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 m) in 1990, and that U.S. cropland lost 0.1%. A key element in this difference was that ~25% of aboveground crop residue in China was returned to the soil, compared to ~90% in the United States. In China, SOC losses were greatest in the northeast $(\sim\!10^3\>kg\>C\!\cdot\!ha^{-1}\!\cdot\!yr^{-1})$, and were generally smaller $(kg\>C\!\cdot\!ha^{-1}\!\cdot\!yr^{-1})$ in regions with a longer cultivation history. Some regions showed SOC gains, generally $kg\>C\!\cdot\!ha^{-1}\!\cdot\!yr^{-1}$. 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.