Soil organic matter pools and carbon fractions in soil under different land uses

Soil

organic

matter

pools

and

carbon

fractions

in

soil

under

different

land

uses

Daniele

Vieira

Guimara˜es

,

Maria

Isido´ria

Silva

Gonzaga

*

,

Ta´cio

Oliveira

da

Silva

,

Thiago

Lima

da

Silva

,

Nildo

da

Silva

Dias

,

Maria

Iraildes

Silva

Matias

a

AgronomyDepartment,FederalUniversityofSergipe,Sa˜oCristo´va˜o,SE49100-000,Brazil

b

EnvironmentalScienceDepartment,FederalUniversityofSemiarid,Mossoro´,RN59625-600,Brazil

c

GeologyDepartment,FederalUniversityofBahia,Salvador,BA40110-060,Brazil

1. Introduction

Soilorganicmatter(SOM)playsanimportantroleinmaintaining theproductivityof tropicalsoils becauseit providesenergyand substrates, and promotes the biological diversity that helps to maintain soilquality andecosystem functionality. SOMdirectly influencessoilquality,duetoitseffectonsoilproperties(Wendling et al.,2010). Once soilis cultivated for agriculturalproduction, especiallyinthetropicsandthesemi-aridregions,SOMisrapidly decomposedduetomodificationsinconditionssuchasaeration, temperature,and watercontent(Ashagrieetal.,2007). Thiscan affect many soil functions that are eitherdirectly or indirectly relatedtoSOM,duetoitscapacitytoretainwaterandnutrients. AlthoughthebreakdownrateofSOMcanbefasterinthetropics, regularinputsoforganicamendmentscanpromotea buildupof SOM(Follett,2001).

Soilswithlownaturalfertilityandreducedwateravailability havebeenbroughtintoproductionbymeansofhightechnology strategiesincludingirrigationandtheintensiveuseoffertilizersand

pesticides.Thesemanagementpracticesareverycommoninthe tropicsandcancontributetonegativechangesinsoilquality,which shouldbeinvestigatedandmonitoredinordertoreduce environ-mentalimpactsofagriculturalactivitiesaswellastopromoteamore sustainableuseofsoils.DuetothecomplexityofSOMcompounds, therelationshipbetweenSOMcharacteristicsandlanduseispoorly understood.Hence, knowledgeofdifferentpools ofSOM should rendertheimpactsofmanagementpracticemoremeasurable.

Soilorganicmatterlabilefractionshavebeenusedinsteadof totalSOMassensitiveindicatorsofchangesinsoilquality(Bayer etal.,2002;Haynes,2005),duetothemanyimportantinteractions ofthesecomponentsinthesoilsystem.Non-humicsubstancesare labilecompoundswithrelativelyrapidturnoverinsoil,sincethey arereadilyutilizedassubstratesbysoilmicroorganisms(Schmidt et al.,2011). Humic substances aremore stableorganic matter compounds,whichmakeupasignificantportionofthetotalsoil organicCandN(Lal,1994;Milorietal.,2002).Humicsubstances canimprovesoilbufferingcapacity,increasemoistureretention, andsupplyplantswithavailablemicronutrients.Moreover,these compounds can alsobind metals, alleviating both heavy metal toxicityandmetaldeficiencyinsoils(McCarthy,2001).

Humicsubstancesrepresentapproximately40–60%ofthesoil organic matter, and include three different fractions, defined

A R T I C L E I N F O Articlehistory:

Received11November2011 Receivedinrevisedform14June2012 Accepted29July2012 Keywords: Humicsubstances Soilquality Tropicalsoils A B S T R A C T

Changesintropicallandusehaveprofoundeffectsonsoilorganicmatter(SOM)status.Ithasbeen suggestedthatalterationsinthedifferentfractionsofSOMaremoreeffectiveinindicatingchangesin soilusethantotalsoilorganicmattercontent.Themain objectiveofthisstudywastoinvestigate changesinthecontentofhumicsubstancesinanUltisolunderdifferentlanduses,inthenortheast regionofBrazil.Soilsampleswerecollectedfromthe0–10and10–30cmlayers,inthreeagricultural areas(conventionalcoconutorchard,integratedcoconutorchard,andcitrusorchard).Anativeforestsoil was used as reference. Organic C and totalN weredetermined to characterize theSOM. Humic substanceswerechemicallyfractionatedintofulvicacid,humicacid,andhumin,basedonsolubilityin acidandalkali.Significantloss(47.5%)ofsoilorganicmatterwasobservedinthesurfacelayersofthe conventionalcoconutandcitrusorchards,comparedto thenativeforest.TherewasincreasedSOM contentintheintegratedcoconutorchardsoil,duetothepresenceofcovercropsaswellasmanagement ofcropresidues.However,inthesubsurfacesoiloftheintegratedcoconutorchard,cultivationmodified thedistributionofthemorelabilefractionsofthesoilorganicmatter,asmeasuredbytheratiobetween humicandfulvicacids(>1.0),indicatingasubstantiallossoffulvicacids.Thedegreeofhumificationwas intherange40–97%.Thedistributionsofthesoilorganicmatterfractionsvariedintheranges12–32.5% (fulvicacids),12–34.5%(humicacids),and40–69.5%(humin).

ß2012ElsevierB.V.Allrightsreserved.

*Correspondingauthor.Tel.:+557921056984;fax:+557921056929. E-mailaddress:[email protected](M.I.S.Gonzaga).

ContentslistsavailableatSciVerseScienceDirect

Soil

&

Tillage

Research

j o urn a l hom e pa g e :ww w . e l se v i e r. c om / l oca t e / st i l l

0167-1987/$–seefrontmatterß2012ElsevierB.V.Allrightsreserved.

according to different stability under acid hydrolysis and permanganate oxidation (Paul et al., 2001). Humin (H) is the insolublefractionof humic substances;humic acid(HA) is the fractionthatissolubleunderalkalineconditions;andfulvicacid (FA)isthefractionthatissolubleunderbothalkalineandacidic conditions(SuttonandSposito,2005).Thechemicallyreactiveand refractory nature of humic substances contributes to their persistencein soils(KiemandKo¨gel-Knabner,2003;Roviraand Vallejo,2007),aswellastotheirimportantroleinnutrientflows throughecologicalsystems,andCemissionstotheatmosphere (Lal,2006).Therelationshipbetweenconcentrationsofhumicand fulvicacids(HA/FAratio)isindicativeofthepotentialmobilityofC inthesoilsystem.Ingeneral,sandysoilspresenthigherratiosdue to the solubility and selective loss of fulvic acids during the decompositionofSOM.The(HA+FA)/humin ratioindicates the degreeofilluviationofSOM(Benitesetal.,2003).

Changesinfieldmanagementpracticescanalterthechemical propertiesofsoilhumicsubstances(Moraesetal.,2011).Spaccini etal.(2006)reportedaprogressivedecreaseinhumicsubstance concentrationsinsoilsthatwereconvertedfromforesttoarable farming. Such decrease is commonly attributed to microbial oxidationoftheorganicmaterialspreviouslyprotectedinthesoil aggregates destroyed by cultivation. Most studies report a reductionin SOM andin its fractionswhen forestis converted tootherlanduses(NavarreteandTsutsuki,2008).

InnortheastBrazil,theproductionoftropicalfruitsoccupiesa largeareadominatedbyUltisols.Managementpracticesandyields varyamongorchards,andsoilqualityhasnotbeenafocusofmany farmers. In this region, there have been few studies that have addressed the impact of land use change on levels of humic substancesinsoils.Theobjectiveofthisstudywasthereforeto evaluateSOMcharacteristicsasinfluencedbylanduse.Thedegree ofhumification of SOM, togetherwiththecontributions of the different humic substance fractions, was determined using an operationallydefinedchemicalfractionationmethod,appliedtoan Ultisolcultivatedwithtropicalfruittrees.Thesamesoiltypeunder nativeforestwasusedasareference.

2. Materialsandmethods

2.1. Sitecharacterizationandexperimentaldesign

TheworkwasconductedbetweenAugustandOctober2010on irrigatedfarmsinthestateofSergipe,inthenortheastofBrazil.The climateistypeBSh,accordingtotheKoppen–Geigerclassification (Peeletal.,2007),withdrysummersandrainfallconcentratedin themonthsofMaytoSeptember.Themeanannualprecipitation andtemperatureare1200mmand308C,respectively.Thesoilis anUltisol(typicHapludult).Physicalandchemicalpropertiesof thesoilunderdifferentusesarepresentedinTable1.Soiltexture

wasanalyzedbythehydrometermethod,pHwasmeasuredusing a 1:2.5 soil/solution ratio, P was extracted using Mehlich 1 solution,andCECwascalculatedbythesumofcations.

The study was carried out according to a fully randomized factorialdesign,withfourtypesoflanduse(conventionalcoconut (Cocus nucifera L.) orchard, integrated coconut orchard, citrus orchard,andnativeforest)and2depthsofsampling(0–10cmand 10–30cm),with3 replicates.Theconventional coconutorchard wasmanagedwithoutanyconservationpractices,usingmineral fertilization (NPK), mechanicaland chemical weed control,and micro-sprinkler irrigation (4.0mm of water). The integrated coconut orchard was managed with mineral fertilization and conservationpracticesthatincludedorganicfertilization(castor beancake,cowmanure),leguminouscovercrops,andmulching. Themaincrop residuesweredispersedonthesoilsurface,and irrigationemployedamicro-sprinklersystem(4.0mmofwater). Thecitrusorchard(CitrussinensisL.Osbeck)wasconventionally managed, with application of mineralfertilizer and pesticides. Weedswerecontrolledusingtillageoperations,andirrigationwas performed via central pivot (4.0mm of water). The native vegetation wasa remnant of the Atlantic Forest. The orchards wereimplementedin1998.

Eachexperimentalareawasdividedintothreeplotsof100m2_.

In each plot,tensoil sampleswererandomlycollected ateach depth, and mixed to obtain a composite soil sample. The soil sampleswerehomogenized,andtransportedinplasticbagstothe laboratory, where theywere air dried,sieved through a 2-mm screen,andstoredpriortoanalysis.

2.2. Chemicalfractionationofsoilorganicmatter

The chemical fractionation was carried out following the methodproposedbytheInternationalHumicSubstancesSociety (IHSS)(Hayesetal.,1989)toobtainhumin(H),humicacid(HA), andfulvicacid(FA)fractions,basedonthesolubilityinacidand alkali. Total soil organic C (TOC) and the C contents of these fractions weredetermined accordingto Yeomans and Bremner

(1988). The relationships HA/FA and (HA+FA)/H were also

calculated.Thedegreeofhumification wascalculated usingthe formula(HA+FA+H)/TOC100.

Soil total N was determined by the Kjeldahl method, as describedbyBremnerandMulvaney(1982).

2.3. Statisticalanalysis

Thedatawereanalyzedfollowingcompleterandomizeddesign andusingsubplots.Thesoilusetypesandsamplingdepthswere considered as primary and secondary effects, respectively. The resultsweresubmittedtovariance analysisforidentificationof relevanteffects,andthemeanswerecomparedusingtheTukey test (P<0.50). All analyses were performed using SISVAR software,version5.0(Fereira,2003).

3. Resultsanddiscussion

3.1. Totalsoilorganicmatterandstratificationratio

Totalsoilorganicmatter(SOM)concentrationsinthe0–10cm soil layer differed significantly among land uses (Fig. 1), and followedtheorder:integratedcoconut>nativeforest>citrus= -conventionalcoconut.Consideringthesoilofthenativeforestasa reference,theintegratedcoconutorchardshowedanincreasein SOMofapproximately37%.Ontheotherhand,intheconventional coconutandcitrusorchardstheSOMconcentrationswerereduced byalmost40%.Thiswasmainlybecausethedifferentsoilusesand managementsaffectedtheinputsoforganicresidues.Evenwithin

Table1

Physicalandchemicalcharacterizationofthesoilsunderdifferentlanduses.

Landuse pH P* (mgkg1 ) CECa (Cmolckg1) Sand (%) Silt (%) Clay (%) 0–10cm Conventionalcoconut 6.4 21.0 6.40 76.4 15.8 7.78 Integratedcoconut 6.5 54.1 10.4 80.1 12.4 7.55 Citrus 6.7 24.5 5.16 83.3 10.8 5.87 Nativeforest 5.0 0.21 5.91 85.6 10.2 4.22 10–30cm Conventionalcoconut 5.5 4.20 4.27 67.6 23.3 9.04 Integratedcoconut 6.3 18.4 7.25 74.8 16.5 8.66 Citrus 6.5 7.80 3.93 80.1 13.4 6.51 Nativeforest 4.7 0.08 3.30 84.4 10.1 5.53 * Mehlich1extractableP. a

thesamelanduse(aswasthecaseforthetwococonutorchards), SOMconcentrationscanbeverydifferentasaresultofdifferent managementpractices.Intheintegratedcoconutorchard,thesoil wascultivatedusingconservationpracticesthatincluded legumi-nous cover crop (Pueraria phaseoloides Benth), mulching, and frequentinputsoforganicandinorganicamendments,whichalso improvedplantbiomassproductionandconsequentlycontributed togreateramountsofresiduesaddedtothesoil.Thesemimicked naturalsystemsbyincreasingresiduereturnsandminimizingC removal,bothofwhicharerecognizedasimportantcomponentsof sustainablesystems(Lal,2008).Thesepracticeswerenotapplied intheconventionalcoconutandcitrus orchards.Thedifferences observedacrosslandusescanalsobeattributedtodifferencesin microclimate, vegetation canopy, and litter input (Yao et al., 2010).In the 10–30cm soil layer, SOM concentrations were significantlylowerthaninthe0–10cmlayer,withvaluesinthe order: native forest=integrated coconut>citrus=conventional coconut.TheresultsobtainedheredifferedfromthoseofSchroth

et al. (2002), who reported no significant difference in C

concentrationsbelow10cm depth.Theirstudyshowedthatthe effect of forest conversion to crops, as a result of agricultural activities,waslargelyrestrictedtothetopsoil.MostoftheSOM content was concentrated near the soil surface (0–10cm), as shown by thestratification ratio,which varied from2.0 to3.4 (Fig.2).AccumulationofSOMatthesoilsurfacewasaresultof surfaceplacement ofcrop residues,togetherwitha lackofsoil disturbancethatkepttheresiduesisolatedfromtherestofthesoil profile.TheSOMstratificationratiowasgreaterunderintegrated coconutcultivationcomparedwithotherlanduses.Accordingto

Schnabeletal.(2001),stratificationofSOMpoolswithsoildepthis commoninmanynaturalecosystems,suchasforests,aswellasin managed grasslands. Besides surface placement of organic

residues, undisturbed soils offer a suboptimal decomposition environment,thuscausingaccumulationofSOMatthesoilsurface (Franzluebbers,2002).

However, care must be taken when evaluating the SOM stratificationratioasanindicatorofsoilquality,becauseahigh stratificationratiodoesnotalwaysreflectgoodsoilquality.High SOM stratification ratios are expected to indicate relatively undisturbedsoilwithgoodwaterinfiltrationcapacity,aggregate stability,microbial activity,andnutrient supply(Franzluebbers, 2002). Inthepresentwork, thiswasasexpectedforthenative forestsoil.However,theconventionalcoconutandcitrusorchard soilsalsopresentedhighSOMstratificationratios,butthistimeas aresultofsoilcompaction(datanotshown),whichreducedthe rateofresiduedecomposition.

3.2. Soiltotalnitrogen

Soiltotalnitrogen(TN)isanimportantcomponentofSOM,and greatlyinfluencesSOMdecompositionandhumificationrates.TN concentrations varied widely among land uses (from 0.58 to 1.14gkg 1_{inthe0–10cmlayerandfrom0.29to0.55gkg} 1_inthe

10–30cm layer)(Fig.3). TotalN valuesweregreateratthesoil surfacethanatdepth(Fig.3).Thesoilunderintegratedcoconut cultivationshowedbestresults,presenting34%moreTNthanthe native forest soil, probably as a result of good management practices and application of N as both inorganic and organic amendments.Thepresenceofleguminousplantspeciesascover crops,frequentinputsoforganicresidue(cowmanureandcastor beancake),andretentionofplantresiduesonthesoilsurfacewere some of the management practices that contributed to higher levelsofsoiltotalN.Undernaturalconditions,soilTNiscontrolled byclimateandvegetation.Intropicalsoils,TNvariesfrom0.02to 0.40%,andis mainly(95%) presentin organicforms(Stevenson, 1994).Thepresentresultswerethereforewithinthenormalrange (0.03–0.14%).Ontheotherhand,verylowTNconcentrationswere observedintheconventionalcoconutorchardsoil.Thissitewas frequently treated with herbicides to control weeds, and no residuemanagement wasemployed.TNconcentrationsreduced significantlywithsoildepth,followingtheCtrend.

3.3. Degreeofhumificationofsoilorganicmatter

The degree of humification of SOM is a measure of the proportions of humic and non-humic substances. Non-humic substancesarelabilecompoundswithrelativelyrapidturnoverin soil,andarereadilyutilizedassubstratesbysoilmicroorganisms

(Masciandaro and Ceccanti, 1999). Humic substances are more

stableorganiccompoundsthatmakeupasignificantportionofthe totalorganicCandNinsoil(Lal,1994).

Fig.1.Soilorganicmatterunderdifferentlanduses,atdifferentsoildepths.Means followedbythesameuppercaseletter(foragivensoildepth),andlowercaseletter (foragivenlanduse)arenotsignificantlydifferentusingTukey’stestatp<0.05.

0 0.5 1 1.5 2 2.5 3 3.5 4

Conventional coconut

Integrated coconut Citrus Native forest

Stratification ratio of soil organic

Land use

Fig.2.Stratificationratioofsoilorganiccarbonunderdifferentlanduses.

Fig.3.Soiltotalnitrogenunderdifferentlanduses,atdifferentsoildepths.Means followedbythesameuppercaseletter(foragivensoildepth),andlowercaseletter (foragivenlanduse)arenotsignificantlydifferentusingTukey’stestatp<0.05.

Soilsfromthedifferentlandusespresentedvarieddegreesof humification(from23.0to86.3%)(Fig.4).Meanvaluesfollowed theorder:citrus(57.4%)>conventionalcoconut(51.2%)>native forest(46.0%)>integratedcoconut(43.8%).Soilsfromthenative forest and the integrated coconut orchard presented a lower degreeofhumificationwhencomparedwiththeotherlanduses. This suggests that there was a greater content of non-humic substances, probably due to non-decayed inputs. Even though these two land uses were very distinct, as far as inputs and decomposition of organic residues and microclimate are con-cerned,goodsoilqualityisfavoredbytheinputoflargeamountsof organic residues, which contributes to improved soil physical, chemical,andbiologicalproperties(Denefetal.,2007). Consider-ingthatthetotalSOMcontentwasgreaterin soilundernative forestand in soil cultivated withcoconut using theintegrated management system, it can be concluded that non-humic substancescomprisedmostoftheSOMinthesetwoareas.This labile fractionof SOM may be physicallyprotected within soil aggregates(Milorietal.,2002),andaccountforthemajorpartof soilC. Inaddition,theintegratedcoconutorchardsoilwaskept constantlyprotectedwithcovercrops,grass,andorganicresidue amendments. This management practice improves soil water retention,protectssoilfromdirectsolarirradiation,andreduces temperatureatthesoilsurface,allofwhichacttoreducetheSOM

mineralization rate. The same features were observed for the nativeforestsoil,forthesamereasons.

Therewasanincreaseinthepercentageofhumicsubstances withsoildepthacrossalllanduses.Thisisacommontraitinmost soils, especially in those from forests (Milori et al., 2002). The buildupofhumicsubstancesinsubsurfacelayersisprobablyrelated totheirrecalcitrance,theiradsorptionbythesoilmineralmatrix, andtheirintra-aggregateocclusion(Mikuttaetal.,2006).Inthesoil evaluatedinthisstudy,therewasanaturalincreaseinclaycontent fromthe surface (0–10cm)tothe subsurfacelayer (10–30cm), whichalsoreinforcestheincreaseinhumicsubstancesatdepth. Therewasalsoagreaterproportionoffreshorganicmaterialinthe surfacelayersascomparedtothesubsurfacelayers.

AccordingtoTeit(1991),thestabilityofhumicsubstancesin soilsisrelated totheregular additionoforganic matter,which determines the formation of new humic substances and their destructionduetotheformationofstablehumusforms.Therefore, evenifthereareminimalalterationsintotalCinputs,changesin managementandCquality,whichinturnaffectbreakdownrates, canalsohaveeffectsonCdynamicsindifferentsystems(Lal,2008).

3.4. Fractionationofhumicsubstancesinsoil

Concentrationsof Cinfulvicacid(FA),humicacid(HA),and humin fractions in the 0–10cm and 10–30cm soil layers are presentedinTable2.MeanvaluesofFA,HA,andhuminvariedfrom 0.46to1.08gkg 1_{,0.41to0.75gkg} 1_{,and1.60to2.52gkg} 1_,

respectively.Asexpected,humincontentswerehigherthanthose oftheHAandFAfractions,foralllanduses(Table2).Thehumic substancesofforestsoilsarecharacterizedbyhighercontentofFA thanHA(GuggenbergerandZech,1994),asobservedinthisstudy. Significant differencesbetween the soilsin terms of humin contentwereonlyobservedforthe0–10cmsoillayer(integrated coconut=native forest>citrus=conventional coconut). The huminfractionislesspronetochangewithmanagementpractices, becauseitisthemoststableandrecalcitrantfraction(Stevenson, 1994).Huminconcentrationsdecreasedsignificantlywithdepth foralllanduses,withtheexceptionofthecitrusorchardsoil.A higherconcentrationofCinthehuminfractionofthesoilsurface layer could be related to intensive microbial activity and consequently a higher SOM decomposition rate. Furthermore,

Fig.4.Percentagesoftotalhumicsubstancesinsoilsunderdifferentuses.Means followedbythesameuppercaseletter(foragivensoildepth),andlowercaseletter (foragivenlanduse)arenotsignificantlydifferentusingTukey’stestatp<0.05.

Table2

Humicsubstancesorganiccarbon(fulvicacids:FA;humicacids:HA;humin)insoilunderdifferentlanduses,atdifferentsoildepths.Meansfollowedbythesameuppercase letterinarow(foragivensoildepth),andlowercaseletterinacolumn(foragivenvariableandagivenlanduse)arenotsignificantlydifferentusingTukey’stestatp<0.05.

Humicsubstances(gkg1

) Soildepth(cm) Landuse

Conventionalcoconut Integratedcoconut Citrus Nativeforest

FA 0–10 0.56Ba 0.60Ba 0.69Ba 1.16Aa

10–30 0.37Cb 0.65Ba 0.52Bb 1.01Aa

HA 0–10 0.59Ba 0.90Aa 0.43Ba 0.92Aa

10–30 0.49Aa 0.40Ab 0.40Aa 0.58Ab

Humin 0–10 2.17BCa 3.37Aa 1.80Ca 2.82ABa

10–30 1.14Ab 1.67Ab 1.41Aa 1.53Ab

HA/FA 0–10 1.06Ba 1.53Aa 0.62Aa 0.79Ca

10–30 1.31Aa 0.61Bb 0.73Aa 0.58Ba

HA+FA/Humin 0–10 0.54ABb 0.44Ba 0.62ABb 0.74Ab

10–30 0.77Ba 0.64Ba 0.54Aa 1.04Aa

HA+FA/TOC 0–10 0.15Ab 0.10Bb 0.14Ab 0.17Ab

10–30 0.25Aa 0.21Aa 0.24Aa 0.26Aa

(HA+FA+Humin)/TOC 0–10 0.44Aa 0.33Bb 0.38ABb 0.40Aa

10–30 0.58ABa 0.54Ba 0.76Aa 0.52Ba

C/N 0–10 12.8Ba 14.7Aa 11.7Ba 14.4Aa

huminconstitutesamajorpartofthetotalorganiccarbon(TOC), and has an intimate association withthe soil mineral fraction (Moraesetal.,2011).

Sincetheconcentrationsofhumicsubstancesareinfluencedby theamountsoforganicresidueinputsandTOC,calculationwas madeoftheproportionofeachfraction(fulvicacids,humicacids, and humin) (Fig. 5). The overalltrend washumin (61.0%)>FA (21.4%)>HA(17.6%).MoreFAthanHAwereobservedinallsoils, withtheexceptionoftheconventionalcoconutorchardsoil.The frequentinputoffreshorganicresidues,especiallyinthecaseof thenativeforestsoil,contributedtothehigherproportionofFA. Irrespectiveofsoildepth,theFAcontentfollowedtheorder:native forest=citrus>integrated orchard=conventional coconut. It is interesting to note that although the citrus and conventional coconutorchardspresentedsimilarconcentrationsofSOM(Fig.1), thedistributionofhumicfractionswasverydifferent(Fig.5).The proportionofFAwassignificantlyhigherinthecitrusorchardsoil thanintheconventionalcoconutsoil.Theinverseoccurredforthe HA.TheHAfractiondidnotdiffersignificantlyamongsoiluses. MaintenanceofhighlevelsofSOM,especiallythemoresoluble fractions,inthecultivatedsoilsinvestigatedinthisstudywouldbe averyhardtask,duetothesandynatureofthesoilcombinedwith thewarmclimateoftheregion.

Thehumin fraction proportionin the integratedorchard was higherthaninthenativeforestsoil,anddidnotdifferamongthe cultivated soils, indicating the occurrence of a more intensive decompositionprocessintheagriculturalsoils.Thisresultconfirms theassertionthattotalSOM isnota verysensitiveindicator for evaluationofchangesinsoilqualityasafunctionofsoilmanagement, becausethepredominantfractionisthemorerecalcitrantpool.

3.5. Ratiosinvolvinghumicsubstances

The relationships between different components of humic substancescanbeofgreatinterestwhenstudyingtheeffectofland useandsoilmanagementonsoilquality.

Theratiobetweenhumicandfulvicacids(HA/FA)reflectsthe mobilityofsoilorganiccarbon.AHA/FArationear1isindicativeof good quality organic material that could enhance soil physical propertiesandimproveplantgrowth.AHA/FAratio>1indicates lossof themorelabile FAfraction, a situationverycommon in sandysoils.Inthisstudy,theHA/FAratiovariedfrom0.62to1.53 (0–10cm),andfrom0.58to1.31(10–30cm),andshowedsome persistenceofFAinsoilundercitruscultivationandnativeforest

(Table 2). More FA than HA indicates low humification rates.

Althoughtheconservationmanagementpracticesappliedinthe integratedcoconutorchardcontributedtoanincreaseinthetotal SOMconcentration,degradationrateswerehighduetothequality of organic residueadded,as wellasthehighN content,which increasedthelossofthemorelabilefractioninthesurfacelayer. Theratiobetween thecombinedhumicandfulvicacids and humin((HA+FA)/humin)canprovideinformationabouttheloss ofSOMthroughthesoilprofile.Forthedifferentlanduses,theratio variedfrom0.44to0.75(0–10cm),andfrom0.54to1.05(10– 30cm),indicatingthepredominantpresenceofhumin(Table2). Thebroadrangeofvaluescouldberelatedtohighheterogeneityof the organic residues applied to the different sites, as well as differences in theSOMhumification process. Thevalues of the (HA+FA)/H C ratio were lower than 1 across all land uses, indicatingagreaterproportionofthehuminfraction.Thesevalues couldalsoberelatedtostronginteractionbetweenSOMandthe soilmineralphase,resultinginhighstabilityofSOMinthemore recalcitranthuminfractions(Canellasetal.,2008).

The rates of decomposition of organic residues and, as a consequence,theaccumulationofSOM,areinfluencedbytheC/N ratio,which variedfrom9.0to14.4.Thehighest C/Nratiowas foundforthenativeforestsoil.Inthecultivatedareas,frequent inputs of N fertilizers contributed to lower C/N ratios, due to stimulationofresiduedecomposition.Thiswasconfirmedbythe reducedTOC andCconcentrationsin theSOMfractionsofsoils fromthecultivatedareas.

4. Conclusions

TheuseofSOMchemicalfractionstoevaluatechangesinsoilC dynamics due to agricultural use was more effective than determination of total SOM. The distribution of C among the different fractions changed when forest soil was brought into cultivation,due totheamountand qualityof organicmaterials addedtothesoils.Amongstthemostlabilefractions,fulvicacid predominated,resultinginaHA/FAratio<1formostlanduses. The predominance of a higher percentage of fulvic acids, as compared to humic acids, indicated eithera slowrate of SOM decomposition,orfrequentinputsoffreshorganicresidues.Fulvic acidwaseffectiveindiscriminatingchangesinlanduse,duetoits characteristiclability.Ontheotherhand,thehuminfractionwas stable,andsimilarforthedifferentcultivatedsoils.Comparedto thenativeforestsoil,cultivationincreasedtheproportionofthe recalcitrantfraction,andledtoareductioninthelabilefractionof SOM. Sustainable management practices should therefore be employed in order to achieve soil stability and biological productivity.

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