NITROGEN BALANCE IN INTENSIVE AGRICULTURE IN THE NORTH CHINA PLAIN

Key aspects of nitrogen (N) cycle were investigated in a 1 ha irrigated wheat-maize rotation field at Luancheng in the North China Plain to estimate the N balance. The main N output items including ammonia volatilization, nitrate leaching, denitrification-nitrification loss, and nitrogen uptake by crop above ground were measured to assess the magnitude of N cycle by different pathway. The main N input items were also investigated, including applied N fertilizer, N in irrigated water and rainfall and N in residue retained to field.The total NH3 volatilization loss was 19.7 kg N ha-1 under wheat and 46.7 kg N ha-1 under maize, accounting for 7.9% and 27.4% of the applied fertilizer N, respectively. Denitrification-nitrification loss was 4.8~7.9 kg N ha-1 for the maize phase, accounting for 2.8%~4.6% of the applied fertilizer N, which was slightly higher than that from the wheat phase (1.0%~1.4%). While the cumulative nitrate leaching losses were 32 kg N ha-1 under wheat and 24 kg N ha-1 under maize, accounting for 12.8% and 14.1% of applied N, respectively.The total N input was 558.5 kg N ha-1y-1, while the total N output was 488.7~492.9 kg N ha-1y-1. N uptake by crop was the main item of N output, accounting for about 73% of the total N output. The total N loss to the environment was 129.7~133.9 kg N ha-1y-1 and ammonia volatilization and nitrate leaching were the main pathways of fertilizer N loss