|
1.4.2. Land-Use Management
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.
|