Effect of N fertilization on grain yield of winter wheat and apparent N losses

Excessive nitrogen (N) fertilizer application to winter wheat is a common problem on the North China Plain. To determine the optimum fertilizer N rate for winter wheat production while minimizing N losses, field experiments were conducted for two growing seasons at eight sites, in Huimin County, Shandong Province, from 2001 to 2003. The optimum N rate for maximum grain yield was inversely related to the initial soil mineral N content (N-min) in the top 90 cm. of the soil profile before sowing. There was no yield response to the applied N at the three sites with high initial soil mineral N levels (average 212 kg N ha(-1)). The average optimum N rate was 96 kg N ha(-1) for the five sites with low initial soil N-min (average 155 kg N ha(-1)) before sowing. Residual nitrate N in the top 90 cm of the soil profile after harvest increased with increasing fertilizer N application rate. The apparent N losses during the wheat-growing season also increased with increasing N application rate. The average apparent N losses with the optimum N rates were less than 15 kg N ha(-1), whereas the farmers' conventional N application rate resulted in losses of more than 100 kg N ha(-1). Therefore, optimizing N use for winter wheat considerably reduced N losses to the environment without compromising crop yields.