Modeling soil organic carbon change in croplands of China

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