Changing land use from cropping or degraded lands to perennial grasslands in response to government policies can increase aboveground and below-ground biomass and soil carbon stocks.
Use and Potential
Set-aside of marginal and/or degraded cropland to grassland is likely to be most predominant in countries with agricultural surpluses, but opportunities for set-asides for environmental protection reasons are possible in all countries. The Conservation Reserve Program (CRP) in the United States has resulted in the set-aside of more than 17 Mha of erodable and environmentally sensitive croplands, most of which have been planted to perennial grasses and legumes. Several estimates of the potential carbon storage on CRP lands have been made, ranging from 12 to 18 Mt C yr-1 (Gebhart et al., 1994; Barker et al., 1995; Follett et al., 2000; Paustian et al., 2000b). Comparable per-unit area increases have been reported for studies of conversions to grasslands in other temperate regions (Paustian et al., 1998b), although low or non-significant increases have been found in some studies in semi-arid climates (Burke et al., 1995; Robles and Burk,e 1998). Globally, estimates of the potential area of cropland that could be placed into set-asides are on the order of 100 Mha (IPCC, 1996b).
Current Knowledge and Scientific Uncertainties
The processes involved in carbon storage with conversion of cultivated land to perennial grasslands are relatively well understood. Rates vary, however, as a function of many site-specific factors, including stand composition and establishment, fertilization, and nutrient availability (Huggins et al., 1998a). Many rate measurements in the literature are based on paired-site comparisons, which increases levels of uncertainty (Huggins et al., 1998a; Robles and Burke, 1998; Follett et al., 2000).
Where direct sampling is used, soil carbon should be measured to sufficient depth (e.g., 1 m) to represent the full rooting zone because of deep carbon storage with some species. Carbon stock changes can be directly measured through repeated sampling at intervals of 3-5 years or more; the length of the re-measurement interval depends on the initial carbon level and the productivity of the grassland conversion. Scaling up can be implemented with the use of models and information on the location of the pertinent areas; time since conversion; and climate, soils, and vegetation composition.
Carbon accumulation to levels comparable to native grasslands may take up to 50 years (McConnell and Quinn, 1988) or longer (Dormaar and Smoliak, 1985; Burke et al., 1995) in temperate regions, but recovery rates will vary according to site and management variables. Rapid rates of carbon accumulation are most likely in highly productive grassland conversions in mesic environments.
Aboveground biomass is subject to removal by livestock and other herbivores, as well as by fire. In the absence of significant erosion or degradation of vegetation, sequestered carbon will be maintained in the soil.
If these lands are returned to crop production, there will be a rapid loss of carbon from the soils (Barker et al., 1995), although re-conversion to no-till cropping may reduce these losses. If continuity of management purpose is maintained, storage of carbon in soils and root biomass can provide pools that persist for years to centuries.
Associated impacts include reduced crop production, increased animal production (if the land is grazed), increased biodiversity of native grass ecosystems (if they are reestablished), increased wildlife habitat, reduced erosion, improved water quality, and reduced dryland salinity downslope (in landscapes with this hazard). If the land is grazed, methane and nitrous oxide emissions may more than offset the sink provided by increasing carbon pools (Crush et al., 1992).
Relationship to IPCC Guidelines
Grassland set-asides are included in the Reference Manual (which refers to "abandonment of managed lands"); the Workbook provides default values of carbon stock changes for set-asides that are less than 20 years old and older than 20 years.
Other reports in this collection