Effects of sugarcane harvesting with bur (2)

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ContentslistsavailableatSciVerseScienceDirect

Agriculture,

Ecosystems

and

Environment

j ou rna l h o m e p a g e :w w w . e l s e v i e r . c o m / l oc a t e / a g e e

Effects

of

sugarcane

harvesting

with

burning

on

the

chemical

and

microbiological

properties

of

the

soil

Rosinei

Aparecida

Souza

a

_,

_Tiago

_Santos

_Telles

a

_,

_Walquíria

_Machado

a

_,

Mariangela

Hungria

b,∗

,

João

Tavares

Filho

a

,

Maria

de

Fátima

Guimarães

a

a_Dept_Agronomia,_Universidade_Estadual_de_Londrina,_C.P._6001,_CEP_86051-990_Londrina,_PR,_Brazil

b_Embrapa_Soja,_C.P._231,_CEP_86001-970,_Londrina,_Paraná,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received20June2011

Receivedinrevisedform13March2012 Accepted16March2012

Keywords:

Soilmicrobialbiomass Sugarcaneharvestingsystems Sugarcanetrashburning Soilqualityindicators Soilorganicmatter

Saccharumspp

a

b

s

t

r

a

c

t

Soilmicrobialbiomassrepresentsanimportantandstrategicreservoirofplantnutrientsthatcanbe quicklyalteredduetodifferentsoilandcropmanagements.Inthiscontext,theaimofthisstudywasto evaluatetheinfluenceofsugarcaneharvestingsystems,withorwithoutburning,onthechemicaland biologicalpropertiesofthesoil.Theexperimentwasconductedonadystrophicredlatosol(Oxisol)soil in2008,inacommercialareaofasugarcanefactoryinthemunicipalityofParaguac¸uPaulista,SãoPaulo state,Brazil.Thetreatmentsincludedareaspreviouslyburned,areaswithmechanicalharvestingandno burningandnativeforest.Soilsampleswerecollectedimmediatelyafterthesugarcaneharvestfromthe treatmentsatadepthof0–20cm.Theparametersevaluatedwere:microbialbiomassCandN(MB-Cand MB-N),totalorganicC(TOC),recalcitrantC(R-C),labile-C(L-C),totalnitrogen(TN),pH,exchangeable cations(Ca2 +₊_Mg2+_and_K+_),_exchangeable_(Al3+₎_and_potential_(H+₊_Al3+₎_acidity,_and_P_available_in_the

soil.Soilchemicalfertilityunderthesugarcanewithoutburningwasbetterthanundersugarcanewith burn.TheTOCvaluesfornativeforestandfortheharvestingwithoutburnwerehigherthanthoseunder thesugarcanewithburn(148%and54%,respectively).ThissuperioritywasalsoconfirmedforTN,L-Cand R-C.Anevenmoresignificantdifferencewasfoundundernaturalforestandsugarcanewithoutburnfor MB-C,whichwas222%higherundernativeforestand102%higherundersugarcanewithoutburnthan thevalueundersugarcanewithburn,confirmingthatMB-Ccouldbeareliableindicatorofsoilquality formonitoringsoilsunderdifferentsugarcaneharvestingsystems.

1. Introduction

Sugarcane(Saccharumspp.)isanativeplantofIndiaandisnow grownthroughouttheworldinregionsbetween22◦_N_and_S_of_the

Equator.Itisconsideredoneofthemostimportantcropsgrown foritsenergyvalue,withaglobalcultivationareaofover20 mil-lionhainmorethan70countries.Brazilisresponsibleforaround aquarterofglobalproduction(FAO,2010).Withthegreatestarea undersugarcaneintheworld,itisestimatedthatinthe2011/2012 seasoncrop8.37millionhawillbeundersugarcaneinBrazil,with annualproductionof571.5millionmetrictons(CONAB,2012).

ThehighestproducingregioninBrazilisthesoutheast, repre-senting62%oftheplantedareainBrazil(withSãoPaulostatealone accountingfor52%ofthistotalarea)andaverageproductivityof

∗_{Corresponding}_author._Tel.:₊₅₅₄₃₃₃₇₁_6206;_fax:₊₅₅₄₃₃₃₇₁_6100. E-mailaddresses:rosysouza@iapar.br(R.A.Souza),tiagotelles@yahoo.com.br

(T.S.Telles),machadowalquiria@ig.com.br(W.Machado),

hungria@cnpso.embrapa.br,hungria@pq.cnpq.br(M.Hungria),tavares@uel.br

(J.T.Filho),mfatima@uel.br(M.d.F.Guimarães).

69.8tha−1_,_followed_by_the_northeast,_with_13%_of_total_area_and

averageproductivityof60.3tha−1₍_CONAB,₂₀₁₂_).

Atpresent,sugarcanecroppingisundergoingaperiodofintense changeinmanagementpracticesduetotheintroductionof mech-anizedharvesting,withnopre-harvestburn.However,sugarcane trash burning, still frequently carried out in most producing regions,hasprofoundimpacts,asitdestroysthesoilorganic mat-ter,leavingitexposedtoerosion,impactingmicroorganismsand causingsignificantpollution.Indeed,impactsofburningmaybe severeandshouldbeconsideredinabroaderview,i.e.,focusingnot onlyonagriculturalsystems,butalsoonnaturalecosystemssuch asgrasslandsandforests(Liuetal.,2007,2010).Interesting,ina temperatesteppeinnorthernChina,inafirstmomentsoil micro-bialbiomasswasincreasedandsoilrespirationwasdecreasedby burning,buttheeffectsweredecreasedwithtime(Liuetal.,2007, 2010).AlsointropicalforestsandnaturalgrasslandsofBrazil, burn-ingmayinafirstmomentstimulatesoilmicrobialbiomass,butit decreaseslater,whenthereadilyavailableCisdepleted(Kaschuk etal.,2010).IntheBrazilianAmazon,decreasesinsoilmicrobial biomassduetoburningcanbeashighas80%inashortperiod (Pfenningetal.,1992).

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2 R.A.Souzaetal./Agriculture,EcosystemsandEnvironment155 (2012) 1–6

In viewofthisscenario,over thelast fewyears thepossible prohibitionofsugarcaneburninghasbeendiscussed.InBrazilian legislation,article2ofLawno.11.241/2002,relatingtothegradual eliminationoftrashburning,setsforthasequenceofprogressive stagesforreducingthepracticeofburninginareasofmorethan 150ha,sothatby2021,theuseofsugarcanetrashburningwillbe completelyeliminated.Ontheotherhand,inareasofmechanized harvestingofsugarcanewithoutburn,theaccumulationoftrash onthesoil’ssurfacecanexceed20tha−1₍_Negrisoli_et_al.,₂₀₀₇_),

creatingathicklayeroforganicwaste.

Itisthoughtthatsomebiologicalpropertiesofthesoilare sen-sitivetochangeswhenthesoilissubjectedtodifferenttypesof managementand,therefore,wouldbebetterindicatorsofsoil qual-ity.Thisappliestomicrobiologicalevaluations,highlysensitiveto anydisturbancesproducedbychangesinsoilmanagement(Tótola andChaer,2002;Kaschuketal.,2010,2011).Consistent results haveshownthatdeterminingsoilbiomassallowsedaphicchanges tobeevaluatedfasterthananalysisbasedonthesoil’schemicaland physicalproperties,andcanbeusedasanindicatorofsoilquality (JenkinsonandLadd,1981;Powlsonetal.,1987;Franchinietal., 2007;Souzaetal.,2008a,b;Hungriaetal.,2009;Kaschuketal., 2010,2011).Thisalsoappliestofewstudiesperformedsofarwith sugarcane(Santˇıanaetal.,2009;Kaschuketal.,2010)

Despite growing interest inaspects related tothe biological functioningofthesoilundernaturalandagriculturalsystems, stud-iesontheimpactofdifferentharvestingsystemsonsoilmicrobial biomassundersugarcanearearecentphenomenon.Againstthis backdrop,ourstudywasdevelopedwiththeaimofassessingthe influence of harvesting, with or without trash burning, onthe chemicalandbiologicalpropertiesofthesoil.

2. Materialandmethods

Theexperimentwasconductedin2008,inParaguac¸uPaulista, stateofSãoPaulo,Brazil(22◦₂₉′_S_and₅₀◦₃₇′_W)._The_soil_is

clas-sifiedasdystrophicredlatosol(Oxisol),ofmediumtexture,with smoothlyundulatingterrain.AccordingtotheKöppen classifica-tion,theclimateismesothermalsubtropical(Cwa),characterized byhot,rainysummers,averagetemperatureshigherthan22◦_C,_and

winterswithaveragetemperaturesbelow18◦_C,_and_clearly_defined

seasons.Meanaltitudeisof470mandaverageannualrainfallis 1500mm(CEPAGRI,2010).Thestudywasconductedina commer-cialareaofthesugarcanefactoryNovaAmerica/COSAN.Thearea undersugarcanetreatmentshad15haandtheremainingareaof nativeforestwasof3ha.Beforebeingturnedovertosugarcane,the areawasusedforgrowingmaize(ZeamaysL.),soybean(Glycine max(L.)Merr.)andcotton(GossypiumhirsutumL.).Theareahad beencroppedtosugarcane(Saccharumspp.;sugarcaneisnamed withoutadefinedspeciesbecausemoderncommercialvarieties arefoundedoninterespecifichybridsbetweenSaccharum sponta-neumandSaccharumofficinarum)since1992,andoneachcycleof fivetosevenslashes,theplantationisoverhauled.Whenthe sam-plesusedinthisstudywerecollected,theplantationwasonthe thirdslash,inthisyearofavarietyofS.officinarum.

Theexperimentaldesignwasfullyrandomized,withthree treat-mentsandtwentyreplications.Sugarcanetreatmentsconsistedof theharvestingsystemswithtrashburning(sugarcanewithburn) andmechanizedharvestingwithouttrashburn(sugarcane with-outburn).Theareashavebeenestablishedwiththesetreatments since1992,thereforefor17years,whenthesamplesweretaken, in2008.Intheareawithburning,sugarcanewasburnedeveryyear fromthesecondyear,therefore16timeswhenthesoilsampling fromthisstudyweretaken.Themechanizedharvestingwithout trash burn started after the burning in 2002; therefore, a six-yearperiodwithoutburningwasaccumulatedatthetimeofsoil

sampling.Theadjacentsavannanativeforest(NF)wasusedasa referencefortheinitialconditionofthesoilbeforeitwasturned overtofarming.Areaswithorwithoutburningwerelocatedside byside,andtheremainingnativeforestwaslocatedperpendicular tobothareas.Declivityofthearearangedfrom3%toamaximum of8%.

Soilsampleswerecollectedimmediatelyafterharvestingthe sugarcane,atadepthof0–20cm.Foreachtreatmentasampling areaof2km×2kmwasestablished,with20samplesrandomly

collected,andcomposedoffourdiscretesub-samples,twofrom thesugarcanerowsandtwofromtheinter-rows.Samplingwas performedwiththeaidofGPS,georeferencedonaregulargridof 20m×10m.Wemetallthebasicprinciplesofexperimentation,

repetitionandrandomization;areasundersugarcanewere homo-geneous,withthesamesoiltype,cultivarofsugarcane,thesame weatherconditions;theyhavealsoreceivedthesameamountof fertilizersandpesticides.

Chemicalanalysiswascarriedoutusingthemethodsdescribed byEMBRAPA (BrazilianAgricultural Research Company)(1997). Basedontheanalysisresults,valueswereestimatedforthesum ofbases(SB),cationexchangecapacityatpH7.0(TpH7.0),base saturation(V%),andaluminumsaturation(m%).Toobtaina phys-icalcharacterization,soiltexturewasevaluatedusingthepipette methodwithorganicmatteroxidation,accordingtoTavaresFilho andMagalhães(2008).

Thefumigation-extractionmethod(Vanceetal.,1987)wasused toassess microbial biomass carbon (MB-C)in thesoil, and the methoddevelopedbyBrookesetal.(1985)toanalyzemicrobial biomassnitrogen(MB-N).LevelsofMB-NandtotalNwere deter-minedbywetdigestioninadigestorat350◦_C_with_concentrated

sulfuricacid(H2SO4)(Bremner,1965),afterwhichthe

concentra-tionof Nwasmeasuredusing thesemi-micro-Kjeldahlmethod (FeijeandAnger,1972).

MicrobialbiomassvaluesareexpressedinmgCandmgNinthe microbialbiomasskg−1_of_dry_soil._The_microbial_quotient₍_q_Mic)

wasalsocalculatedusingtheformula(MB-C/TOC)×100(Anderson

andDomsch,1990).

Labilecarbon(L-C)wasdeterminedaccordingtotheprocedure describedbyBlairetal.(1995).Recalcitrantcarbon(R-C)was deter-minedusingthedifferencebetweenTOCandlabileC.Theresults forlabileandrecalcitrantCareexpressedingkg−1_.We_met_all_the

basicprinciplesofexperimentation,replicatesandrandomization. Areaswerehomogeneousintermsofshowingthesamesoiltype, cultivarofsugarcane,thesameweatherconditionsandreceivedthe sameamountsoffertilizersandpesticides.Theareaunder remain-ingforestwasadjacenttothesugarcaneareasandalsopresented thesamesoiltypeandweatherconditions.Theanalysisof vari-ancewasperformedwiththeSISVARprogramandtheTukeyˇıstest (p≤0.05)(Ferreira,2008).

3. Resultsanddiscussion

3.1. Physicalandchemicalproperties

Topography,climaticconditionsandsoiltypewereverysimilar intheareas,thereforesoilgranulometrywasnotverydifferent,as follows(gkg−1_of_clay,_silt_and_sand,_{respectively,}_at_the_0–20_cm

depth):sugarcanewithburn(387,234,379);sugarcanewithout burn(372,243,385);nativeforest(379,239,383).

Inrelationtosoilchemicalproperties,themajorityofvariables analyzeddifferedaccordingtotheharvestingsystemused(Table1). Areasundersugarcanewithoutburningexhibitedthehighest val-uesforpH,P,K,Ca,Mg,sumofbases,basesaturation(V%)andlower valuesforpotentialacidity(H+Al),Al3+_and_aluminum_saturation

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Table1

Chemicalpropertiesofdystrophicredlatosol(Oxisol)underdifferentsugarcaneharvestingsystemsandnativeforest(reference),atadepthof0–20cm.

Management pH P K+ _Ca+2 _Mg+2 _H₊_Al _Al3+ _SB _T _V _m

mgdm−3 _cmolc_dm−3 _%

Sugarcanewithburn 4.52Ba _10.17B _0.05B _0.71B _0.21B _3.72B _0.19B _0.97B _4.69B _20.98B _16.38B

Sugarcanewithoutburn 5.13A 17.17A 0.10A 1.27A 0.38A 3.18C 0.10C 1.75A 4.93B 35.20A 5.40C Nativeforest 4.01C 5.09C 0.04C 0.44C 0.14B 6.70A 0.55A 0.62C 7.32A 8.47C 47.01A CV(%) 0.32 28.99 29.10 25.03 29.77 7.05 4.89 21.96 6.98 20.10 20.53

a_Averages_followed_by_different_letters_in_the_columns_differed_{statistically}_in_the_Tukey_test₍_p_<_0.05).

Therewasnodifferenceinthecationexchangecapacity(T)for thetwosugarcaneharvestingsystems(withorwithoutburn),butT washigherthanintheareaundernativeforest,whichexhibitedthe highestvaluesforH+Al,Al3+_and_m_%._However,_the_higher_T_value

undernativeforestis directlyrelatedtoitshighpotential acid-ity(6.70cmolcdm−3)andthelowsoilfertility.ThehigherP,K,Ca

andMglevelsundersugarcanewithoutburnareduetothehigher contributionofnutrientsfromthetrashleftonthesoil(Table1).

ThedropinTintheareaundersugarcanewithoutburncan beattributedtoalterationsinlevelsoforganicmatter(OM), par-ticularlyinareascultivatedforlongperiods.Theseresultsarein agreementwiththosereportedbyCorrêaetal.(2001),whoalso observedthatthesugarcanewithoutburnmanagementresultedin increasesinpH,P,Ca,Mg,SandV%,incomparisonwiththenative forest,butindecreasesinOMcontent,T,exchangeableAlandm%. Accordingtotheseauthors,thelowerOMcontentinsoilunder sug-arcanewithoutburnisbelievedtoreflectthelowercontributionof OMovertheyearsofcultivation,comparedtothesoilundernative forest.AsimilarpatternwasalsoreportedbyCasagrandeandDias (1999).

The results of our study are also in agreement with those obtainedbyCanellasetal.(2003),whoobservedsignificantchanges andincreasesinthelevelsofK,MgandCainareasofsugarcane withoutburnincomparisonwithburnedsugarcane.Theaddition ofthesugarcanewithoutburnresiduescouldincreasethepH, lead-ingtotheformationofcomplexofH andAlwithplantresidue composts,leaving more Ca,Mg and K free in thesoil solution. AccordingtoPavinatoandRosolem(2008),itisusualtoobserve anincreaseintheavailabilityofPinthesoilwiththeadditionof plantresidues,boththroughthereleaseofPfromtheresidueandby competitionamongorganiccompoundsforadsorptionsitesinthe soil.Forthesugarcanewithoutburnmanagement,themicrobial attackoforganicresiduesfromthesugarcanecouldalsoincrease thepHbydecarboxylationoforganicacids,whichconsumes pro-tons.Mendonzaetal.(2000)alsoobservedhigherMglevelsfor sugarcanewithoutburn but,in contrasttoourstudy,observed higherlevelsofPandKforsugarcanewithburn.Theauthorsexplain that,whenthesugarcanetrashwasburned,althoughPandK lev-elswerehigher,itispossiblethat,inthelongterm,soilfertility willdropsincethenutrientsintheasharelikelytobelostthrough leachingorerosion,especiallyinsoilswithlowlevelsofOM.

Thehighestlevelsoftotalorganiccarbon(TOC)showninTable2

werefoundundernativeforest(20.10gkg−1_),_followed_by_the

sug-arcanewithoutburn(12.48gkg−1₎_and_the_sugarcane_with_burn

(8.10gkg−1₎_management._Marchiori_and_Melo₍₂₀₀₀₎_studying_a

purplelatosol(Oxisol),alsoobservedhigherTOCvaluesfornative forestand lower values under sugarcane.Turning nativeforest areasovertoagricultureisgenerallyaccompaniedbyadropinthe quantityofOM,whichisexplainedbyanincreaseinthe mineraliza-tionrate,amongotherfactors,whichcanhaveasignificantimpact onOMlevels,andevenmoredrasticresultsinsomeecosystems (Kaschuketal.,2010,2011).InthestudyconductedbyMarchiori andMelo(2000),theareaundersugarcaneexhibiteddecreasesof 41.3%to49.1%atadepthof0–10cmand,inthe10–20cmlayer, thedropwasoftheorderof23.7%to35.8%.Thesevaluesareclose

tothosefoundinourstudy,withdropsof37.9%undersugarcane withoutburnand59.7%undersugarcanewithburn.

Inthecomparisonbetweentheharvestingsystems,mechanized harvesting(withoutburning)ontheplantationprovidedadense layeroftrashoverthesoil,increasingTOClevels.Thiscouldbedue tothehighercontributionofplantmaterial,tothelowerlevelof tillageandtothenon-destructionofplantmaterialandOMbyfire. SeverelossesofTOCunderthecaneplantationsbycomparisonwith nativeforestwerealsoreportedbyCerrietal.(2010),andgiventhe highplantbiomassproductioncapacityofsugarcanecultivation, theselosseswereattributedtoburning.

In astudyconductedin AustraliabyNobleetal.(2003),the authorsobservedanincreaseinTOCcontentfrom14.7gkg−1 _in

treatments with trash burning, to 30.0gkg−1 _under _sugarcane

withoutburn.InthesoilsofSouthAfrica,Dominyetal.(2002)also foundhighlevelsofTOCundersugarcanewithoutburn(45gkg−1_),

whereasundertheburnedsugarcane,TOCdidnotexceed30gkg−1_.

AssessingstocksofCandNinsoilunderdifferentmanagement sys-temsanduses,RangelandSilva(2007)observedthatstocksofTOC weresignificantlyaffectedbythemanagementsystem,andthose involvingno-tillageorminimumtillageexhibitedatendencyto storemoreTOC.Similarresultswereobtainedinameta-analysis ofvarioustrialsconductedinBrazil(Kaschuketal.,2011).

Thetreatmentsinvolvingsugarcanewithoutburningandthe nativeforestreferenceproducedsignificantlydifferentresultsin termsoflabileandrecalcitrantCbycomparisonwiththe sugar-canewithburning.Inthesugarcanewithburnmanagement,the greatest proportionoftheTOC (74%)wasreadilymetabolizable labileC,butforsugarcanewithoutburningthisvaluewasof54%. Undernativeforest,only36%oftheTOCwaslabileC.Theseresults indicatethatburningresultsinhigheravailabilityofmineralizable andeasily-decomposedC.Therefore,greaterproportionsof recal-citrantCwereobservedundernativeforest(64%)andsugarcane withoutburn(46%)and,accordingtoLossetal.(2009),itresultsin theslowerreleaseofnutrientswithalowerrateofplantresidue decompositionandconsequently,lowerlossofC.

However, in an integrated production system, it would be advantageoustobalancetheCintheoxidizablefractions,withthe sameproportionsofCdistributedbetweenthefractions.Therefore, partoftheorganicmatterwouldbeeasilydecomposableby min-eralizationofnutrientsandtheotherpartwouldbemoreresistant, helpingtoimproveormaintainthephysicalpropertiesofthesoil (Lossetal.,2009).

MineralizationofSOMis responsibleforannuallyconverting from2%to5%oforganicNintomineralN.Thisprocessis regu-latedbysoilmanagementanduse(D’andréaetal.,2004;Kaschuk etal.,2010,2011).Inourstudy,thesoilTNcontentwasalso influ-encedbythetypeofmanagementsystem,withthehighestvalues observedundernativeforest(3.14gkg−1_),_followed_by_the

sugar-canewithoutburntreatment(2.10gkg−1_),_and_then_the_sugarcane

withburn(1.13gkg−1_)._In_the_soil_under_native_forest,_there_may

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Table2

Averagevaluesfortotalorganiccarbon(TOC),totalnitrogen(TN),labilecarbon(L-C),recalcitrantcarbon(R-C)andC/Nratioinadystrophicredlatosol(Oxisol)underdifferent sugarcaneharvestingsystemsandnativeforest(reference).

Management TOC TN L-C R-C C/Nratio

gkg−1

Sugarcanewithburn 8.10Ca _1.13C _6.01C _2.10C _7.20A

Sugarcanewithoutburn 12.48B 2.10B 6.81B 5.66B 5.93C Nativeforest 20.10A 3.14A 7.37A 12.73A 6.41B

pValues 0.0000 0.0000 0.0000 0.0000 0.0000

CV(%) 4.14 3.85 5.01 8.67 2.20

a_Averages_followed_by_different_letters_in_the_columns_differed_{statistically}_in_the_Tukey_test₍_p_<_0.05).

sourceoforganicresiduesinthesoilisassociatedwiththenatural depositionofplantresidueswhichreachthesoil,aswellasorganic substancesfromrootdecomposition.Whencomparingsugarcane harvestingsystems,Canellasetal.(2003)alsoobservedanincrease of47%and50%atdepthsof0–20cmand20–40cm,respectively, inTNlevelsundersugarcanewithoutburnincomparisontothe figuresobtainedundersugarcanewithburn.

ThehighestC/Nratiofigureswereobtainedundersugarcane withburning.ThismaybeduetoacceleratedlossofNcausedby burning.ThefiguresfortheC/NratiointheareasstudiedbyCanellas etal.(2003)undercontinuoussugarcanecultivationvariedfrom 7.6to10.2fortreatmentswithoutadditionofvinasseand sugar-canewithoutburn,respectively,andaccordingtotheauthors,these figuresindicatethepresenceofstableorganicmatter.

3.2. Microbiologicalproperties

MicrobialbiomassconsistsofthelivingfractionoftheOMin thesoilandingeneralcontainsfrom1%to4%Cand3%to5%N. Itthereforerepresentsareservoirofplantnutrients,contributing totheprocessesoforganicmatterdecomposition,andenhancing biologicalsustainabilityandecosystemproductivity(Schloteretal., 2003;Ferreiraetal.,2007).

First,itisimportanttopointoutthatthecoefficientsof varia-tionforcarbonandnitrogeninthemicrobialbiomass(MB-Cand MB-N)areacceptableandinagreementwiththeworkcarriedout bySouzaetal.(2008a).Accordingtotheseauthors,themaximum acceptableCVfortheseparametersis35%.Therewasasignificant differenceinMB-Cbetweenthetreatmentsevaluated(Table3), withthenativeforestexhibitingthehighestlevel(523.79mgkg−1_),

sugarcanewithoutburninsecondposition(328.45mgkg−1_),_and

thelowestlevelsinthesugarcanewithburn.Thelevelsobserved reflectbetterenvironmentalconditionsforthedevelopmentofthe microbialpopulation. According toKaschuket al.(2010, 2011), cultivated systemsgenerally exhibit lower levels of MB-Cthan soilsundernativevegetation.Undernativeforest,abundantplant residuesprovideasubstrateforsoilmicroorganismsto accumu-latebiomassrapidlythroughthedecompositionofresidues.The lowlevelofMB-Cunderthesugarcanewithburncanbeexplained bythehighimpactanddisruptionthatthissystemcausestothe

microbialcommunity,stressingmicrobialpopulationsasaresult ofhigherswingsintemperature,moisturecontentandaeration.

The results of our study are in agreement with those of

Mendonza et al. (2000),who verified that thesugarcane with-out burn system used for cultivation in a Yellow Podzolic sandy/mediumsoilsinthestateofEspíritoSanto,Brazil,resultedin anincreaseinMB-Clevelsatadepthof0–20cmincomparisonwith thesugarcanewithburning.Withtheadditionsofsugarcanetrash, thereisapredominanceofcarbonimmobilizedinthemicrobial biomass,mainlyinthetop5cm.

OurresultsdifferedfromthosereportedbyMarchioriandMelo (2000),whoevaluatedtheeffectsofdifferenttypesofsoil man-agementonorganicmatterandmicrobialbiomassinaredlatosol (Oxisol)andobtainedhigherMB-Cfiguresinsoilsundersugarcane thanundernaturalforest.Accordingtotheauthors,thisincrease inmicrobialbiomassundersugarcanecouldbeduetotheintrinsic characteristicsofthecrop,suchastheorganicsubstances gener-atedbytheplantscultivated,especiallytheroots.However,inthe greatmajorityoftrialsconducted,MB-Cwasfoundtobehigher undernativeforest,andespeciallyprimaryforest.

Ingeneral,MB-CandMB-N aremoresensitivetochangesin environmentalconditionsthanthesoilchemicalandphysical prop-erties(Franchinietal.,2007;Souzaetal.,2008a,b;Kaschuketal., 2010,2011).Forinstance,instudiescomparingdifferentsoil man-agement systems, conservationist systems produced significant increasesinMB-CandMB-N(D’andréaetal.,2002;Franchinietal., 2007;Hungriaetal.,2009;Silvaetal.,2010).

TheaveragefiguresforMB-Nvariedamongthetreatments eval-uated(Table3),withhighervalues undernativevegetationand sugarcanewithoutburning,indicatinghigherimmobilizationofN underthese conditions.Nimmobilization bymicrobial biomass isatemporaryphenomenon,sinceasthemicroorganismsdie,it ismineralizedandtheimmobilizednutrientsreleased,whichis whymicrobialbiomassisconsideredanimportantcomponentof potentiallymineralizableN(Perezetal.,2005).

Deforestationandburningplantmaterialcausemodifications insoilbiologicalcharacteristics.Temporaryincreaseshavebeen relatedintemperateregions(Liuetal.,2007,2010),butingeneral, thesepracticesdecreasemicrobialactivity(MoreiraandMalavolta, 2004; Kaschuk et al., 2010), which was confirmedby the data

Table3

Averagevaluesformicrobialbiomasscarbon(MB-C)andnitrogen(MB-N),microbialquotient(qMic)andC/NratiooftheMBunderdifferentsugarcaneharvestingsystems andnativeforest(reference).

Management MB-C MB-N qMic MB-C/MB-N

mgkg−1 _%

Sugarcanewithburn 162.70Ca _57.80B _2.01B _2.82B

Sugarcanewithoutburn 328.45B 79.45A 2.61A 4.13B

Nativeforest 523.79A 83.87A 2.64A 6.80A

pValues 0.0000 0.0090 0.0014 0.0001

CV(%) 13.96 25.21 15.90 37.35

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obtainedinourstudyforMB-N.Hernández-Hernándezand López-Hernández(2002)alsoreportedthat,inundisturbedareasinwhich plantresidueswereleftonthesurface,therewasagreater concen-trationofMB-N.

TheC/Nratioofthebiomassisoftenusedtodescribethe struc-tureand conditions of themicrobial community (Moore et al., 2000).ThisratioindicatesthepotentialforNmineralizationand alterationsinmicrobialcomposition,withvaluesabove10 indicat-ingpredominanceoffungi,andbelow10predominanceofbacteria (Campbelletal.,1991;Lietal.,2004).

Inourstudytherewerenosignificantdifferencesinthebiomass C/N ratio betweenthe managements withor without burning. However,thesoilundernativeforestexhibitedahigherbiomass C/Nratio,possiblyduetothelowerdecompositionrate(Table3). InastudyconductedbyMoreiraandMalavolta(2004)onasoil subjectedtoasuccessionofplantcoversandmanagementsystems inWesternAmazonia,theprimaryforestalsoexhibitedahigher biomassC/Nratioandsimilarresultsareavailableinliterature.

Themicrobial quotient(qMic,MB-C/TOC) expressesthe effi-ciencyofthemicrobialcommunityinimmobilizingCinorganic residuesinthesoil(Sparling,1992;Gama-Rodriguesetal.,2008a). Furthermore,itindicateshowmuchofthemicrobialbiomass rep-resentsa labilereservoir inthedynamicsoftheorganicmatter (Gama-Rodriguesetal.,2008b),andthisiswhyitisusedtorelate microbialbiomasstotheavailabilityoforganicCinthesoil(Ferreira etal.,2007).ThedramaticimpactofsoilmanagementonqMicwas demonstratedby Franchiniet al.(2007),in thestateofParaná, Brazil,where,inaconservationistsystemwithcroprotationthe qMicwas5.2%,whereasinatillagesystemandwithsoybeanand wheat(TriticumaestivumL.)croppedserially,thisquotientwasof only1.7%.

In regard tothe effects of the treatments on the qMic, the soilundernativevegetationandthesugarcanewithoutburn sys-temexhibitedvaluesof2.64%and2.61%,respectively,significantly higherthanthevalueforthesugarcanewithburnsystem(2.01%) (Table3).Asithasbeendiscussedbefore(AndersonandDomsch, 1986,1993;Ferreira etal., 2007;Maluche-Barettaet al.,2007), higherqMicfiguresindicatehigher availabilityofsubstratesfor microorganisms,havingapositiveinfluenceonmicrobialbiomass. Wecanthereforeinferthat,inourstudy,nativeforestandthe sug-arcanewithoutburningarethemostfavorable tomaintainand increasethesoilmicrobialbiomass(Table3).However,itwasclear thattherewasagreaterbalanceinthemicrobialbiomassunder nativeforestfromtheanalysisoftheMB-C/MB-Nratio,whichwas statisticallyhigherthanthoseofthecultivatedareas.

Asconcludingremarks,wemaysaythattheresultsreported inourstudyshowthatbyeliminatingtheburningmanagement at the sugarcane harvesting there is an improvement in soil fertility—mainly in terms of organic matter—andsoil microbial biomass.However, it is stillpossible toimprove soilquality in comparisontotheconditionsundernativeforest.TOClevelsunder nativeforestandsugarcanewithoutburnwerehigherthanthose undersugarcanewithburnby148%and54%,respectively,andthe soilTN,L-CandR-Cparameterswerealsobetter.Bycomparison withthesugarcanewithburn, thenativeforestand the sugar-canewithoutburnalsoresultedinhighervaluesforMB-C(222% and102%,respectively)andMB-N(45%and34%,respectively).The highersensitivityofmicrobiologicalparametersinresponseto dif-ferentsugarcaneharvestingsystemsalsoshowsthepotentialof usingtheseparametersasindicatorsofsoilquality.

Acknowledgments

Our thanks to Usina Nova América/COSAN for permission to carry out the experiment on their fields. M. Hungria and

M. F. Guimarãesacknowledge research fellowships from CNPq (ConselhoNacionaldeDesenvolvimentoCientíficoeTecnológico, Brazil).

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