The current global arable land area is approximately 1.5 Gha, or 11 percent of total terrestrial area (FAO, 1999). Most of the carbon stocks in croplands are in the soil because of frequent biomass removal during harvest. Soils now contain about 8-10 percent of total global carbon stocks (Cole et al., 1993). For the purposes of this Special Report, arable land is divided into conventional (aerobic) systems and (anaerobic) paddy rice systems. The remainder is classified as agroforestry and degraded lands, which are discussed as separate activities.
The conversion of natural systems to cultivated agriculture results in losses
of soil organic carbon on the order of 20-50 percent from the pre-cultivation
stocks in the surface 1 m (Davidson and Ackerman, 1993; Cole et al.,
1997; Paustian et al., 1997a; Lal and Bruce, 1999). Loss rates are higher
during the first years of cultivation and then tend to level off after 20-50
years (Díaz-Raviña et al., 1997; Lal et al., 1998). There are
several exceptions to this rule. One is with paddy cultivation (flooded for
extended periods), where decomposition rates are retarded, thereby maintaining
or increasing carbon stocks (Greenland, 1985; Bronson et al., 1997).
In contrast, when organic soils (Histosols) are drained and brought into cultivation,
losses of soil carbon may continue as long as the soil is exposed (Lal et
al., 1998). When arid or semi-arid soils are brought under irrigation, increased
organic matter input from roots and crop residues may increase soil carbon stocks
(Leuking and Schepers, 1985). This increase could be partially offset by a concomitant
increase in residue decomposition resulting from added moisture. On a global
basis, the cumulative historic loss of carbon from agricultural soils has been
estimated at 55 Gt C (IPCC, 1996b)-nearly one third of total carbon loss from
soils and vegetation (150 Gt C) (Houghton, 1995).
Studies suggest that most of the world's agricultural soils have not reached saturation of their carbon stocks; therefore, most soils are potential carbon sinks (Kern and Johnson, 1993; Donigian et al., 1995). Future increases in carbon stocks are likely to be smaller in areas that have been highly productive during past decades than in areas that currently are experiencing sharp increases in crop yields, as well as those moving into conservation tillage; the magnitude of these increases is highly dependent on inputs and management, however. Barriers to adoption of soil carbon-enhancing activities include a perceived lack of profitability and land managers' lack of understanding and acceptance of improved techniques for controlling pests and weeds, applying fertilizers, and switching to crop varieties that are matched to different soil moisture and temperature conditions. Because these barriers are addressable by public policies and programs, increasing the adoption of cropland management activities appears to be an opportunity for a broad range of Parties. Virtually everything that occurs in arable lands is "human-induced;" therefore, that criterion, if chosen, should pose no technical difficulty with regard to these activities.
Global warming is expected to increase yields in higher latitude cropland zones by virtue of longer growing seasons and CO2 fertilization (Cantagallo et al., 1997; Travasso et al., 1999). At the same time, however, global warming may also accelerate decomposition of carbon already stored in soils (Jenkinson, 1991; MacDonald et al., 1999; Niklinska et al., 1999; Scholes et al., 1999). The net effect of these changes on carbon sequestration in croplands is likely to impact some geographic areas more than others, but there are no reliable estimates of those future impacts. Although much work remains to be done in quantifying the CO2 fertilization effect in cropland, van Ginkel et al. (1999) estimate the magnitude of this effect (at current rates of increase of CO2 in the atmosphere) at 0.036 t C ha-1 yr-1 in temperate grassland, even after the effect of rising temperature on decomposition is deducted.
Efforts to improve soil quality and raise SOC levels have been grouped into three practices: agricultural intensification, conservation tillage, and erosion reduction. Agricultural intensification (Section 18.104.22.168) involves the use of improved water management, new or higher yielding varieties, integrated pest management, judicious use of organic and inorganic amendments, crop rotations, and other sound technologies that are available now, except for conservation tillage. Conservation tillage (Section 22.214.171.124) includes various forms of reduced tillage and is discussed as a distinct practice because of its specific effects of accumulating soil carbon and reducing soil erosion. Soil erosion control and improved water management are discussed in Section 126.96.36.199. Care has been taken so the areas listed under each practice are not double-counted.
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