Quantifying economically and ecologically optimum nitrogen rates for rice production in south-eastern China

China consumes 32% of the worldʹs total synthetic fertilizer nitrogen (N). Overuse of fertilizer N has become widespread, resulting in severe environmental problems. Based on a set of statistical models, we quantified the optimum N rates for rice production in terms of economic and ecological benefits. Model fitting results suggested that the dependence of rice yield, N uptake and N loss on fertilizer N application rates can be well determined by a quadratic polynomial function, a logistic function and a power function, respectively. Using these functions, the economically optimum and ecologically optimum N rates in south-eastern China were estimated to be 180–285 kg ha−1 and 90–150 kg ha−1, respectively, depending on rice subspecies, varieties and cropping systems. A case study in Jiangsu Province, where single rice with conventional japonica varieties is dominated, suggested that current N rates (∼390 kg ha−1) could be cut by 26% and 61% when the economically and ecologically optimum N rates, respectively, are adopted, saving 189 × 103 and 442 × 103 metric tons per year, respectively. Cutting one-third of the N use would not reduce rice yield but is expected to mitigate negative environmental impact in this province.