfall,fromyearonecovercrops(legumeandgrass)and

fromcrops/coverplantsresiduesrepresent49.5–51.2%,15.8–16.4%and34.6–32.4% oftotalinputs,respectively.

Afterthefirstharvestofmaize,thequantitiesofaerialbiomasswhichwere returnedtothesoilinNTplots(8.5MgDMha−1₎_were_{significantly}_higher_than inDT(4.9MgDMha−1_)._Restitutions_after_the_other_crops_were_not_{significantly} differentforDTandNTtreatmentsbecausetheestablishmentofthecoverplant(U.

Table3

Drymatter(Mgha−1₎_and_C_and_N_contents_(g_kg−1₎_of_above-ground_biomass_returned_to_soils_or_removed_from_plots,_during_the_three_years_after_the_clearing_of_the_forested site.

Date Typeofabove-groundbiomass n Drymatter(Mgha−1₎

Mean±SD

TotalN% Mean±SD

TotalC Mean±SD

C/N Mean±SD

T0(October08) Forestchopped-biomass 15 24.1±14.3 0.5±0.1 46.9±2.5 108±31

Forestlitter 15 9.4±5.2* 1.1±0.3** 49.2±1.9**

T1(November09) Coverplants 9 10.7±3.4 0.9±0.1 45.5±0.3 53±5

T1.5(April10) MaizeNT(Stovers,leavesandspathes) 4 2.0a_±_3.4 _0.5_±_0.1 _45.3_±_0.4 ₈₅_±₁₀

UrochloaruziziensismulchNT 4 6.5±2.0 1.1±0.2 44.2±0.5 42±6

Residualcoverplantandwoodchips n.m. n.m. n.m. n.m.

MaizegrainsharvestedNT 4 6.5a±2.0 1.3±0.3 43.9±0.4 34±7

MaizeDT(Stovers,leavesandspathes) 4 3.9b_±_0.6 _0.4_±_0.1 _45.6_±_0.4 ₁₁₇_±₂₃

Urochloaruziziensis−regrowthDT 4 1.0±0.7 1.5±0.4 43.3±1.0 30±7

MaizegrainsharvestedDT 4 5.7b_±_2.4 _1.1_±_0.1 _44.0_±_0.4 ₄₀_±₄

Grassrestitution(onMay2010) n.m. n.m. n.m. n.m.

Hayharvested 8 5.5±2.1 1.6±0.4 44.7±0.5 29±6

T2(October10) SoybeanstemsNT 4 0.3a_±_0.1 _0.5_±_0.1 _46.3_±_0.5 ₉₆_±₁₅ Soybeanleaves+residualmulchNT 4 2.8±0.1 0.7±0.1 46.2±0.3 71±12

SoybeangrainsharvestedNT 4 2.8a_±_0.4 _6.5_±_0.2 _53.3_±_0.4 ₈_±₀ SoybeanstemsDT 4 0.7b±0.1 0.5±0.1 45.6±0.3 104±23

SoybeanleavesDT 4 1.4±0.3 0.6±0.0 44.3±0.3 70±2

SoybeangrainsharvestedDT 4 2.0b_±_0.1 _6.4_±_0.3 _53.0_±_0.3 ₈_±₀

Grassrestitution No

Hayharvested 8 11.0±1.7 0.5±0.1 45.6±0.2 104±26

T2.5(April11) MaizeNT(Stovers,leavesandspathes) 4 7.5±0.9 0.8±0.1 45.6±0.2 61±7

Urochloaruzi+residualmulchNT 4 0.9±0.3 2.2±0.1 43.0±0.7 19±1

MaizegrainsharvestedNT 4 5.1±0.9 1.5±0.1 43.7±0.8 30±2

MaizeDT(Stovers,leavesandspathes) 4 7.9±1.3 0.7±0.1 45.8±0.3 64±12

MaizegrainsharvestedDT 4 5.0±1.0 1.6±0.1 43.9±0.2 28±2

Grassrestitution(onJanuary11) 8 2.6±0.3 1.2±0.1 44.8±0.3 38±5

Hayharvested 8 2.2±0.4 1.2±0.1 44.9±0.7 37±4

T3(November11) Crotalaria+StylorestitutionNT 4 5.4±1.2 1.5±0.1 45.6±0.1 31±3

MaizeresiduesNT n.m.n.s. n.m. n.m. n.m.

Crotalaria+StylorestitutionDT 4 6.2±0.2 1.4±0.2 45.6±0.2 32±3

Grassrestitution 8 No

Hayharvested 3.8±0.5 1.0±0.1 45.9±0.3 46±4

n.m.,notmeasured.T1.5:thequantitiesofaerialbiomass(maizestovers,leavesandspathes+Urochloaruziziensis)andthequantitiesofmaizegrainsharvestedwere signifi-cantlydifferentinNTandDTtreatmentsasindicatedbydifferentlowercaseletters(Tukeytest,p<0.05).T2:thequantitiesofaerialbiomass(soybeanstems+leaves+residual mulchforNT)andthequantitiesofsoybeangrainsharvestedweresignificantlydifferentinNTandDTtreatmentsasindicatedbylowercaseletters(Tukeytest,p<0.05).

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Fig.3. Soilbulkdensity(Mgm−3₎_and_C_content_(g_kg−1₎_of_the_soil_fraction_<2_mm_in_the_0–5,_5–10,_10–20_and_20–30_cm_layers_in_Combi_site_for_forest,_grassland_(G), maize/soybeancroprotationplotswithdisktillage(DT)andwithnotillageindirectseeding(NT).Meanandstandarderror.Meanvaluesfollowedbythesamelowercase letterforthesamelayerandthesamesamplingdatedidnotdiffersignificantlybytheTukeytest.Meanvaluesfollowedbythesameuppercaseletterforthesamelayerdid notdiffersignificantlyfromreferenceforest(TukeyandDunnetttests),p<0.05.

ruziziensis)wasunsuccessfulduetofailureoftheseedstogerminate.Hencethis treatmentishereconsideredasasimpledirectseedingmanagementwhichdiffers frommulch-baseddirectseeding.

Although litter was not sampled in our forested site, various data can be found in previous studies at the Guyaflux experimental site in Paracou (5◦₁₆′₅₄′′_N,₅₂◦₅₄′₄₄′′_W), _especially _in_forested_areas _showing_the _same_soil

parameters(http://www.gip-ecofor.org/f-ore-t/paracou.php),andthesametypical forest species composition (Petitjean, 2013). Janssens et al. (1998) reported 0.94±0.52kgm−2 of dry litterfall. A total forest litterfall of 8.3Mgha−1yr−1

wasestimatedby Chaveetal.(2010).Hättenschwiler etal. (2011)measured 492±19gkg−1ofC(drymatter)and11±3gNkg−1drymatterinleaflitterfrom

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108 184 (2014) 101–114

Carboninputsfromforestlitter,forestchopped-biomassandyearonecover cropsamountedto0.462±0.256,1.128±0.670,and0.487±0.153kgCm−2,

respec-tively.Thesethreeinputtypesinducedbylanduseconversionmethodaccounted for65.4%and67.6%oftotalaerialCinputstosoilsbetweenT0andT3forNTandDT, respectively.

Choppedforest-biomasshadhighC:Nratios(108.5±31.0)similartomaizeand

soybeanstems(85–117)whichwouldindicatelowmineralizationrates.U. ruz-iziensisingrasslandorasmulchinNThadC:Nvaluesoflessthanhalfthesevalues (Table3).

3.3. Soilbulkdensityandcarboncontentafterforestconversion

AtsamplingdateT1,bulkdensitiesrangebetween1.04and1.22Mgm−3_in_the 0–5cmlayerandincreaseprogressivelyto1.50–1.56Mgm−3_in_the_20–30_cm_layer foralltheagriculturalsoils(Fig.3).Bulkdensitiesdidnotdiffersignificantlybetween plotsofallthetreatmentswithinagivenblock(datanotshown),orconsideringthe wholesiteinthe0–30cmlayer.Thesevaluesincreasewithtimeforeachsoillayer. Comparedtoforestsoils,NTplotsweresignificantlymorecompactedinthe0–5and 10–20cmsoillayersatallsamplingdates,inthe20–30cmatT2andinthe5–10cm layeratT3.Disktillage(DT)soilshadasignificantlyhighermeanbulkdensitythan forestinthe10–20,20–30and0–5cmsoillayersatT1.5,T2andT3,respectively. FromT1toT3,foragivensoillayer,differencesinmeanbulkdensitiesremain smallbetweentreatments.Inno-tillageplots,meanbulkdensitiesweresignificantly higherinthe0–5cmlayeratT1thanintheothertreatments.Significantlylower valueswerefoundforgrasslandsoils(G)atT1.5below10cmcomparedtoother treatments.HoweveratT2soilbulkdensitieswerenotsignificantlydifferentfor anytreatmentsineachlayer.AtT3,datarangesbetween1.12and1.30Mgm−3_in the0–5cmandbetween1.47and1.57Mgm−3_in_the_deepest_layer_(20–30_cm)._DT wassignificantlylesscompactedthanothertreatmentsinthe5–10cmlayer.In grasslandplotsinT3,DbweresignificantlyhigherthaninNTsoilsinthe5–10cm layerandlowerthanforothertreatmentsbetween10and20cm.No-tillageplots showedthehighestmeanvaluesatT3forallsoillayers(althoughnotsignificantly so).

Soilcarboncontents(C<2mm)(Fig.3)increasedsignificantlyforalltreatments

inT1andT1.5comparedtoforestbetween20and30cm(exceptforDTinT1.5) andinT1.5between5and10cm.InT2,onlygrasslandshowedsignificantlyhigher valuesthaninforestbelow20cm.Incontrast,Ccontentswerelowerthaninforest inthe0–5cmlayerforalltreatmentsinT2andT3,forgrasslandinthe20–30cm layerinT2andforNTinthe10–20cmlayerinT3.Significantdifferencesbetween treatmentscanbeobservedafterT2withlowervaluesforNTinT2below10cmand inT3above20cm.DTandNTplotspresentsignificantlylowervaluesinthe0–5cm layerinT3comparedtograsslandandforest.

3.4. Carbonmassesinsoilfractions<2mmand>2mm

Forthestudiedtreatments,carbonconcentrationsinsoillayersfollowed sim-ilartrendsasCstocksinsoilsbecausebulkdensitiesshowonlysmallvariations (seeSection3.3).InforestsoilsatT0(October2008),C<2mmmeanstockswere1.24, 1.05,1.76,1.44and5.50kgCm−2_in_the_0–5,_5–10,_10–20,_20–30_and_0–30_cm_soils layers,respectively.Morethanayearlater(414days,atT1),C<2mmmeanstocks (Table4)didnotdiffersignificantlybetweentreatmentswhenconsideringeach layerseparately.Valueslaybetween6.14and6.40kgCm−2_in_the_0–30_cm_layer. HigherC<2mmmeanstocks(1.69to1.76kgCm−2)werefoundinthe20–30cmlayer ofagriculturalplotsthaninforestsoils(significantforGandNTtreatments).After forestconversion,thehighestC<2mmstockswerefoundatT1.5,withmeanvalues of1.24–1.40,1.33–1.42,2.04–2.15and1.56–1.86kgCm−2_in_the_0–5,_5–10,_10–20, and20–30cmlayers,respectively,foragriculturalsoils.Atthissamplingdate, sig-nificantlymorecarbonwascontainedinthe<2mmfractionsof5–10cmlayersfor allagriculturalplots(whichdidnotdiffer)thaninforestsoils.Whenconsidering thesurface30cmofsoils,convertedplotscontainedhigherC<2mmmeanstocks thaninitialforestsoilsbutthisdifferenceissignificantforGsoilsonly(Table4). Afteroneyearofmaize/soybeanrotation(atT2),C<2mmwaslowerforeachsoil layerthanatT1.5althoughnotsignificantlyso.AtT2andT3,C<2mmmeanstocks weresignificantlylowerinthesurface5cmforallconvertedsoils(between0.86 and1.01kgCm−2₎_than_in_forest_soils₍_Table₄_)._NT_plots_contained_less_carbon_in the<2mmsoilfractionineachlayerthangrasslands(significantexceptin0–5cm forT2andin20–30cmforT2andT3).DTalsohadslightlylowervaluesthanGbut notsignificantlyso.

Themassesofcarboncontainedinthe>2mmsoilfractions(C>2mm)were

sig-nificantlyhigheroneyearafterdeforestationinthesurface30cmofallconverted soils(1.23–1.39kgCm−2₎_than_in_forest_soils_(0.63_kg_C_m−2₎₍_Table₄_)._At_T1.5, sig-nificantlymorecarbonwascontainedinthe>2mmfractionofGsoilsinalllayers comparedtoforestsoil(notsignificantinthe0–5cmlayer).DTandNTsoilshad intermediatevalues;themassofC>2mminthe0–30cmlayerwasthesameat T1.5forbothcroppedtreatmentsbutatT2,NThadvaluesabout27%lowerthanDT (notsignificant).AtT3,themassofC>2mmwassignificantlylowerinNTsurfacesoil (0–30cm)thaninotherconvertedsoilsduetolowervaluesinthe0–10cmlayer (significantfor0–5cmlayer).C>2mmforNTdidnotexceed0.19kgCm−2(0–5cm) andtotalledonly0.46kgCm−2_in_the_top₃₀_cm._C

>2mmmeanvalues(kgCm−2)per layerwereintheranges0.13–0.37,0.05–0.32and0.07–0.19forGandDTandNT,

Fig.4. Changerateofcarbonmasses(tn−t(n−1),inkgCm−2yr−1)inthesoilfractions <2mmand>2mminsurface30cmforgrassland(G),maize/soybeancroprotation underdisktillage(DT)andundernotillagewithdirectseeding(NT)duringthe3 yearsfollowingforestconversionatCombisite.Mean±standarderror.T0is

samp-lingdatejustbeforelandconversion.Tn:samplingdatewherencorrespondsto year(s)afterlandconversion.Periodduration(year):T0toT11.13years;T1toT1.5 (maizecycle)0.43year;T1.5toT2(soybeancycle)0.44yearandT2toT31.09years. Foragivensoilfraction,significantdifferenceofsoilcarbonmassesbetweentwo samplingdatesarereportedbyasterisks(*p<0.05;***p<0.0001).Foreachperiod, differencesbetweentreatmentswerenotsignificantasassessedbyTukeytest.

respectively.Themassofcarboninthesurface30cmofthesoilsdidnotexceed 0.83kgCm−2_.

Overall,thehighesttotalsoilcarbonvalues(Ctot)(inkgCm−2₎_which_were significantlyhigherthaninforestsoils(6.12±0.21)weremeasuredatT1forDT

(7.65±0.43)orNT(7.72±0.42),andatT1.5forG(8.19±0.37)(Table4).Later,Ctot

fellineverylayerforallconvertedsoils.AtT2andT3,valuesinNTsoilswerelower thanthoseofotheragriculturalplots(significantexceptinthe20–30cmlayer).At T3,CtotwassignificantlylowerforNTsoilsthaninforestsoils.

Forsurface5cmintheforestsoil,20%and5%oftotalCmass(0–30cm)were containedin<2mmand>2mmfractions,respectively(Table5).Afterlanduse con-version,thepercentagecarbon(Table5)inthe<2mmfractionofthe0–5cmsoil layerfellto16%oftheCtotcontainedinthe0–30cmsoillayerwhilstthatofthe >2mmfractionsroseto8%.Thedistributionofcarboninsoilsurface30cmisalso slightlymodifiedinthe5–10cmlayer,withahigherproportionofcarboninthe >2mmfractionincultivatedsoils(maximum6%)thaninforestsoil(2%).Soil lay-ers10–20cmand20–30cmcontained26–29%and21–25%oftotalcarboninthe <2mm,respectively,and2–6%and1–3%infraction>2mm,respectively.

3.5. Ratesofchangeincarbonmassesofsoilfractions<2mmand>2mm

Fig.4showstheratesofchangeofcarbonmasses(tn−t(n−1)C)duringsuccessive periodsoftheexperiment.Unlessotherwisespecified,allvaluesmentionedinthis paragraphareinkgCm−2_yr−1_.

Duringthe414daysperiodwithcovercropsfollowingforestconversion,carbon accumulatedsimilarlyinthe0–30cmlayerofalltreatments(Fig.4)whetherinthe <2mm(0.76forGto1.05forDT)orthe>2mm(0.66forDTto0.83forNT)soil fractions.ThesecarbonvariationsbetweenT0andT1arenotsignificant,probably duetothehighvariabilityinvalues.BetweenT1andT1.5,C<2mmandC>2mmvaried less(−0.23forNTinthe>2mmfractionto+0.52forGinthe<2mmsoilfraction).

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Table4

Massesofcarbon(kgCm−2₎_contained_in_the_soil_fractions_<2_mm_and_>2_mm_for_surface_5,_10,₂₀_and₃₀_cm_of_soil_for_grassland_(G),_{maize/soybean}_crop_rotation_with_disk tillage(DT)andwithno-tillageindirectseeding(NT).Valuesaregivenforsoils1,1.5,2,and3yearsafterforestconversion(T0)withoutburninginCombisite.

Soillayer(cm) n Forest T1(November09) T1.5(April10) T2(October10) T3(November11)

G DT NT G DT NT G DT NT G DT NT

Soilfraction<2mm

0–5 24 1.24a 1.11aA 1.29aA 1.20aA 1.24aA 1.31aA 1.40aA 0.93aB 0.95aB 0.90aB 1.01aB 0.86bC 0.88bC

(0.06) (0.07) (0.09) (0.09) (0.07) (0.07) (0.10) (0.04) (0.04) (0.04) (0.04) (0.03) (0.03)

5–10 24 1.05a _1.30_aA _1.41_aA _1.34_aA _1.42_aB _1.37_aB _1.33_aB _1.16_aA _1.10_abA _1.02_bA _1.15_aA _1.08_aA _0.97_bA

(0.06) (0.11) (0.13) (0.11) (0.05) (0.08) (0.07) (0.04) (0.04) (0.04) (0.04) (0.03) (0.02)

10–20 24 1.76a _1.96_aA _2.02_aA _2.04_aA _2.15_aB _2.06_aA _2.04_aA _1.97_aA _1.76_abA _1.59_bA _1.79_aA _1.69_abA _1.46_bB

(0.06) (0.08) (0.12) (0.11) (0.09) (0.13) (0.10 (0.09) (0.09) (0.04) (0.10) (0.08) (0.03)

20–30 24 1.44a _1.76_aB _1.69_aA _1.75_aB _1.86_aB _1.56_aA _1.68_aA _1.57_aA _1.43_aA _1.34_aA _1.48_aA _1.35_aA _1.23_aA

(0.04) (0.08) (0.09) (0.10) (0.09) (0.10) (0.10 (0.09) (0.07) (0.04) (0.10) (0.07) (0.03)

0–30 5.50A 6.14aA 6.40aA 6.33aA 6.66aB 6.29aA 6.45aA 5.63aA 5.24abA 4.84bA 5.43aA 4.98abA 4.54bB

(0.16) (0.25) (0.39) (0.35) (0.23) (0.33) (0.32) (0.21) (0.21) (0.12) (0.24) (0.17) (0.09)

Plantdebrisofthesoilfraction>2mm

0–5 24 0.31*_A _0.48_aA _0.72_aB _0.53_aA _0.52_aA _0.55_aA _0.42_aA _0.44_aA _0.49_aA _0.30_aA _0.37_aA _0.31_aA _0.19_bA

(0.06) (0.09) (0.13) (0.07) (0.08) (0.09) (0.07) (0.04) (0.08) (0.04) (0.04) (0.05) (0.03)

5–10 24 0.13*_A _0.33_aA _0.23_aA _0.31_aA _0.41_aB _0.29_aA _0.23_aA _0.45_aB _0.31_abA _0.20_bA _0.16_aA _0.32_bB _0.09_aA (0.03) (0.08) (0.05) (0.06) (0.08) (0.05) (0.03) (0.10) (0.04) (0.03) (0.02) (0.04) (0.01)

10–20 24 0.13*a _0.19_aA _0.22_aA _0.35_aA _0.38_aB _0.17_bA _0.28_abA _0.43_aA _0.21_aA _0.20_aA _0.13_aA _0.15_aA _0.11_aA

(0.02) (0.06) (0.04) (0.13) (0.07) (0.04) (0.07) (0.15) (0.04) (0.05) (0.02) (0.02) (0.04)

20–30 24 0.06*_A _0.24_aB _0.15_aA _0.19_aA _0.23_aB _0.12_aA _0.23_aB _0.24_aA _0.20_aA _0.16_aA _0.15_aA _0.05_aA _0.07_aA (0.00) (0.08) (0.03) (0.04) (0.06) (0.03) (0.05) (0.08) (0.06) (0.06) (0.07) (0.01) (0.03)

0–30 0.63*_A _1.23_aB _1.24_aB _1.39_aB _1.53_aB _1.15_aA _1.16_aA _1.51_aB _1.20_aA _0.87_aA _0.81_aA _0.83_aA _0.46_bA (0.08) (0.17) (0.13) (0.21) (0.18) (0.11) (0.19) (0.26) (0.15) (0.12) (0.10) (0.10) (0.08)

Totalsoilcarbon

0–5 24 1.55A 1.60aA 2.01aA 1.73aA 1.75aA 1.86aA 1.82aA 1.36abA 1.44aA 1.18bB 1.38aA 1.17bB 1.07bB (0.08) (0.14) (0.20) (0.13) (0.12) (0.13) (0.14) (0.07) (0.09) (0.06) (0.07) (0.06) (0.05)

5–10 24 1.17A 1.60aA 1.72aB 1.65aB 1.83aB 1.65aB 1.56aB 1.59aB 1.42abA 1.22bA 1.31aA 1.40aA 1.05bA (0.08) (0.14) (0.17) (0.11) (0.10) (0.09) (0.08) (0.12) (0.07) (0.06) (0.05) (0.06) (0.02)

10–20 24 1.90A 2.17aA 2.22aA 2.40aB 2.53aB 2.24aA 2.33aA 2.40aB 1.98abA 1.79bA 1.93aA 1.84aA 1.57bB (0.07) (0.08) (0.14) (0.17) (0.14) (0.15) (0.15) (0.20) (0.11) (0.07) (0.11) (0.09) (0.05)

20–30 24 1.50A 1.98aB 1.82aA 1.94aB 2.07aB 1.68aA 1.92aB 1.84aA 1.63aA 1.51aA 1.63aA 1.40aA 1.31aA (0.04) (0.11) (0.10) (0.11) (0.13) (0.11) (0.12) (0.16) (0.10) (0.10) (0.17) (0.07) (0.05)

0–30 6.12A 7.23aA 7.65aB 7.72aB 8.19aB 7.39aA 7.61aB 7.14aA 6.45abA 5.72bA 6.24aA 5.81aA 4.99bB (0.21) (0.29) (0.43) (0.42) (0.37) (0.36) (0.40) (0.39) (0.29) (0.20) (0.32) (0.22) (0.12)

Standarderrorsaregivenbetweenbrackets.Meanvaluesfollowedbythesamelowercaseletterforaspecificsoillayeratagivensamplingdatedidnotdiffersignificantlyby theposthoctestofTukey(p<0.05).MeanvaluesfollowedbythesameuppercaseletterforthesamelayerdidnotdiffersignificantlyfromreferenceforestbytheDunnett test(p<0.05).

*_Estimated_using_C_content_of_forest_roots_measured_in_Bréchet₍₂₀₀₉₎_. periodasindicatedbyasterisksinFig.4)weremeasuredbetweenT1.5andT2with

−1.03,−1.05and−1.61forG,DTandNT(p<0.0001).BetweenT2andT3,depletion ratesofC<2mmmeanstocks(0–30cm)werelower(0.20,0.26and0.33forG,DTand NT,respectively)thanduringthepreviousperiod.Bycontrast,thehighestcarbon decreasesforC>2mmoccurredbetweenT2andT3,withvaluesof0.70,0.38and0.41 forG,DTandNT,respectively.Carbonchangeratesinthiscoarsersoilfractionwere significantlydifferentcomparedtothoseofthepreviousperiod(asterisksinFig.4).

Table6showsthatthree years after forestconversion tocultivation,the C<2mmstockinthe0–30cmlayerwasunchangedforGandsignificantlydepleted by0.49and0.96kgCm−2_for_DT_and_NT_soils,_respectively_(highly_significant forNT,p<0.0001).TheC>2mm poolsofconvertedsoilsdidnotchange signifi-cantlyduringthisperiod.BetweenT1andT3,C<2mmwasdepletedby0.71and 1.42kgCm−2_for_G_and_DT,_respectively_{(significant,}_p_<_0.05)_and_by_1.80_kg_C_m−2 forNT(highlysignificant,p<0.001).Duringthesameperiod,significantdecreases

Table5

Distributionoftotalcarbonin<2mmand>2mmsoilfractionsinthe0–10,10–20and20–30cmsoillayersforforest(F),grassland(G),maize/soybeancroprotationunder disk-tillage(DT)andunderno-tillagewithdirectseeding(NT)atdifferentsamplingdatesbeforeandafterforestconversionwithoutburninginCombisite.Valuesarein percentagerelativetototalCmassinthe0–30cmlayer.

Soillayer(cm) Soilfraction T0 T1 T1.5 T2 T3

F G DT NT G DT NT G DT NT G DT NT

0–5 <2mm 20 15 17 16 15 18 18 13 15 16 16 15 18

>2mm 5 7 9 7 6 7 6 6 8 5 6 5 4

5–10 <2mm 17 18 18 17 17 18 17 16 17 18 18 19 19

>2mm 2 4 3 4 5 4 3 6 5 4 3 6 2

10–20 <2mm 29 27 26 26 26 28 27 27 27 28 29 29 29

>2mm 2 3 3 5 5 2 4 6 3 4 2 3 2

20–30 <2mm 24 24 22 23 23 21 22 22 22 23 24 23 25

(10)

110 184 (2014) 101–114

Table6

Changerateofsoilcarbonmasses(inkgCm−2₎_between_sampling_dates_for_the_soil

fractions<2mmand>2mminsurface30cmforgrassland(G),maize/soybeancrop rotationunderdisktillage(DT)andundernotillagewithdirectseeding(NT)during the3yearsfollowingfire-freeforestconversionatCombisiteandstatisticalanalysis.

T0toT3 T1toT3

To:samplingdatejustbeforedeforestation.Tn:samplingdatewherencorresponds toyearsafterdeforestation.Foragivenperiod(e.g.T0toT3)andasoilfraction, treatmentswiththesamecaseletterindicatethatmeansoilcarbonchangeratesdo notdiffersignificantlyasassessedbytheTukeytest(p<0.05).Foragivenperiod, soilfractionandtreatment,significantdifferenceinsoilcarbonmassesbetweenthe twosamplingdatesoftheperiodarereportedbyasterisks(*p<0.05;***p<0.0001), n.s.notsignificant.

forC>2mmwereasfollows(inkgCm−2_):_DT_(0.41)_<_G_(0.53)_<_NT_(0.93)_(highly

significantfor NT, p<0.0001). Thechangein C<2mm andC>2mm did not

dif-ferbetweentreatmentsforthesuccessiveperiodsofthestudy.However,when consideringlonger periods(T1 toT3 orT0to T3),thechangeinsoil carbon stocks(<2mmsoilfraction) forNTplotsdifferedsignificantlyfromthatofG (Table6).

4. Discussion

4.1. Soilcarbonstocksandpoolsinforestsoils

Themeansoilcarbonstock(C<2mmonly)inthesurface0–20cm

layer of neotropical forest at Combi site (4.05kgCm−2₎ _was

much higher than that reported by Desjardinset al. (2004) in

eastern Amazonia (2.92kgCm−2_,_in _the_Brazilian _state _of _Pará,

sandy–clayeyacrisols).C<2mmstockinthe0–30cmsoillayerin

Combi(5.50kgCm−2₎_is_similar_to_values_measured_in_Rondônia

byCarvalhoetal.(2010)onJulianafarmonaRhodicKandiudox

soil(5.63kgCm−2₎_and_Maia_et_al.₍₂₀₁₀₎_on_oxisols_(Santa_Luzia

D’Oeste,5.30–5.57kgCm−2_)._Considering _the_pedon,_the_C

<2mm

stockinthe0–2msoildepthatCombi,14.16kgCm−2_,_corresponds

tothemorefrequentrangeofvalues(10–20kgCm−2₎_estimated_for

67sitesacrossAmazoniabyQuesada(2010).Inourstudy,about39%

ofthecarbonestimatedforthe0–2mforestprofileiscontainedin

thesurface0–30cm.

Estimatesby Bréchet(2009) for roots biomass in theupper

30cmofforestsoilsatParacousiteareinthesamerangeasour

values(Table4),with0.547and0.640kgCm−2_contained_in_roots

ofdiameters<2mmand>2mm,respectivelyforthe0–30cmsoil

layer.BouletandHumbel(1980)measuredatotalrootbiomassof

1.63kgm−2_(i.e._0.69_kg_C_m−2_assuming_42.7%_of_C_in_roots)_for

sim-ilarwell-drainedferralsolsinthe0–2mlayer,ofwhich75%and83%

werelocatedinthesurface20and40cmofthesoil,respectively

andonly6%inthe1–2mlayer.

4.2. Forestconversionintograsslandandcropland

Totalabove-ground biomassin forestsofFrench Guianahas

beenestimatedinpreviousstudiesonsimilarsoilsto318±17mg

ofdrymatterperhectare(Sarrailh,1984;Puigetal.,1990),what

correspondstoahighvalueforAmazoniaasreportedbyAnderson

etal. (2009).The quantityofchopped biomass thatwas

incor-poratedinto thesurface20cm ofsoilsrepresents3–13%ofthe

estimatedtotalabove-groundbiomassofnativeforest.Asreported

byDenich et al. (2004), theefficiency of the chopping process

dependsonplantmorphologicalparameterssuchasstem

diame-ter,heightandbiomassofthepredominanttreesandshrubs.Inthe

caseofneotropicalforest,theuseofamorepowerfulwoodcrusher

adaptedtogrindthecrown(stemsandleaves)oncethetreesare

felledcouldincreasethequantitiesofforestbiomassreturnedtothe

soiltoabout100MgDMha−1_(our_estimate,_according_to_Sarrailh,

1984).

Inourexperiment,thechoppedbiomassinputandfreshgrinded

vegetationdidnotdisturbthedevelopmentorthegrowthofthe

covercropsduringthefirstyearorofthefollowingcrops,although

smallmassofmineralnitrogenwereappliedtosoils.Later,crops

didnotshowvisualsignsofdeficiencyandgrainyieldswere

sat-isfactory (Table 3)except for thefirst maize crop, which faced

uncontrolled competitionby regrowth of U. ruziziensis.

Experi-mentsreportedinDavidsonetal.(2008)andComteetal.(2012)

involvedsecondaryforestconversionwithmulchinputsashigh

as99.6±19.5MgDMha−1withoutapparenteffectonyieldswhen

mulchedfieldswerefertilizedwith60,60,and30kgha−1_of_N,_P_and

K,respectively.InKatoetal.(1999),incorporationormulchingof

thechoppedvegetation(crushedto2–5cmwoodchips)with

min-eralfertilizationdidnotsignificantlyinfluencecropyields,total

drymatterproductionornutrientaccumulation(N,P,K)ofrice,

cowpeagrainsandcassavafreshrootsduringtwosuccessive

crop-pingperiods.Thus,chop-and-mulchpracticesdonotappeartohave

negativeimpactsongrassandcropsproductions.

Chop-and-mulch clearing caused limited soil compaction as

shownbythesmallchangesinsoilbulkdensitiesbetweenT0andT1

(Fig.3),unlikeothermechanicaldeforestationmethods(e.g.Alegre

etal.,1986;Laletal.,1986;Alegreetal.,1990;AlegreandCassel, 1996;Chauveletal.,1991)orcomparedtoslash-and-burnwith

manualfelling(Ghumanetal.,1991;Desjardinsetal.,2004).This

resultis inaccordwithanotherstudyonchop-and-mulchland

clearingmethodofa12–15year-oldfallowforestintheeastern

Amazononsoilswithsimilartexturalparameters(Comteetal.,

2012).Inourstudy,themulchformedbythechoppedvegetation

mayhaveprotectedsoilfromcompactionduringtheclearingphase

aswellasthetypeoftractor(track-type).Soiltillagewith

incor-porationofchoppedvegetationintothesurfacelayer(0–20cm)

ofsoilsjustaftertheclearingundoubtedlymodifiedsoilporosity

andcouldhavestimulatedsoilbiologicalactivityduetoincreased

substrateavailability.AsshowninFig.1B,woodchipsstillcovered

muchofthesoilsurfaceevenaftertheirincorporationinto0–20cm

layerandcouldhavehelpedtoreducephysicalconstrainscausedby

rainfallduringthefirstyear.Furthermore,covercropsrootsmight

havehelpedtorestoresoilporosity.

Nevertheless,compactionincreasedslightlyduringthe3years

following land use change, especially in NT soils. Compaction

increasedbetweenT2andT3inthe0–10cmlayer(Fig.3),

prob-ablyduetomoreintenseclimaticeventsrecordedinApril–May

2011aftermaizeandgrassharvestsandatsowingofsoybean.Soil

surfacecompactionmighthavebeencausedbymachinerybeing

usedinwettersoilconditionsthanduringpreviouscrops.

4.3. Shorttermrelativedepletionofsoilsurfacecarbon

4.3.1. Firstyearafterclearing

During this experiment, land clearing with the

chop-and-mulch method led to the incorporation of 1.59kgCm−2 _(i.e.

0.462±1.128kgCm−2)fromtheabovegroundforestbiomassinto

the0–30cmsoillayer,inadditiontotheinitial0.63kgCm−2_mainly

containedinforestroots.Thegrindingoftheaerialpartsofcover

cropsincorporated0.487kgCm−2 _more_in_the_0–5_cm_and_litter

layer.Deathofandexudationbycovercroprootsalsoincreased

soilcarbonmasses.Thus,duringthefirstyear,atleast2.71kgCm−2

wasaddedtothesoils,increasingCmassesinthewholesoil

pro-filebyabout0.61–2.23kgCm−2_(72–98%_of_the_C_tot_in_the_<2_mm

fraction)comparedtoforestsoils.Wecanassumethatlitterand

amajorpartofforestrootsweredecomposedrapidlyenoughto

(11)

thedecompositionratesmeasuredinnearbyGuianeseneotropical

forest.Indeed,Sarrailh(1990)measuredthatforestlitterfallwas

completelydecomposedafter195days.Onceincorporatedintosoil,

littermightshowaslowerorfasterdecomposition/mineralization

ratebutwecanhypothesizethatitsdecompositionwaslargely

completedinT1.Moreover,Bréchet(2009)measuredaturnover

rateof0.59–0.84peryearforfineforestroots(diameter<2mm)at

Paracou.Inaddition,thelowmeanC:Nratioofcovercrops

indi-catesprobablerapiddecomposition(Table3).Thesethreeinput

types(litterfall,roots,covercrops),whichhavefastdecomposition

potentialsgiventhetemperature/climateregimesinFrenchGuiana

andsoilfaunaactivityundertropicalhumidconditions(e.g.Tian

etal.,1992,1993;Tian,1998),totalize1.58kgCm−2_and_are_likely

tohavesuppliedthe<2mmsoilfractionduringthefirstyear.

4.3.2. Grasslandandcroplandwithandwithouttillagepractice

Literaturedataaboutcarbonstockchangesduringtheperiod

inferiorto3yearsfollowing landclearinginAmazoniaarevery

scarce.Publisheddataconcernsoilsafter2–3yearsofgrasslandor

croplandinstallationfollowingslash-and-burnandarebasedon

chronosequenceapproachesthatincreaseuncertaintyoncarbon

stocksmeasuresbecauseofspatialheterogeneityofsoils.

Inourstudy,forallconvertedsoils,themostrapidlydecreasing

ratesofCoccurredbetweenT1.5andT2inthefinefraction(<2mm)

andduringthe3rdyearafterlandconversioninthecoarsefraction

(>2mm).However,forthethreetypesoflanduse,ratesofchange

ofcarbonmassesdifferedforbothsoilfractions(Fig.4).Asfaras

weknow,thishasnotbeenobserveduntilnow.

In our study, the grassland system is characterized by

large amounts of above-ground biomass export, equivalent

to 0.27–0.75kgCm−2_yr−1_, _and _a _low _level _of _restitutions

(0.12kgCm−2_the_2nd_year)._Two_years_after_{establishment,}_this

systemresultedinsimilarsoilcarbonmassesasforforestinboth

soilfractions. InSouth Americansavannas, deep-rootedgrasses

suchasUrochloaspp.havebeenreportedtosequestersignificant

massesoforganiccarbondeepinthesoil(between100and507Mt

yearly)(Fisheretal.,1994).TheturnoverrateofUrochloaishigh

(e.g.Salimonetal.,2004).Inthesoilsofourexperiment,thecarbon

returnedbyrootsisfoundinboth<2mmand>2mmsoilfractions,

whichprobablyoffsetspartofthelossofCfromforest.

Variations ofcarbon stocksweremeasuredin forest to

pas-tureconversionchronosequencesatNova-Vida farm(Rondônia,

Brasil)inthetop30cmofUltisols(deMoraesetal.,1996;Neill

etal.,1996,1997).In2chronosequences,Neilletal.(1996,1997)

observedCstockincreasesof0.23and1.23kgCm−2_in₃_year-old

pasture (compared toa closeforested area). In the same farm

andthesametype ofsoil,Fernandesetal. (2002)measuredan

increase of 0.8–0.9kgCm−2 _in _a ₃ _year-old _pasture. _Using _the

Centurymodel,Cerrietal.(2004)basedonNova-Vidafarmdata,

simulatedadeclineofcarbonstocksduringthefirstyearsafter

con-versionwithslash-and-burnofforesttowell-managedpastures.

Thisdeclinewasfollowedbyaslowincreaseofcarbonstocks.In

thesamestudy,theempiricalmodelfittedbyregressiondidnot

simulatedeclineduringthefirstyears.InAmazonia(Marabá,Pará

state,Brazil),Desjardinsetal.(2004)measuredanincreaseintheC

stockinferiorto0.5kgCm−2_in_the_0–20_cm_soil_layer_after_3,₉_and

15yearsofgrasslandinstalledaftermanualclearingwithburning.

ThesesoilshavecomparabletexturalpropertiestothoseatCombi,

butagriculturalpracticesdiffered(noliming,nofertilizationand

withUrochloahumidicola).InParáState(nearTailândia),increases

ofabout0.62and0.23kgCm−2 _(compared_to_forest)_were

mea-suredinthe0–20cmsoillayerofmediumtexturedTypicHapludox

after8and13yearsrespectivelyofpasture(Brachiariabrizantha,no

limeorfertilization),establishedafterforestclearingwithburning

(deSouzaBrazetal.,2013).Inthesamestudy,Cstocksdecreased

slightlyin15year-oldpastureinreasonofintensiveuseand/or

mismanagement.Inameta-analysisbasedonresultsfrom

Mato-grossoandRondôniastates,Maiaetal.(2009)concludedthatthe

effectofforestconversiontopastureonsoilCstocksofAmazonia

andCerradobiomesshowscontrastingresults,dependingonthe

managementappliedtothepasture.

Incontrasttograssland,thedecreaseinsoilcarbonstocksin

ourexperimentissignificantforcroplands.Comparedtothe

disk-tillagetreatment,no-tillagepracticesinducedthehighestdepletion

ratesinbothsoilfractions.Thisdifferencecannotbeexplainedby

smallerCinputsinNTplots.BetweenT1and T3,slightly larger

C quantities were indeed returned tosoils with above-ground

biomassunderno-tillage plots(1.15for NTcf.0.96kgCm−2 _for

DT). Moreover, successful U. ruziziensis association betweenT1

and T1.5in NTcertainly increasedtheamountofC inputsinto

soilsfromrootscomparedtoDT.Wecanthensupposethatfaster

decomposition and mineralization processes occurredin no-till

plotscomparedtotillageplotscontrarilytowhatisusuallyreported

intheliteraturefortillageversusno-tillagepractice(e.g.Balesdent

etal.,2000).

In Amazonia, studies oncropsystems and tillage effects on

soilsafterclearingarescarce. Inthehumidzone ofNigeria, on

Ultisols, after 4 and 5 years of cropping (maize/cassava

with-outfertilization),thequantityof mostof thelabile carbonpool

wasnotfoundsignificantlydifferentafterslash-and-burnor

bull-dozednon-windrowedforestclearing(Okoreetal.,2007).After

forestconversionwithburning,cropsuccessionsof2and6years

underno-tillage causedthedepletionof 0.29 and0.71kgCm−2

in the C stock of the0–30cm soil layer(Carvalhoet al., 2010,

Rondônia).After1and5yearsofconversiontoanIntegrated

Crop-LivestocksystemunderNT,soilcarbonstocksdecreasedby0.62and

increasedby0.51kgCm−2_respectively_compared_to_native_forest

(Carvalhoetal.,2010).Maiaetal.(2010)observedinthewestern

partofBrazil(RondôniaandMato-Grossostates)thatfull-inversion

tillagetendedtodecreasesoilcarbonstocksbutthatthepotential

forCsequestrationinno-tillsystemsofannualcropsunderthese

particularclimaticconditionsremainsunclear.Theseauthorsdo

notmentionwhetherornotthesesystemswerebasedonmulch,

orthequantitiesofbiomassrestitutions.Recently,Maiaetal.(2013)

reportedthat moreresearchisneededtounderstandprocesses

governingsoilorganiccarbonstockdynamicsintheAmazonbiome

whereNT-basedagriculturalsystemsdonotseemtohavethesame

effectonsoilcarbonstockscomparedtotheCerradobiome.

Mulch-based direct seeding cropping systems have been

described intheCerrado biomeof Brazilasa meansof

preser-ving carbon stocks in soils(e.g. Corazza et al., 1999; Bernoux

etal.,2006;Corbeelsetal.,2006;Marchãoetal.,2009;Netoetal.,

2010).Recently,severalmeta-analysesdealingwithlong-termfield

experiments(>5years)comparingno-tillandfull-inversiontillage

management(Luoetal.,2010;AngersandEriksen-Hamel,2008;

Virtoetal.,2012)haveshownthatno-tillagepracticecouldnot

bedirectlylinkedtohigherCstorageinsoilsbutthedifferencein

inputswouldexplainthevariabilityinCstoragebetweenthetwo

managementstypes(Virtoetal.,2012).

FromthreegeographicalregionsoftheBrazilianAmazonBasin,

Koutikaet al.(1999)showedthatthe“Cdecompositionof

top-soilsfromthreegeographicalregionsoftheBrazilianAmazonBasin

primarilydependedonsoiltexture,especiallythesand/clayratio.

However,theinfluenceofthelocalclimate,i.e.mainlyannual

pre-cipitation,maybealsoimportantinthetopsoilsofcoarsetexture

withasand/clayratiomorethan2,suchasthoselocatedinthe

westernandeasternBrazilianAmazonBasin.”IntheCombi

exper-iment,ourobjectivewastoestablishamulch-baseddirectseeding

treatmentbutproblemswithplantingofcovercropsafterT1.5led

tomuchlowerbiomassrestitutionstosoilsthanexpected.This

unforeseensituationprobablyincreasedsoilorganiccarbon