[Effects of nitrogen application rate on soil nitrate nitrogen accumulation under vegetable-paddy rice rotation system].

A 2-year field experiment of mustard-cabbage-early rice rotation was conducted to investigate the effects of nitrogen application on yield and accumulation of nitrate nitrogen in the soil. The results showed that the applications of 150 kg N x hm(-2) for mustard and cabbage respectively and 90 kg N x hm(-2) for early rice were the best economic application mode, which could increase the net profit by 0.2%-75.6% compared with other application modes. Nitrogen application rates were positively correlated with NO3(-)-N concentration in the soil and in the percolating water. The vegetable-paddy rice rotation decreased the surplus of nitrogen in the soil. The average soil NO3(-)-N concentration was 29.7 mg x kg(-1) under the rotation of mustard-cabbage-early rice, which was only 84.4% of that under the continuous cropping of mustard-cabbage. The average NO3(-)-N concentration in the percolating water under mustard-cabbage-early rice rotation was little different from that in basal soil. Therefore, with the optimum nitrogen application mode, the vegetable-paddy rice rotation could gain the best economic benefit while significantly decrease the accumulation of nitrate nitrogen in the soil to effectively control non-point source pollution of nitrogen from vegetable fields.