Amounts, dynamics and sequestering of carbon in tropical and subtropical soils

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Amounts, dynamics and sequestering of carbon in tropical and subtropical soils

  1. 1. Royal Swedish Academy of Sciences Amounts, Dynamics and Sequestering of Carbon in Tropical and Subtropical Soils Author(s): Wim G. Sombroek, Freddy O. Nachtergaele, Axel Hebel Source: Ambio, Vol. 22, No. 7, The Royal Colloquium (Nov., 1993), pp. 417-426 Published by: Springer on behalf of Royal Swedish Academy of Sciences Stable URL: http://www.jstor.org/stable/4314120 . Accessed: 15/06/2011 23:43 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=springer. . Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Springer and Royal Swedish Academy of Sciences are collaborating with JSTOR to digitize, preserve and extend access to Ambio. http://www.jstor.org
  2. 2. WimG. Sombroek, Freddy0. Nachtergaele and Axel Hebel Amounts, Dynamics and Sequesterin of Carbon in Tropical and Subtropical Soils L] < 3.6 3.6-7.5 7.6-15.0 15.1-35.0 in >35.0 Figure 1. Soil organic carbon pool in kg C m-2up to I m depth. The organiccarbon pool inthe upper 1 m of the world'ssoils contains 1220 Gtorganic carbon, 1.5 times the total forthe standing biomass. In the widespread deep soils in the tropicsthe carbon stored below 1 m may add about 50 GtC. Thecontributionsofcharcoal, roots and soil fauna should be added to these totals. The much less dynamic carbonate- carbon pool amounts to 720 Gt C. Changes in land use, particularlybyclearing of forests, reduce organic carbon by 20 to 50% inthe upper soil layers, but littleindeeper layers. On the other hand, there are indications that a human- inducedenrichmentofsoil organic mattercan be maintained over centuries. Research on the causative soil processes should be supported, because an improved understanding of this phenomenon might lead to better management strategies and sound programs to stimulate organic carbon storage and fertilitylevels in tropical and subtropical soils. Recent research data on the C02 fertilizationeffect and the associated antitranspirationeffect due to an increase ofC02 inthe atmosphere indicate that a positive influence on soil organic carbon levels can be expected. INTRODUCTION Soil carbonis an importantpartof the terrestrialcarbonpool. Itssize hasbeenestimatedbetween700 and3000 Gt C (1 Gt = i015 g C) as organiccarbonand 780 to 930 Gt C as calcium carbonate.Othercarbonpoolsaretheoceans(38 000 GtC),fossil carbonreserves(6000 GtC), CO2in theatmosphere(720 GtC), andbiomassof plants(560 to 835 GtC) (1). Thebeneficialeffects for agriculturalproductionof soil car- bon in its organicform(freshorganicmatter,humus)arewell- known: source of nutrientsgradually becoming available to plants;increasedaggregatestability;increasedmoisturestorage capacityandincreasedmicrobialdiversityandactivity.Higher percentagesgenerallyimprovethe chemical, physical andbio- logical statusof the soil. In spite of this, soundorganic-matter managementpracticesremaintheexceptionratherthantherule, particularlyin tropicaland subtropicalenvironments.More re- cently, soil organic matterhas received attentionin scientific circles for its role in the sequestrationof the greenhouse gas CO2,andtheemissionof C02, CH4andN20. Theobjectivesof thisarticleareto refinecurrentestimatesof theorganicsoil carbonpools by usingtherecentlycorrectedand digitizedFAO/UnescoSoil Mapof the World(2), to emphasize the role of sound organic-mattermanagement,to explore the possibilitiesfor soil-carbonsequestering,andto drawattention to possible effects of global climaticchangeon the soil-carbon pool. CARBONPOOLSINTHEWORLD Average Soil Carbon Contents Based on the FAO/Unesco Soil Map of the World Organic Carbon Estimatesofthestorageoforganiccarboninsoilsonaglobalscale varybetween700 and3000 GtC(1, 3-1 1).Thislargevariationis explainedby thedifferencein basemapsselected(FAO/Unesco SoilMapof TheWorld,HoldridgeLifeZones,VegetationMaps) andthe variousassumptionsmadeaboutsoil attributesdirectly relatedto thetotalcontentof organic-matterpersoil typesuchas organicmattercontentanddistributionin the soil profile,bulk densityof soil layers,andaveragesoil depths. In the present approach,the correctedand digitized FAO/ AMBIOVOL.22 NO. 7, NOV. 1993 417
  3. 3. Unesco Soil Mapof the World(2, 3) was used to estimatethe extent of each soil unitin the mappingunits.A studyof about 400 soil profiles groupedper FAO soil unitgave an indication of the rangeandmedianvalues for organic-carboncontentand bulkdensityfor each soil unit(Table 1). These medianfigures were used to calculatethe total soil organic-carboncontentfor eachmappingunitandwerecorrectedforanassumedsoil depth as indicatedby specific soil phases or soil names on the map: Lithosols(10 cm), RendzinasandRankers(30 cm), petrocalcic, petrogypsic,duripanand petroferricphases (half 75 cm deep, half 30 cm deep), lithic phases (30 cm deep). Othersoils were considered100cm deep.Litterlayerswerenotconsideredinthe calculation,butHistosols wereincluded. Theapplicationof thismethodologyyieldeda globalestimate of about1220Gtorganiccarbon.Thisfigureis somewhatlower thanmost recently publishedestimates (1400-1700 Gt C) but very close to the one obtainedby using soil informationand Holdridgelife zones (1270 Gt C; 13). The resultsfor the world arepresentedin Figures1and2. Individualcountryresultswere also estimatedandareavailablefromthe authorsuponrequest. Theyvaryfromabout2.5 kg m-2forcountriesin aridareas(e.g. Libya,Mauritania)to about25 kg m-2in FinlandandSweden. Itshouldbe notedthatourestimateis significantlylowerthan theone resultingfroma recentstudyby a USA teamof soil sci- entists (14). They arrivedat 1576 Gt of organiccarbonin the world'ssoils to I-m depth.The difference is due to the use of anothersoils database, viz. a world mapof majorsoil regions preparedby the USDA Soil ConservationService, still in press (15). It uses the USDA Soil Taxonomysystemof classification (16) ratherthanthe FAO-Unesco one, and dataof 1000 indi- vidualsoil profilesof 45, mainlytropicalcountries,in addition to 15000 profiles of continentalUSA. The divergency of the figuresshouldbe due to differencesin the criteriafor soil clas- sification,mappingscales, degree of representativenessof soil profilesused anddegreeof incorporationof new soil inventory information.The discrepancymay be solved in the nearfuture by contactsbetweenFAO, USDA andISRIC(the International Soil Reference and InformationCentre of Wageningen, The Netherlands),underthe aegis of the IGBPStandingCommittee on DataandInformationSystems. FAO andISRICarealready engagedin updatingof its georeferencedsoil databaseforLatin America;thedataon thatregionfortheexisting 1:5Mapof that regionare35-yearold, andin themeantimemuchimprovedin- formationhas become availablethroughthe efforts of national soils resources institutionsin most of the regions, especially concerningthe Amazonregion. Organic Soil Carbon Distribution with Depth Calculationsof theorganiccarbonpool intheworld'ssoils have traditionallyprovided results to 1 m depth. This facilitates comparisonof resultsbuthasthedisadvantagethatnotthewhole soil-carbonpoolistakenintoconsideration.Thereislittlescientific justificationtoconsideronlythefirst 1mof thesoil profileinthe calculation,becausemanysoilsaredeeperandcontainconsiderable amountsof carbon(organicandinorganic)inthesubsoil.Indeed, althoughorganic-carboncontentsgenerallydecreasewithdepth insoils (withtheexceptionof somealluvialandorganicsoils),the contributionof carbonin the deep subsoil-below 1 m-to the totalcarbonpool is importantparticularlyin soils of the tropics andsubtropicswhicharegenerallydeepto verydeep. It was pointedout (17, 18) thatthe age of soil organiccarbon increases with depth. This indicates that in the tropical soils deeper layers contain an appreciableamount of soil organic matteras stable,relativelyinerthumuswhich would not be af- fected by a changein landuse. Detailedanalysesof the carbon profiles in these soils supportthatfinding. Indeed,the organic carboncontent in the uppermeter of the majortropical soils Vertisols, Andosols, Ferralsols,Arenosols, Acrisols and Luvi- sols is 53, 66, 69, 77, 80 and 82%,respectively,of the amount containedup to 2 m depth(Table2). This implies thatthe total globalamountof soil carbonis largerthanthe 1220Gtestimated fortheupper1m andmaybe as muchas 1270 GtC, takinginto accounttheproportionof these deeptropicalsoils. A calculationon about30 soil profiles of the Amazon area sampledduringFAO forest inventoriesin the early 1960s and some morerecentsurveysshow thatmorethan90%of the up- land (terrafirme) soils, Ferralsolsand FerricAcrisols mainly, with samplingdown to 200 cm or morehave total carboncon- .. .. ... ... . ....................... .............. .................... . . ........ . ... . . .. ................... . ............. .... ....................... ....... .......................................... ............ .... .. .. ... ... ... ........... ...... .. ... . . .... ..... . ................................. .......... . .. ....... . .......... .. .... .... .. . .... .. ..... ............................. Up, . .... .... .... .... ... ....... .... . . ........ . . ....... .. ....... . . .. .... . . ....... ... ... ...... .... ... . .. . ............ .... ............ ................... ........ ........ ... ... ... ... .......... .... .. .... ... .. .. ............ ... II .. ... ................ ........ .... ... . ....... ... .... .... ........ .................. .. ............................. .............. . . . ......... ..................... . ...... ... .... ........ ..... ..... ... .... .... ....... .. ......... ... .. .. ....... ...... ........ .. ....... . .. .......... .. . ... ................. ............................. ............................ 1-1.1-11.1-1 ...... .... .................. ........... .. ... ... . .. ... .......................... .S ......... .... .. .. ... ....... ...... . . . 13 . . ........ . ....... . .... ......... . aR............... .... .... ........ . ............I...... .... 2-1::-44- . ........ ......... ... .... ......... .... ......... ... .............. .... .................................... ........ . ......... ........ ............. gp . ........................ ..... ....... ...... .... .......................................... ........... ....... .. .............. ....... .... . ...... ..... ... ... ... ............... ............ ... . ....... .......... . ............ .. .... .... .. .. .... ..... ........ . .......... .... . ... . ....... ... .... .. . ... ..... . ........... . . ....... . ....................................... .. .... . .. .... ... .... .... - - - - - - ............................. .................. . ...... ... .... ................ .............. . ........ ... ... ..... .. ................ .... . ... ..... ................ .. .......- .............. ..................... .. ... ... ... ........ r%p - --- --- ------- 418
  4. 4. tentsthatvarybetween 13 kg m-2,for the veryclayey soils, and total8 kg m-2 for the sandy or moderatelydeep soils, with the upper25 cm accountingfor only one thirdof the amountsto 2 m depth (43% of amountsto 1 m depth), unless the soils are shallow or imperfectly drained(19, 20). These data compare well with recentcalculationson carboncontentof all recorded soil profile analyses carriedout in the frameworkof the radar supportednatural-resourcesurveysof the BrazilianAmazon in the 1970s (21). A similarcalculationfor 53 profilesof Suleimi series, a Vertisol occupying extensive areas of Sudan central clayplain(22) showedthattotalcarboncontentswerestrikingly uniform,withall horizonsto 1.5 m depthcontributingevenly to the total pool of soil carbonand contents in deeper layers di- minishingby one third.The churningprocesses thatcharacter- ize these smectitic soils arelargelyresponsiblefor the homog- enizationof the organic-carbonprofile.Consequently,the con- tributionof the upper25 cm to the carbonpool is only about 13%of the whole profile down to 2-m depth(26%of the total to 1-mdepth). In Arenosolsthe soil textureis by definitioncoarse (sandor loamysand),andthese soils normallycontainverylittleorganic carbon.However,dueto deeppercolationof rainwaterthiscar- boncontentremainsremarkablyuniformbelow the top 25 cm. If analyzedto 2 m depth,the topsoil contributionof Arenosols tothetotalcarbonpool is about40%of the total(halfof the to- talto 1m). A different picture emerges for non-ferralic Acrisols and Luvisols,which areextensive in the tropicsin savannaandsa- vanna-forestareas.In these soils, contributionof deep subsoil carbonis less, andthebulkof thecarbonpool is concentratedin thetop25 cm. Note thatin Histosols the upperlayers are stronglyconsoli- datedduringcultivationby compactionand settling underthe influenceof gravity after drainage.Therefore,arbitrarydepth boundariesto express carbon content do not make much sense in these soils, and direct estimationof carbonlost by ac- celeratedoxidationis essential, as well as the estimationof the totalcarbonstock down to the underlyingmineralsubstratum, regardlessof thedepthatwhich thisoccurs. Soil Organic Carbon Versus Biomass Carbon Theglobalbiomasscarbonpool hasbeenestimatedat about835 Gt C (23), althoughthis figure remainsdebateddue to the ge- neral lack of agreement on definitionsof vegetation types andintheabsenceof anadequate global landcover and land use map.On a global basis the soil carbonpoolcomprisesabout1.5 times the carbon stored in the vegetation. Matchingsoil groupswiththe normal(simplified) ecosystem types under which they occur leads to the following results (Table 3). Soils contain nearly as muchcarbonas the vegeta- tion underrainforest,but con- siderably exceed the biomass carbonin otherecosystems, by Figure 2. Percentages of land with different organic carbon classes for Africa, America and Asia in kg C m-2up to 1 m depth. _~~~~~~~~~~~~~~~~~~~~~~~~~~~.... ... _ ......... .........__I,. .. . _ ___,l. . . . . . . . . _ . . . . . . ., . . .. . . ... ... ... ... ...Il!|l. . . . _. . . . . . . . . . . }_ !-_... . . . . . . . . . . .....E _!.t_............. :::::::::::::::::::::::::1..... . ........ ... .. . . . ... ...:::::::::::::Z i_ki>~~~~~~~~~~~~~~~~.t . W=_.. . . . j,, X:..::..::.:::-: -:...Asia.... > 35 1. 5 76_1 . . . .:....:..g...:...:...:....:.:e...:.>: :e.:::.>::.: :::.::0: :::0: : m: ::. ..': :e ' 'O':C: :... :0.: :.:e: :e.:e:..:..: ::.:: .: .: :::: :::: '.: - ::::: ::' :::' ::::: '::s:::: :.. ....:0':"::.....::::: * ' ''! ! *!.''i'!>,'!-''!''"^''',''','''''8':'*2-s'!'::-s-0OtCO?oo3;8<c;us*M-!8<,,e?8>i?~~~~~~~~~~~~~~~~ ::o?.z?.z?o?.o:uous;abe!>. : !; ~~~~~....... ... ... ... ......... ..........S .... ... ... .... .. ... ... S'SS|..?'S'S' ....r . ...y ...# ..<sl ... ............... tws... ...;8s# @u u X S : # #b ee e WSn ' x 8 ' ................_ ,,.. . G.....I,--, . .. _1_i l?1 s -|#oZ R S .-' ..XRl .. .. .fg > X r- e b w .3....... . ... .. ...........m S S& s^ R , ER | .R?-@WOXPSSSRXwXw(XBNBb2SwbRRSa..ZSa=XwWNO#SwwpMu#O>#bxp.WTssonSR:axe.wSRSWjww.m-SYS-Z-----... -OO<O>XOM@S2#8aNlE8ors1ae!2g!1!!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ?2XXZ.: a2S|.s||8#R o.<.>.oiii_# |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ AMBIOVOL.22 NO. 7, NOV. 1993 419
  5. 5. ............................... ................. ............................ ... .. ...... M---M g :w AN .............................................. ------------- min= Mg a factor2 to 10. Note thatthe figurefor tropicalforestgiven by (24)-18 kg C m-2-and quotedby (12) is controversial.16 kg C m-2is usedforglobalmodellingas well (25). FortheAmazon regiona figureof 11.7 kg C m-2 is given (26). It can be argued thateven thelatterfigureis stilltoo highdueto biasof theforest samplingtechniquesappliedin the area(20). Carbonate Carbon Carbonatecarbonhas received little attentionin carbon-cycle research.Schlesinger(27)concludedthatindesertandsemidesert areas where the accumulation of pedogenic carbonates is pronounced,carbonatecarbonexceedsorganiccarbonbyafactor 10ormore.Inotherclimaticregionssoils prevailwithoutanyor withonlylittlecarbonatedueto solubilizationandleachingof the carbonatesthatmayhavebeen presentin the weatheringparent materialorbroughtalongby dustorflowing water.Heestimated theglobalcarbonate-carbonpool in soils atbetween780 and930 GtC.To estimatetheglobalcarbonatecarbonstocksin soils in a mannerconsistentwiththeorganic-carbonestimates,theaverage carbonate-carboncontents for soil types given by Schlesinger (27) wereusedincombinationwiththesoil unitareascalculated fromthe mappingunitsof the digitizedSoil Mapof the World. Resultsforcontinentsandmajorpartsof continentsarepresented in Tables4 and5. We arriveat an estimated global carbonate-carbonpool in soils of 720 Gt C. This is slightly less thanSchlesinger's esti- mate,butcan be explainedby differencesin soil areaestimates betweenthetwo approaches.Indeed,desertareashavea signifi- cantextentof sanddunes androcks which arecountedas soils in the Schlesinger approachand are excluded in the present study. However, a certain underestimationin our approach should be noted:we did not consider soils which may be cal- careous, but for which this characteristicis not consideredin theirtaxonomicname,e.g. certainArenosolsin the 1974 FAO/ Unesco Legend (3, Vol. I). Also, soils developed on coral are probablynot adequatelyrepresentedon the Soil Map of the World. For converting carbonateto carbon, all carbonatewas as- sumedto contain 12%C (thecase for CaCO3),consideringthat dolomiteandMgCO3areof minorimportanceon a global scale comparedwith calciumcarbonate.No attemptwas madeto es- timatecarbonin carbonaterocks. Problems Severalproblemsrelatedto theestimationof theglobalorganic- carbonpool deservemoreattention.Some of thesehavealready beentoucheduponintheprevioussection,suchasdifferencesin approachesforareaestimatesandtheuseofdifferentkindsofbase maps.Inthisrespectanintegratedapproachusing soil, climatic andland-usedatain combinationwould probablyyield thebest results. However, the lack of an adequateland-use map will remainamajorconstraintforsuchanapproachinthenearfuture. The establishmentof global pointdatabases by international researchand modelling organizationssuch as IGBP (Interna- tional Geosphere-BiosphereProgramme)and others, deserves supportas theywill allowbettercarbonestimatesby soil typeor vegetationtypeto be madein duecourse. The use of organiccarbonandrelatedparametersto inferthe fertilitystatus,organicmatterturnoverandbiological activityin tropicalecosystems remainsproblematic,becausethe causalor statisticalrelationshipsbetweenthemarepoorlyknown.This is due to the paucityof investigationson the subject,but also to lackof standardizationin methodsandlackof linkagewith ma- jor soil units. Recent researchwithin the TSBF (TropicalSoil Biology andFertilityProgramme)of Unesco/IUBS/ISSSandat national researchinstitutionsaims to demonstratethe impor- tanceof soil organicmatterin general.Recommendationson the use of standardizedlaboratorymethodsforthedeterminationof organiccarbon,soil respiration,litterdecomposition,nitrogen mineralization,root length and distribution,etc. are given in (28). A majorproblemremainsthe lack of sufficientdataon bulk densityof soil horizons.Organiccarbonandcalciumcarbonate determinationsareconsideredroutineanalyses in soil surveys, butaredone on weight basis. Bulk densitiesareonly rarelyde- termined,however,andas thesevaluesarecrucialto recalculate carbon data on a kg m-2 basis, this lack hampers consistent treatmentof results.Bulkdensitiescanbe estimatedwitha vari- able level of confidence (29). However, the wide variationof bulkdensitiesas a functionof themoisturecontentatwhichit is determined,is anaddedproblem,especiallyforsoil groupssuch as Vertisols. Weathering-resistantcharcoalhasbeen identifiedas one kind of soil carbonthatis rarelyif evermeasured.Withtheexception of specialpurposestudies(30) suchcharcoalpieces areignored by soil laboratories, because only the fine-earth fractions (smallerthan2 mm aftersieving) areanalyzed.Forinstance,it is indicated(31) thatcharcoalresultingfrom burningthrough lightningorearlyAmerindianoccupationis commonin thesoils of the Amazon basin under naturalrainforests.For a well- drained("terrafirme")soils of the upperRio Negro areaof the ColombianandVenezuelanAmazonregionthecharcoalcontent variesfrom3 to 24 t/ha,with33 to 86%of it concentratedin the upper50 cm (32). Undercurrentshiftingcultivationpracticesin 420 AMBIOVOL.22 NO. 7, NOV. 1993
  6. 6. ............. .... .... .. ...... ........... .... ......... xom: X yx ---------- ---- .............. m m d'- -- ---------- ............ ,,mg M W zp i W R5,10 R:t,-m ....................................... ... . ......... ...... gqgv IEMg - Z'sx:1c ...... ... ... ... ...... tropicalforest areas,and in savannaecosystems subjectto re- peatedburning(naturalorhuman-induced),thetotalaccumula- tion of charcoalpieces in the soil can be very substantial,but again,factualmeasurementsarerare(33). A finalproblem,particularlyrelevantwhensoil mapsareused as a basisfor carbonestimations,is the lack in most soil classi- fication systems of due attentionto topsoil characteristicsin generaland humus characteristicsin particular.The develop- mentof a propertopsoil characterizationas proposedby FAO (34) is worthwhile,not only for a betterunderstandingof the agriculturalpotentialof thesoil butalsoforecological studieson aglobalscale suchas thepresentone. FACTORSINDUCINGCHANGESINSOIL CARBONLEVELS Theambientorganic-mattercontentin the soil is determinedby thesupplyof soilorganicsubstances,includingsoil substancesof the precedingyears and newly added undecomposedorganic materials,combinedwiththerateof decomposition.The supply dependsontheinsituproductivityintermsofbiomassproduction and on man-madeadditionalorganic inputs. Decomposition, whichcanbe measuredin termsof CO2productionby thesoil, is influencedby variousbiotic andabioticconditions.The factors controllingsupplyanddecompositionhavebeenreviewedinthe literature(35-37). Some factorsof particularimportanceforthe tropicsandsubtropicsarebrieflydiscussedin thefollowing. EnvironmentalConditions Theactualsoil organic-mattercontentandits changeswithtime arestronglyinfluencedby the interactingfactorsclimate,relief andparentmaterial,floraandfaunaandby the soil itself. A combinationof high temperaturesand large amountsof precipitationinducesenhanceddecompositionrates.Therefore, in thehumidtropicsthose ratesarehigh unless soils areimper- fectly drainedand oxygen deficiencies reduce decomposition rates.This situationfrequentlyoccurs in valleys and depres- sions. For instance,an overlay of the wetlands map of Africa (FAO,unpublished)with the organiccarbonmap reveals that relationship.The decreaseof soil organic-mattercontentin the humidtropicsdue to decompositionis counteractedby a large supplyof organic substancesfrom biomass productionunless thevegetationis cleared. Frequentdryingandmoisteningof the soil-often occurring in thesemi-aridtropics-also bringsaboutrapiddecomposition of organic substances. Soils derived from volcanic materials (Andosols) have much highercarboncontents thansoils from otherparentmaterials,mainlybecausethestrongbondsbetween amorphousmineralconstituentsanda majorpartof the organic matterinhibitsits decomposition.Relief, nutrientstatus(in par- ticular phosphorus), vegetative productivity and soil fauna (earthworms,termitesin thetropics)affectamountanddistribu- tion of organiccarboncontentin theprofileandtype of organic compoundsformed,butarenotdiscussedin detailhere. Coastalareasoften exhibitecological conditionsthatdeviate fromtheirsurroundings. Lower riverplains and deltas have nearlyeverywherelarge amountsof soil organicmatter,formedin situ ordepositedwith the sediments(andthen occurringirregularlywith soil depth), even whereadjacentuplandsoils arelow in organicmatter,as in aridclimates. Human-induced Factors Thekindof landuse stronglyinfluencesthe amountsof organic carbon in the soil. Any land use in the long run leads to an equilibriumin soil organic-mattercontents.Conversionsof land use candrasticallychangeorganic-mattercontents(38). Land-use and Management The clearingof forests or woodlandsandtheirconversioninto farmlandin the tropicsreducesthe soil-carboncontent,mainly throughreducedproductionof detritus(Table3), increasedero- sionratesanddecompositionof soil organicmatterby oxidation. Variousreviews (e.g. 1, 27) agreethatthe loss amountsto 20 to 50%of theoriginalcarboninthetopsoil,butdeeperlayerswould be little affected,if atall. It was calculated(39) thatdeforestationin the tropicscaused a netreleaseof between0.1 and0.3 GtC soil carbonperyearin theyearsaround1980(comparedwithamountsbetween0.3 and 1.3GtC by burninganddecayof clearedvegetation,and5.3 Gt C peryearby fossil-fuel consumption). The decreaseof organicmatterin topsoils can have dramatic negativeeffects on waterholdingcapacityof the soil, on struc- turestabilityandcompactness,nutrientstorageandsupplyand on soil biological life such as mycorrhizasand nitrogen-fixing bacteria(38, 40). Otherpropertiesbeing equal,thereis often a clearcorrelationbetweenorganic-carboncontentin topsoilsand cropyields, also in semiaridareas(22). The dynamics of organic carbon in the various systems of landuse afterdeforestationneedsto be systematicallystudiedto makemorereliableprognosesfor the futuredistributionof car- bon pools in the world.In a studyof representativesoil profiles forthewhole of Brazilit was shown(41) thatonly thehumusof the topsoil (A horizon)is in directequilibriumwith the present AMBIOVOL.22 NO. 7, NOV. 1993 421
  7. 7. dayvegetativecover;thatof deeperlayersis largelybeyondthe influenceof the vegetation. Therearemanystudies(42-44) on thesoil nutrientdynamics underthe shiftingcultivationsystemof cropping,in its various forms,fromwhichthe soil-carbonbehaviourcanbe derivedfor thatlanduse. Less systematicobservationshavebeenmadeon theorganic- carboncycling underpermanentcrops,especially treeperenni- als suchas oil palmandrubber. Pasturesare an increasingly widespreadland-use type fol- lowingdeforestation,establishedeitherimmediatelyorafterone or morecycles of shiftingcultivation.In the lattercase the or- ganic-mattercontentsremainlow. However,pastureestablish- ment immediately after deforestation,especially when using species with high percentagesof below-groundbiomass pro- duction,may actuallyincreasethe soil organic-mattercontent. This is confirmedby several studies in the BrazilianAmazon (45) and elsewhere in Latin America (46) and in East Africa (47). Twenty years old pasturesin the Rondonianpartof the BrazilianAmazonprovedto haveto 50%moresoil carbonstock to 30 cm depth,in comparisonto forestedsoils of the samena- ture.It shouldbe notedthatthis does not yet fully compensate for the loss of carbonstoragein the above-groundbiomass,and the economic value of the establishedgrasslandsmay be short- lived. This because it is difficult to maintainthe palatibilityof suchgrasslandsaftera numberof yearsbecauseof the growing competitionof various species (woody herbs;hardyperennial grasses). Some of these species, such as Imperatacylindrica ("alang-alang",cogon grass) do provide a protective ground coverandarelativelylargeamountof above-andbelow-ground carbonstorage,butdirectbenefitsforthehumanpopulationsare nil, except wherethe grassis used forthatchingorpapermanu- facture.An interestingalternativeis the developmentof inten- sive livestockproductionatthe marginsof tropicalforests,em- ployingpreseminalcropwithhighbiomassproductionpotential (sugar-cane,forage trees) for the feeding of livestock species managedin confinement(48). Applicationof fertilizerinducesenhancedbiomassproduction andmay thereforeresultin largercarboninputsin the topsoil. When organicfertilizersare applied-household refuse, urban waste, agro-processingwaste, greenmanure,forestlitteror sod from landelsewhere-then the effect on a longer-termbasis is obvious.Theage-oldPlaggensoils of TheNetherlands,Flanders and NorthernGermanyareperhapsthe best-knownexamples, Figure3. RelationshipbetweenPercentagesof SoilOrganicCarbon andCationExchangeCapacity(CEC)interra-preta-do-lndiosoil profilesof the Amazonregion.(AdaptedfromSombroek,1966.Five profiles;nonsettleduponnowadays. m.e./100 g soil 40 - A A A A 30 - O20a ' ' ] 0 1 2 -3 4 5 6 _.- Organiccarbon but similarsoils exist in the tropics and subtropicsin spite of higherdecompositionrates.A case in point(16) arethe "terra- preta-do-Indio"soils onhighriverbanksandotheruplandsin the Amazonregion.Thesesoils, originallyaspoorchemicallyasthe surroundingkaolinitic soils, were purposely enriched by the early Amerindianpopulation with organic matter from sur- rounding land or aquatic grasses and phosphates-the latter from hunting and fishing-and they have maintained their highertrophiclevel even aftertheirabandonmentcenturiesago. The few analyses availablefor these soils indicatenot only a doublingof the organic-mattercontentin the upper0.5 to 1 m butalso ahighercation(nutrient)exchangecapacitythanwould be expectedfromthesumof thecolloidalactivityof theorganic matterand the kaolinitic clay mineralsindividually.A stable complexof theaddedorganicmatterandthekaolinite,underin- fluence of the addedphosphorusand possibly colloidal silica, may be the cause (Fig. 3). A recentstudy(41) indicatesthaton these"terra-preta-do-Indio"soils theCO2productionis loweras comparedto comparableadjacentsoils withouthumusenrich- ment. This possibly is inducedby a higherstabilityof the soil organic matter.This conclusion supportsthe idea that stable complexesof organicmatterandkaolinitemayoccur.Someba- sic researchon theseterra-pretasoils mayprovideaclueto what specific land-managementpracticesareneededto emulate,in a few years time ratherthanin centuries,this increasedfertility level andstableorganic-carbonstorage. Liegel indicatesthatsoil carbonaccumulationratesby pur- posefulsoil managementin PuertoRico arecomparableto those in temperateregions(46). Therearenumerousexamplesof soil organicmatterenrichmentunderintensiveirrigatedricecultiva- tion and underhydromorphicsoil conditions in general. Pur- posely stimulatingthiswouldhoweverincreaseemissionsof the greenhousegas methane(anpossibly also of nitrousoxide) un- dertheprevailingsoil andwatermanagementpractices. EFFECTSOFCLIMATECHANGE CO2Fertilization Effect Thereareseveralreviews on the likely effect of the anticipated human-inducedglobal climatic change on soil conditions in general, and its carbon pool in particular(1, 51, 52). Most attentionhasbeendevotedto thelikely negativeeffects of rising temperatures:more rapid breakdownof soil organic matter; decreasingsoil moisturestoragebecauseof lowerprecipitation/ evapotranspirationratioswithconsequentlyless newproduction of organicmatter;depletionof plantnutrientsdue to increased erosion, againentailingless organicmatterproduction;loss of essentialsoil floraandfauna,includingsymbioticorganismsand the anticipatedshiftingof agroecologicalzones, with sloweror unbalancedlitterdecomposition;andloss of landduetosea-level rise,causinga decreasein terrestrialbiomassproduction. However,recentexperimentson thebehaviorof plantsunder elevated atmosphericCO2levels, in greenhouses,in open-top field chambersas well as in free-airCO2enrichment(FACE) field experiments,coupled with observationson plant growth near sites with naturallyhigh CO2concentrations(some vol- canic lakes) all pointto a substantiallyhigherprimaryproduc- tion, above-groundand below-ground, especially of the so- called C3 plants(CO2fertilizationeffect) anda moreeconomic use of water by plants, especially those of the C4 type (CO2 antitranspirationeffect). Under a doubled atmosphericCO2 concentrationthis increasemay be 30%oor morein bothcases, and slightly higher temperaturesmay furtherstrengthenthese effects (61-63). The CO2fertilizationeffect is likely to have a significantim- pact on plantgrowthin the humidtropicsin a CO2enrichedat- mosphere,yielding extra soil organic matterthroughlitterfall 422 AMBIOVOL.22 NO.7. NOV. 1993
  8. 8. Some examples of the variation in amount and rMvertical distribution of organic matter in tropical and subtropical soils. Fl 1. Very low amount, with gradualverticaldecrease -i.' concentrated in topsoil (luvic Arenosols, Amazon,1 Brazil). 2. Fairamount, with gradual verticaldecrease (rhodic : 3. Fairlyhigh amount, with gradual vertical decrease ~~~~. (xantho-humic Ferralsol, southern Brazil). 4. Highamount,withvery ;LOn gradual vertical decrease .- (humic Nitisol, Kenya). 5. Highamount, concentrated in subsurface horizon (humic Alisol, - Brazil). .. 6. Fairlyhigh amount, regularlythroughout the profile (eutric Vertisol, Ethiopia). 7. Highamount, irregularly throughout the profile (mollic Andosol, Ecuador). 8. Highamount in upper subsoil (dystric Planosol, t Uruguay). 9. Highamount irregularly in subsoil (carbic Podzol, Amazon, Brazil). A ~~~~~~~~~ .~~~~~~~ 9
  9. 9. andcropresidues,andespecially througha morevigorousroot growth.Nutrientsmaycome in shortsupplyatsuchhigherrates of biomassproduction,butthiscan,inprinciple,be redressedby larger applicationsof fertilizers and promotionof integrated plant-nutritionsystems in agro-ecosystems.Therearesome in- dications that in naturalecosystems higher atmosphericCO2 concentrationinducesmoremycorrhizalactivitystimulatingre- lease of occludedphosphorus,morebiologicalnitrogenfixation fromthe air,andmorepotassiumreleaseby increasedweather- ing of the saprolitebelow the soil (56, 63, 64). According to some estimates (61) the CO2fertilizationeffect would already havecontributed,by 10%ormore,to the approximatedoubling of theworldwideagriculturalproductivityin thepast 100years. The CO2antitranspirationeffect would be of particularsig- nificance in the semiaridregions of the tropicsand subtropics: plants would grow more vigorously with the same amountof water, and some plant growth would become possible where hithertothe land surfaceis bare,due to climate- or salinity-in- ducedaridity.A bettergroundcover would be the result,limit- ing soil-erosionhazards,loweringthe soil-surfacetemperatures andprovidingfreshorganicmatterforincorporationin the soil. The two plant physiological effects of higher atmospheric CO2concentrationswill in practicebe counteractedandlimited by a numberof associatedfeaturesof Global Change,such as higherUV-B values, highertropospheric03, temperaturesnear theupperlimitforgraindevelopment,etc. (62, 63). Thereis still some doubtaboutthe persistenceof the effects over successive generationsof annuals,andthe laterstages of life of perennials-including forests-and hence the experi- mental data recording will have to extend over many years, partlyin costly field set-ups in naturalor plantedforests. But even if theneteffectwill be only afractionof theexperimentally obtainedgrowthincreasesof 30%or more, an increasein soil organiccarbonstoragemay be expected in an environmentof higheratmosphericCO2.Threeyearsonly of FACEexperimen- tation with cotton, using 13C,showed that 10%of the soil or- ganiccarbonin theupper30 cm was "new"(58). Carbon Sequestering in Soils Onthebasis of the admittedlyfew indicationson thepossibility to increase carbon storage in tropical soils by adaptedland- managementtechniquesthatemulatethealreadyhuman-enriched soils, combinedwith the existence of the CO2fertilizationand antitranspirationeffects, we herebyplea for furtherresearch.In agriculturalecosystemspotentialstudiesshouldincludecorrelating totalsoilcarbonandaccumulationrateswithclaycontentandsoil mineralogy,relatingobservedtotalcarbonpools to past/present croppingsystems andlanduse changes anddeterminingwhich kinds of soils are most suitablefor carbon/nitrogenstorageby fungi and other rhizosphere organisms (46). In particular experimentationon the terra-pretaenrichmentprocess and the subsequentdevelopmentofalarge-scalefieldprogramtostimulate organiccarbonstoragein tropicalandsubtropicalsoils is highly desirable. Fortemperateareasthereareanumberof historicalexamples. A recentpublicationof the US EnvironmentProtectionAgency (63) gives an overview of practical possibilities for carbon sequesteringin arablesoils, mainlythroughconservationtillage techniques.Ofthese,theno-tillmanagementtechniques-leaving nearly all of the crop residues on the land, with minimum disturbanceonly at seeding the next cropwould be the most promising,because it reduces soil temperature,increases soil moistureand stimulatessoil faunato transportthe residueun- derground.Theideawastakenupinasimulationstudy(64)forthe main agriculturalsoils of the formerSoviet Union: Complete conversionof all climaticallysuitableland(181 M. ha)to no-till management,resultingin a 10%increasein soil carbon,would imply a sequesteringof 3.3 Gtof carbon. No-till soil managementin the tropicsandnon-tropicsseems feasible,too,thoughsomesoils needtobecultivatedregularlyto providea good seebed ("tilth"),andburningof the residuesof somecropsisrequiredtopreventthesurvivalofdiseasesandpests in monocultures.In additionto no-tilling, as much as feasible withinthe local transportfacilities, urbanandperi-urbanwaste andsludgeshouldbeusedbecauseofitsinherenthighconcentration of organiccarbonandplantnutrients.Insteadof thiswastebeing dissipatedinaquaticsystemsandultimatelyincoastalwatersand sediments, it should be processed on the spot and separated acordingto its quality,includingits contentsof pollutants,and made thereupon availableto improvesoils in ruralareason a permanentbasis. Algae or grassy aquaculturesmay be an intermediatestageof suchaprocessingof wasteforaproduction factorin agriculture.A majorfield programin the above sens couldbe startedin the moist lowlandsavannazones of West or SoutheasternAfrica,aspartofFAO'sinternationalschemeforthe conservation and rehabilitationof African lands (ISCRAL). Especially when in combination with additionalphosphorus supply-for instancethroughtheapplicationof locallyavailable rockphosphates-it wouldservetwo purposes:theresilienceof tropicalsoilsandtheirproductivecapacityforastronglygrowing humanpopulationwould be improved,and there would be a substantialsequesteringof carbonin soils which,by its natureis of a morepermanentcharacterthanstoragein living vegetation. It would form an automaticand gradually strongerbraking mechanismontheanticipatedhuman-inducedriseofatmospheric co2. Theaboveis nota pleafora "business-as-usual"use of fossil fuels.Amorefrugaluseofthisnon-renewableresource,especially in alreadyindustrializedcountriesof temperatezones, remains essentialtoslackenthecurrentriseof atmosphericCO2tosuchan extent that adverse effects of human-inducedglobal climatic change,suchas sealevelriseandintensificationof local climate variability,maybe keptdownto manageableproportions. CONCLUSIONS (i) The organic-carbonpool in the upper1 m of the world's soils is 1220 Gt, 1.5 times higher than that of the standing biomass(naturalvegetationandcrops).Inthe soils undertropi- cal rainforeststhis pool is about equal to that of the above- groundbiomass. (ii) In the tropicsmany soils aredeep andthe storageof or- ganiccarbonbelow 1m depthis substantial;about50 Gtof the global pool of carbonis storedat between 1 and 2 m depthin tropicalsoils. (iii) Charcoalstoragein manytropicalandsubtropicalsoils is notinsignificant,even in soils underpristinevegetation,butit is rarelymeasuredquantitatively. (iv) In the tropicaland subtropicaldrylandregions the car- bonate-carbonstoragein soils amountsto about720 Gt;it is lit- tle dynamic. (v) The short-termdynamics of organic carbonare largely restrictedto the upper30 to 50 cm in most tropicaland sub- tropicalsoils. (vi) At clearing of tropicalforests or woodlandsfor arable cropping(permanentorshiftingcultivation)thereis a20 to 50% reductionin organiccarbonin theseuppersoil layers.Establish- mentof pasturesimmediatelyafterdeforestation,using grasses with a high percentageof below-groundbiomass production, canresultinmaintenanceorevenincreasein soil organic-carbon content, though the palatabilityof the above-groundbiomass oftendecreasesgreatlyaftera few years. (vii) Organicmatterin most tropicalandsubtropicalsoils is essentialformaintenanceandimprovementof waterinfiltration 424 AMBIOVOL.22 NO. 7, NOV. 1993
  10. 10. 3?:.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. 4'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1 r Difference in soil organic carbon content between an anthropogenically enriched soil of the Amazon (Terra-preta- do-indio) and an adjacent nonenriched soil of the same physiographic position, clay content and clay minerology (xantic Ferralsol). andwater-holdingcapacity,for structurestability,for adequate nutrientstorageandsupply,andfor a healthysoil biological ac- tivity(mycorrhizalphosphorusrelease,biologicalnitrogenfixa- tion).Thepurposefulsequesteringof organic-carboncontentsin suchsoils is thereforeworthwhilein its own right. (viii) Some tropical soils traditionallyenriched in organic matter,nutrientsand cation-exchangecapacity due to ancient humanoccupationgive indicationsthatsuchanenrichmentcan be maintainedover severalcenturies;the supplyof phosphorus wouldseemto be a key conditioningfactor. (ix) It is often assumedthathuman-inducedglobal climatic changewill have an overall negative effect on the amountof organiccarboninsoils. However,if onetakesintoaccountrecent researchdata on the so-called "fertilizationeffect" and the associated"antitranspirationeffect"of higheratmosphericCO2 levels on plant growth, above- ground and especially below- ground,thesepositiveeffectsmaymitigatethenegativeeffectson soil-organiccarbonlevels oreven overcomethem. (x)TheCO2fertilizationeffectanditsinfluenceonsoil organic matterstorageimplies an automaticcurbingof any excessively risingatmosphericCO2levels as causedby increasedfossil fuel use andbiomassburning. (xi) Therefore, large-scale field programmesto stimulate organic-carbonstorage in tropical soils, especially when in combinationwithphosphorussupply(forinstancethroughlocally availablerockphosphates)wouldservetwopurposes:thequality of tropical soils and their productivecapacity for a strongly growing populationwould improve, and at the same time the currentriseinatmosphericCO2levelswouldbeattenuatedtosuch anextentthatadverseeffects of human-inducedglobalclimatic change,suchas sea-levelriseandintensificationof localclimate variability,maybe keptdownto manageableproportions. AMBIOVOL.22 NO. 7, NOV. 1993 425
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Gaston,G.G.,Kolchugina,T. andVinson,T.S. 1993.Potentialcffectof no-tillmanage- mentoncarbonintheagriculturalsoilsof theformerSovietUnion.Agriculture.Ecosvst. Environ.45, 295-309. 65. Acknowledgement.This article was originally presentedin the Royal Colloquium 1993.Itwas preparedby W.G.Sombroek,F.O,NachtergaeleandA. Hebelof the Food andAgriculturalOrganizationof theUnitedNations(FAO),andis reproducedhereby permissionof thatorganization. WimSombroek holds a PhDfromWageningen Universityin agriculturalsciences. From1959 to 1979 he participatedin tropicalsoil and land-evaluationprojects in LatinAmerica (Brazil,Uruguay)and Africa(Nigeria,Kenya).From1979 till 1991, he was Directorof the InternationalSoil Reference and InformationCenter(ISRIC)inWageningen, and during most of those years he also served as Secretary-Generalof the InternationalSociety of Soil Science (ISSS),which is a scientific associate of ICSU.He now leads the Landand WaterDevelopment Division of the Food and Agriculture Organizationof the UnitedNations and is also the Organization'sFocal Pointon ClimateChange matters. FreddyNachtergaele has a PhDin agronomy fromGent University.Since 1989 he has been Technical Officer,Soil Resources, withthe Landand WaterDevelopment Division of the Food and AgricultureOrganization.Before thattime, he worked mainlyas a land resources expert in several FAO field projects in northernand eastern Africaand southeast Asia. Axel Hebel is MScgeographer from BerlinUnviersity withspecialization in landscape ecology and regional planning. From1989 to 1992, he workedat the Universityof Hohenheimin Germanyand at ICRISATSahelian Centerin Nigeron soil fertility.His PhDin agriculture/soilscience to be defended in 1993. Since 1993, he has been workinginthe Soil Resources Groupof the Soil Resources, Management and Conservation Service, Landand WaterDevelopment Divisionat FAOHeadquartersin Rome. Theiraddress: Land and WaterDevelopment Division, FAO,Viadelle Termedi Caracalla,00100 Rome, Italy. 426 AMBIOVOL.22 NO. 7. NOV. 1993

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