Rice is the staple food in China, especially in the south. Over the last twenty years, nitrogen fertilizer applied in paddy field has been increased considerably to keep up with growing food demand. It is believed that this change has been causing a series of environmental consequences, including groundwater pollution, nitrogen leaching, runoff, and greenhouse gas emissions. Previous studies concluded that most farmers in economically developed areas have been applying too much more fertilizer than crop needs -- hence researchers suggested the government should regulate nitrogen fertilizer useto avoid serious environmental problems. However, farming practice data used for those studies were aggregated statistical data at or above the county level and, therefore, their conclusions may not be applicable at local or individual farm levels given the spatial heterogeneity in a region. In this research, crop management data was collected at the village level in Anji County in Zhejiang Province, China to examine the spatio--temporal variation of nitrogen use efficiency of paddy rice systems. A biogeochemical model, DNDC, was used to simulate paddy rice systems from 1991 to 2009. The results show that spatial variability had significant impacts on nitrogen use efficiency estimate at county level. Further, as an effort to close the knowledge gap between site-based experiment findings and regional estimation, this study assessed the sensitivity of site-based modeling results to various spatial interpolation approaches. Results indicate that the spatial distribution of paddy field has significant impacts on regional estimates of N use efficiency and must be taken into account for extrapolation of site-specific results to the county level.
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