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[Export of nitrogen by lateral seepage from paddy field in Sanjiang Plain].

The present study aims to discuss the export mechanism for nonpoint source pollution of nitrogen from paddy field in Sanjiang Plain. Different scales of in-situ experiments were performed to study the concentrations of nitrogen, the intercepting ratio of bund to nitrogen, and the lateral seepage velocity during the export process of nitrogen in the field-bund-trench system. The annual export load of nitrogen was also quantified. The results showed that the width of near-trench bund affected the lateral seepage velocity and the intercepting ability of bund to nitrogen obviously. With the increasing of the width, the velocity decreased, however, the intercepting ability increased. The intercepting ratio of bund to ammonia nitrogen was bigger than that of total nitrogen and nitrate nitrogen. In order to control the nitrogen loss, the width of near-trench bund and the depth of trench were suggested to be at 80-130 cm and 150 cm, respectively. Under the current cultivation mode, the export load of total nitrogen from lateral seepage was 11 kg x hm(-2), accounting for 6.7 percent of the total fertilizer applied, demonstrating that lateral seepage was an important pathway of nitrogen loss. And the export load of total nitrogen was about 25.3 kg x hm(-2), accounting for 15.3 percent of the total fertilizer applied, the agricultural nonpoint source pollution of the large scale development of paddy fields in Sanjiang Plain should not be neglected.