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Many Great Plains soils have declined in organic carbon (C) content and nutrient supplying capacity since they were initially cultivated. Haas et al. (13) summarized early studies documenting the effects of cultivation on soil carbon and nitrogen from 24 Great Plains Research Stations and reported losses of 24 to 60% from soils cultivated 30 to 43 years. More recent studies from the Great Plains (1, 3, 6, 10, 16, 17, 24, 25) report similar losses of soil organic carbon (SOC) with cultivation. Current estimates (7) suggest that cultivated croplands in the United States lose about 2.7 million metric tons C y sup -1 (6 billion lbs C yr sup -1 ). An additional 35.4 million metric tons C (78 billion lbs C) are released to the atmosphere every year from agricultural fossil fuel use and manufacture of nitrogen fertilizers (7). These data indicate that agricultural activities are significant contributors of CO sub 2 gas to the atmosphere.
One of the major goals of the Conservation Reserve Program (CRP) is to reduce water and wind erosion through the establishment of perennial grass cover on more than 17 million ha (45 million ac) of highly erodible and environmentally sensitive croplands (14). Once established, however, these new grasslands also may have the potential to concomitantly reduce atmospheric CO sub 2 levels and increase SOC levels due to accumulation and incorporation of litter into surface soils and the relatively large amounts of net primary production allocated toward root growth in grasslands (2,7). McConnell and Quinn (18) showed that SOC contents of the surface 0 to 25 cm (0 to 10 in) of croplands abandoned and/or reseeded to perennial grasses were similar to adjacent native rangeland following about 50 years of recovery. Conversely, surface SOC of cropland continually cultivated was substantially lower than that of native rangeland and abandoned and reseeded cropland. Dormaar and Smoliak (12) observed similar trends for abandoned cropland and native rangeland in Alberta, Canada, and suggested that more than 55 years are required for SOC in revegetated, abandoned croplands to approach that of native rangeland. Based on these results, the land use change from crop production to perennial grass cover associated with CRP may sequester atmospheric carbon back into the soil carbon pool, thereby changing soils...