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Abstract

Using 1990 conditions, we modeled carbon (C) and nitrogen (N) biogeochemical cycles in croplands of China (and, for comparison, the United States) to estimate the annual soil organic-carbon (SOC) balance for all cropland. Overall, we estimate that China's croplands lost 1,.6% of their SOC (to a depth of 0.3 in) in 1990, and that U.S. cropland lost 0.1%. A key element in this difference was that similar tO₂5% of aboveground crop residue in China was returned to the soil, compared to similar to90% in the United States. In China, SOC losses were greatest in the northeast (similar to10(3) kg C.ha(-1).yr(-1)), and were generally smaller (<0.5 X 10(3) kg C.ha(-1).yr(-1)) in regions with a longer cultivation history. Some regions showed SOC gains, generally <10(3) kg C.ha(- 1).yr(-1). Reduced organic-matter input to China's cropland soils, and lower overall SOC levels in those soils, led to lower levels of N mineralization in the simulations, consistent with higher rates of synthetic-fertilizer application in China. C and N cycles are closely linked to soil fertility, crop yield, and non-point-source environmental pollution.