Management of forests, croplands, and rangelands affects sources and sinks of CO2, CH4, and N2O. On land managed for forestry, harvesting of crops and timber changes land cover and carbon stocks in the short term while maintaining continued land use. Moreover, most agricultural management practices affect soil condition. A forest that is managed in a wholly sustainable manner will encompass stands, patches, or compartments comprising all stages from regeneration through harvest, including areas disturbed by natural events and management operations. Overall, a forest comprising all stages in the stand life cycle operates as a functional system that removes carbon from the atmosphere, utilizing carbon in the stand cycle and exporting carbon as forest products. Forests of such characteristics, if well managed, assure rural development through working opportunities at the beginning and establishment of forest industries in later stages of the development process. In addition, such forests provide other benefits, such as biodiversity, nature conservation, recreation, and amenities for local communities. For historical and economic reasons, however, many forests today depart from this ideal and are fragmented or have strongly skewed stand age distribution that influences their carbon sequestration capability.
Forest soils present opportunities to conserve or sequester carbon (Johnson, 1992; Lugo and Brown, 1993; Dixon et al., 1994a). Several long-term experiments demonstrate that carbon can accrete in the soil at rates of 0.5 to 2.0 t ha-1 yr-1 (Dixon et al., 1994b). Management practices to maintain, restore, and enlarge forest soil carbon pools include fertilizer use; concentration of agriculture and reduction of slash-and-burn practices; preservation of wetlands, peatlands, and old-growth forest; forestation of degraded and nondegraded sites, marginal agricultural lands, and lands subject to severe erosion; minimization of site disturbance during harvest operations to retain organic matter; retention of forest litter and debris after silvicultural activities; and any practice that reduces soil aeration, heating, and drying (Johnson, 1992).
Cropland soils can lose carbon as a consequence of soil disturbance (e.g., tillage). Tillage increases aeration and soil temperatures (Tisdall and Oades, 1982; Elliott, 1986), making soil aggregates more susceptible to breakdown and physically protected organic material more available for decomposition (Elliott, 1986; Beare et al., 1994). In addition, erosion can significantly affect soil carbon stocks through the removal or deposition of soil particles and associated organic matter. Erosion and redistribution of soil may not result in a net loss of carbon at the landscape level because carbon may be redeposited on the landscape instead of being released to the atmosphere (van Noordwijk et al., 1997; Lal et al., 1998; Stallard, 1998). Although some the displaced organic matter may be redeposited and buried on the landscape, in general the productivity of the soil that is eroded-and its inherent ability to support carbon fixation and storage-is reduced. Losses through leaching of soluble organic carbon occur in many soils; although this leaching is seldom a dominant carbon flux in soils, it is a contributor to the transport of carbon from the terrestrial environment to the marine environment via runoff (Meybeck, 1982; Sarmiento and Sundquist, 1992; cf. runoff in Figure 1-1). Soil carbon content can be protected and even increased through alteration of tillage practices, crop rotations, residue management, reduction of soil erosion, improvement of irrigation and nutrient management, and other changes in forestland and cropland management (Kern and Johnson, 1993; Lee et al., 1993; Cole et al., 1996).
Livestock grazing on grasslands, converted cropland, savannas, and permanent pastures is the largest areal extent of land use (FAO, 1993). Grazing alters ground cover and can lead to soil compaction and erosion, as well as alteration of nutrient cycles and runoff. Soil carbon, in turn, is affected by these changes. Avoiding overgrazing can reduce these effects.
Croplands and pastures are the dominant anthropogenic source of CH4 (Section 1.2.2) and N2O (Section 1.2.3), although estimates of the CH4 and N2O budgets remain uncertain (Melillo et al., 1996). Rice cultivation and livestock (enteric fermentation) have been estimated to be the two primary sources of CH4. The primary sources of N2O are denitrification and nitrification processes in soils. Emissions of N2O are estimated to have increased significantly as a result of changes in fertilizer use and animal waste (Kroeze et al., 1999). Alteration of rice cultivation practices, livestock feed, and fertilizer use are potential management practices that could reduce CH4 and N2O sources.
Ecosystem conservation may also influence carbon sinks. Many forests, savannas, and wetlands, if managed as nature reserves or/and recreation areas, can preserve significant stocks of carbon, although these stocks might be affected negatively by climate change. Some wetlands and old-growth forests exhibit particularly high carbon densities; other semi-natural ecosystems (e.g., savannas) may conserve carbon simply because of their large areal extent.
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