Lal, Carbon Sequestration, PhilTransRoySoc 2008

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Downloaded from rstb.royalsocietypublishing.org on April 14, 2010Carbon sequestration R. Lal 821uptake in C-3 dominated tropical regions in eight of the10-year study period. Their data showed a possibleexistence of a large equatorial terrestrial CO 2 sink.Pacala & Socolow’s (2004) estimated managementof temperate and tropical forest is one of the 15 optionsto stabilize atmospheric CO 2 concentration at 550 ppmby 2050. They estimated that 0.5 Pg C emission wouldbe avoided if the current rate of clear cutting of primarytropical forest were reduced to zero over 50 years by2050. Another 0.5 Pg C yr K1 would be sequestered byreforesting or afforesting approximately 250 Mha in thetropics or 400 Mha in the temperate zone. Anadditional 0.3 Pg C yr K1 can be sequestered by establishingapproximately 300 Mha of plantations ofnon-forested lands (Pacala & Socolow 2004). Theafforestation may account for a total of 25 Pg Csequestration between 2000 and 2050.However, afforestation on a large scale can impactwater resources. Jackson et al. (2005) documentedsubstantial losses in stream flow, and increased soilsalinization and acidification, with afforestation. Theyobserved that establishment of tree plantationsdecreased stream flow by 227 mm yr K1 globally(52%), with 13% of streams drying completely for atleast one year. Thus, any plans of large-scale afforestationfor C sequestration must consider the possibleadverse impact on availability of water.Bunker et al. (2005) addressed the concern ofdecline in tropical forest biodiversity due to expansionof monoculture plantations. Thus, resulting effects ofmonoculture plantations on key ecosystem services(e.g. biodiversity, water resources, elemental cycling, Csequestration) must be critically assessed. There is astrong need for developing regulatory policies, includingthe transaction costs of trading C credits andobtaining the permits. The cost of C sequestration visà-visthe opportunity cost must also be considered(McCarl & Schneider 2001).Wetlands and the associated soils or histosolsconstitute a large pedologic pool estimated at approximately450 Pg (Gorham 1991; Warner et al. 1993).Wetland soils may contain as much as 200 times moreC than the associated vegetation (Milne & Brown1997; Garnett et al. 2001). Gorham (1991) and Kobaket al. (1998) estimated that C sequestration in wetlands/peatsoilssince the post-glaciation period resultedin the C accumulation at the rate of 0.1 Pg C yr K1 over10 000–18 000 years. However, drainage of peatlandsand their subsequent cultivation made these ecosystemsa net source of CO 2 . Large areas of wetlands havebeen drained worldwide for agriculture (Armentano &Menges 1986) and forestry (Paavilainen & Paivanen1995). Drained wetland soils decompose and subsideat the rate of approximately 1–2 cm yr K1 primarily dueto oxidation (Rojstaczer & Deverel 1995; Hillman1997; Wosten et al. 1997). Restoration of wetlands canlead to reversal of the process and make restoredwetlands once again a sink of atmospheric CO 2 .However, there is a long time lag after the restorationuntil processes in restored wetlands become similar tothose of natural wetlands.Soil C sequestration. Implies enhancing the concentration/poolsof SOC and SIC as secondary carbonatesthrough land-use conversion and adoption ofrecommended management practices (RMPs) inagricultural, pastoral and forestry ecosystems andrestoration of degraded and drastically disturbedsoils. Formation of charcoal and use of biochar as afertilizer is another option (Fowles 2007). In contrastto geological sequestration that implies injecting CO 2at 1–2 km depth, the SOC sequestration involvesputting C into the surface layer of 0.5–1 m depththrough the natural processes of humification. Mostsoils under the managed ecosystems contain a lowerSOC pool than their counterparts under naturalecosystems owing to the depletion of the SOC pool incultivated soils. The most rapid loss of the SOC pooloccurs in the first 20–50 years of conversion of naturalto agricultural ecosystems in temperate regions and5–10 years in the tropics (Lal 2001). In general,cultivated soils normally contain 50–75% ofthe original SOC pool. The depletion of the SOCpool is caused by oxidation/mineralization, leachingand erosion.Strategies for increasing the SOC pool are outlinedin figure 4. There is a wide range of degraded soils witha depleted SOC pool. Important among these are thosedegraded by erosion, nutrient depletion, acidificationand leaching, structural decline and pollution/contamination(figure 4). Restoring degraded soils andecosystems is a strategy with multiple benefits forwater quality, biomass productivity and for reducingnet CO 2 emission. Grainger (1995) estimated thatthere are approximately 750 Mha of degraded lands inthe tropics with potential for afforestation and soilquality enhancement. With a sequestration potential ofapproximately 0.5 Mg ha K1 yr K1 as SOC and anadditional 1.0 Mg ha K1 yr K1 as biomass, a terrestrialC sequestration potential of 750 Mha is approximately1.1 Pg C yr K1 . Lal (2001) estimated the SOC sequestrationpotential of 0.4–0.7 Pg C yr K1 through desertificationcontrol in soils of arid and semi-aridregions. Similar estimates were provided by Squireset al. (1995).West & Post (2002) assessed SOC sequestration rateupon conversion of plough tillage to no-till farmingthrough analyses of data of 67 long-term experimentsfrom around the world. They reported the mean rate ofSOC sequestration of 570G140 Kg C ha K1 yr K1 ,which may lead to the new equilibrium SOC pool in40–60 years. Lal (2004a,b) estimated the global SOCsequestration potential, 0.4–1.2 Pg C yr K1 or 5–15%,of the global fossil fuel emissions. Pacala & Socolow(2004) estimated that conversion of plough tillage tono-till farming on 1600 Mha of cropland along withadoption of conservation-effective measures could leadto sequestration of 0.5–1 Pg C yr K1 by 2050.Integrated nutrient management (INM) is alsoessential to SOC sequestration. The humificationprocess can be severely constrained by the lack of N,P, S and other building blocks of soil humus (Himes1998). The efficiency of C sequestration is reducedwhen C and N are not adequately balanced (Paustianet al. 1997). Therefore, the SOC sequestration rate isenhanced by an increase in the application of biomassC(Campbell et al. 1991; Janzen et al. 1998) andN(Halvorson et al. 1999, 2002). Liebig et al. (2002)observed that high N rate treatments increased SOCPhil. Trans. R. Soc. B (2008)
Downloaded from rstb.royalsocietypublishing.org on April 14, 2010822 R. Lal Carbon sequestrationstrategies ofSOC sequestrationrestoration of soildegraded byadoption of RMPs onagricultural and forest soilserosionintegrated nutrientmanagement (INM)restoring physical, chemical andbiological quality of degraded soilssalinization andalkalizationnutrient depletionacidification andleachingcompetition,crusting andstructural declineno-till farming withresidue mulch andcover cropsdiverse croprotations,agroforestryand charcoalprecision farming andfertilizationagricultural intensification toincrease productivitypollution andcontaminationintegrated pestmanagement (IPM)converting surplus agricultural land for natureconservancy and environmental improvementsFigure 4. Management strategies for soil carbon sequestration through restoration of degraded soils and adoption of RMPs onagricultural soils.sequestration rates by 1.0–1.4 Mg C ha K1 yr K1compared with unfertilized controls. Similar observationswere made by Dumanski et al. (1988),Schjonning et al. (1994), Gregorich et al. (1996),Bowman & Halvorson (1998), Studdert & Echeverria(2000), andJacinthe et al. (2002). Malhi et al. (1997)reported that SOC sequestration depended both on therate and the source of N application. In Victoria,Australia, Ridley et al. (1990) observed that applicationof P and lime increased the SOC pool in the 0–10 cmlayer by 11.8 Mg ha K1 over a 68-year period at anaverage rate of 0.17 Mg C ha K1 yr K1 .Application of manures and other organic amendmentsis another important strategy of SOC sequestration.Several long-term experiments in Europe haveshown that the rate of SOC sequestration is greaterwith application of organic manures than with chemicalfertilizers (Jenkinson 1990: Witter et al. 1993; Christensen1996; Korschens & Muller 1996; Smith et al.1997). Increase in the SOC pool in the 0–30 cm depthby long-term use of manure compared to chemicalfertilizers was 10% over 100 years in Denmark(Christensen 1996), 22% over 90 years in Germany(Korschens & Muller 1996), 100% over 144 years atRothamsted, UK (Jenkinson 1990) and 44% over 31years in Sweden (Witter et al. 1993). The data fromMorrow plots in Illinois indicated that manured plotscontained 44.6 Mg ha K1 more SOC than unmanuredcontrol (Anderson et al. 1990). In Hungary, Arends &Casth (1994) observed an increase in SOCconcentration by 1.0–1.7% by manuring. Smith et al.(1997) estimated that application of manure at the rateof 10 Mg ha K1 to cropland in Europe would increasethe SOC pool by 5.5% over 100 years. In Norway,Uhlen (1991) and Uhlen & Tveitnes (1995) reportedthat manure application would increase SOC sequestrationat the rate of 70–227 Kg ha K1 yr K1 over 37–74-year period.Soils under diverse cropping systems generally havea higher SOC pool than those under monoculture (Dicket al. 1986; Buyanoski et al. 1997; Drinkwater et al.1998; Buyanoski & Wagner 1998). Elimination ofsummer fallow is another option for minimizing lossesof the SOC pool (Delgado et al. 1998; Rasmussen et al.1998). Growing a winter cover crop enhances soilquality through SOC sequestration. In the UK, Fullen &Auerswald (1998) reported that grass leys set asideincreased SOC concentration by 0.02% yr K1 for 12years. In Australia, Grace & Oades (1994) observed thatthe SOC pool in the 0–10 cm layer increased linearlywith increase in frequency of pasture in the crop rotationcycle. In comparison with continuous cropping, incorporationof cover crops in the rotation cycle enhancedSOC concentration in the surface layer by 15% inSweden (Nilsson 1986), 23% in The Netherlands(Van Dijk 1982) and 28% in the UK (Johnston 1973)over 12–28-year period. Similar results were reported byLal et al. (1998) for the US cropland.The rate of sequestration ranges from negative orzero under arid and hot climates to approximatelyPhil. Trans. R. Soc. B (2008)
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Lal, Carbon Sequestration, PhilTransRoySoc 2008

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