Quantifying nitrogen leaching response to fertilizer additions in China's cropland

Agricultural soils account for more than 50% of nitrogen leaching (L-N) to groundwater in China. When excess levels of nitrogen accumulate in groundwater, it poses a risk of adverse health effects. Despite this recognition, estimation of L-N from cropland soils in a broad spatial scale is still quite uncertain in China. The uncertainty of L-N primarily stems from the shape of nitrogen leaching response to fertilizer additions (N-rate) and the role of environmental conditions. On the basis of 453 site-years at 51 sites across China, we explored the nonlinearity and variability of the response of L-N to N-rate and developed an empirical statistical model to determine how environmental factors regulate the rate of N leaching (LR). The result shows that L-N-N-rate relationship is convex for most crop types, and varies by local hydro-climates and soil organic carbon. Variability of air temperature explains a half (similar to 52%) of the spatial variation of LR. The results of model calibration and validation indicate that incorporating this empirical knowledge into a predictive model could accurately capture the variation in leaching and produce a reasonable upscaling from site to country. The fertilizer-induced L-N in 2008 for China's cropland were 0.88 +/- 0.23 TgN (1 sigma), significantly lower than the linear or uniform model, as assumed by Food and Agriculture Organization and MITERRA-EUROPE models. These results also imply that future policy to reduce N leaching from cropland needs to consider environmental variability rather than solely attempt to reduce N-rate. (C) 2016 Elsevier Ltd. All rights reserved.