Soil nitrate nitrogen buffer capacity and environmentally safe nitrogen rate for winter wheat-summer maize cropping in Northern China

Maintaining high crop yield and meanwhile reducing the risk of environmental pollution are of paramount importance for food security and sustainability. The aims of this study were (i) quantify soil NO3- -N buffer capacity under wheat-maize cropping system in Northern China, and (ii) establish agronomic optimum and environmentally safe N rates to guide N recommendation for the region. We compiled data of 76 sets of wheat and 73 sets of maize experiments spanning about 20 years, where N rate experiments conducted and soil NO3- -N content were measured immediately after crop harvest under conventional practice. By applying linear-plateau and plateau-linear models, we computed the soil NO3- -N buffer capacity, agronomic optimum N rate and environmentally safe N rates. Soil NO3- -N buffer capacity ranged from 10 to 221 kg ha(-1) for wheat season and from 14 to 212 kg ha(-1) for maize season, which were mostly less than 100 kg ha(-1) in the region. The agronomic optimum N rates for wheat and maize were 46-221 and 44-250 kg N ha(-1) with means of 110 and 127 kg N ha(-1), respectively. The environmentally safe N rates were 19-248 kg N ha(-1) with mean of 107 kg N ha(-1) for wheat, and 36-209 kg N ha(-1) with mean of 99 kg N ha(-1) for maize. It was found that agronomic optimum N rates from 61% of wheat and 52% of maize were higher than the environmentally safe N rates, implying that current N recommendation had the high potential risk of nitrate leaching. Hence, it is suggested that N recommendation should consider not only the agronomic optimum N but also environmentally safe N rate, especially in areas where are vulnerable to nitrate leaching the latter should be placed at the top priority.