Fertilization and nitrogen balance in a wheat-maize rotation system in North China

Over N fertilization is a common problem for the winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) rotation system in the North China Plain. A field experiment which included control (no N), conventional N (Con. N) fertilization, and optimized N (N-min) fertilization treatments, was conducted from 1999 to 2003 near Beijing, China. Soil nitrate (NO3) dynamics were measured and N balance was calculated for the period of the eight successive cropping seasons. Soil NO3-N in the 0- to 90-cm profile for the Con. N treatment ranged from 157 to 700 kg ha(-1) during the eight successive cropping seasons, much greater than those in the no N and optimized N treatments. Large amounts of soil NO3-N were detected in the 90- to 200-cm layer under the conventional N fertilization treatment, especially in the summer maize season. For the N-min treatment, the total amount of N applied was 511 kg N ha(-1) in the eight successive crops as compared with 2400 kg N ha(-1) of the Con. N treatment. Grain yields were not different between the fertilized treatments except for maize in 2003. Soil NO3-N in the root zone under conditions of optimized N fertilization was maintained at a relatively low level as compared with the Con. N treatment, therefore dramatically decreasing NO3-N movement to deeper soil profile. This study indicates that soil NO3 movement out of the effective crop root zone is an important pathway of N losses in this winter wheat-summer maize rotation system in the North China Plain and the optimized N fertilization by an improved N-min method shows high potential of reducing N-leaching losses.