ASSESSMENT OF C BUDGET FOR GRASSLANDS AND DRYLANDS OF THE WORLD

Intergovernmental Panel on Climate Change (IPCC) estimates indicate th at potential changes in seasonal rainfall and temperature patterns in central North America and the African Sahel will have a greater impact on biological response (such as plant production and biogeochemical c ycling) and feedback to climate than changes in the overall amount of annual rainfall. Simulation of grassland and dryland ecosystem respons es to climate and CO2 changes demonstrates the sensitivity of plant pr oductivity and soil C storage to projected changes in precipitation, t emperature and atmospheric CO2. Using three different land cover proje ctions, changes in C levels in the grassland and dryland regions from 1800 to 1990 were estimated to be -13.2, -25.5 and -14.7 Pg, i.e., a n et source of C due to land cover removal resulting from cropland conve rsion. Projections into the future based on a double-CO2 climate inclu ding climate-driven shifts in biome areas by the year 2040 resulted in a net sink of +5.6, +27.4 and +26.8 Pg, respectively, based upon sust ainable grassland management. The increase in C storage resulted mainl y from an increase in area for the warm grassland sub-biome, together with increased soil organic matter. Preliminary modeling estimates of soil C losses due to 50 yr of regressive land management in these gras sland and dryland ecoregions result in a 11 Pg loss relative to curren t conditions, and a potential loss of 37 Pg during a 50 yr period rela tive to sustainable land-use practices, an average source of 0.7 Pg C yr-1. Estimates of the cost of a 20 yr rehabilitation program are 5 to 8 x 10(9) US$ yr-1, for a C sequestering cost of approximately 10 US$ per tC.