稻田空间分布格局对三峡库区农业小流域径流氮排放的影响

Impact of Spatial Distribution Pattern of Paddy Fields on Runoff Nitrogen Export from a Small Agricultural Watershed in the Three Gorges Reservoir Area

对三峡库区涪陵段两个毗邻集水域的汇水口径流进行了持续1年的高频(每日)采样监测,以对比分析稻田空间分布格局对径流氮素质量浓度和输出强度的影响.所选择的两个集水域稻田面积比例相近,但其中一个(记为A)的稻田分布零散,破碎度高,另一个(记为B)的稻田则连片分布在其末(底)端,破碎度低.结果表明两集水域不同作物生长季径流氮素的输出均以硝态氮为主(占总氮的60%~81%).对比两个集水域,A在全年和不同作物生长季的径流氮素平均质量浓度都相应地高于B.同样,集水域A的年径流量(1 431 m^3/hm^2)显著高于集水域B(840 m^3/hm^2),因而前者径流总氮的年输出通量(16.1kg/hm^2)远大于后者(5.21kg/hm^2).鉴于两个集水域的气候、土壤类型、地形地貌和农田耕作管理方式相同,单位土地面积的施肥量以及稻田面积比例也相近,只是稻田空间分布格局迥异,汇水口径流氮素的质量浓度和输出通量的差异显然是由于集水域末端连片分布的人工湿地—稻田比零散分布的稻田对集水域内的径流及随其迁移的氮素有更佳的拦截和净化作用所致.

A high frequency （daily） monitoring was carried out in two adjacent catchments in Fuling of the Three Gorges Reservoir for a year with the aim of analyzing comparatively the impact of spatial distribu- tion pattern of paddy fields on nitrogen concentration and export fluxes in the runoff. The rice paddy fields of a catchment （marked as A） were scatteringly distributed with a high fragmentation index, while the paddy fields of the other catchment （marked as B） were continuously distributed at the bottom of the catchment, with a low fragmentation index. The results showed that the form of the exported nitrogen was dominated by nitrate nitrogen （accounting for 60%-81% of the total nitrogen） in different crop growing seasons in both catchments. But the average concentration of nitrogen in the runoff of A was higher than that of B throughout the year and in different crop growing seasons. Similarly, the amount of runoff in A （1 431 m^3/hm^2） was significantly higher than in B （840 m^3/hm^2）, and the total nitrogen export fluxes of A （16.1 kg/hm^2） was much higher than of B （5.21 kg/hm^2）. In view of the fact that the two catchments shared the same climate, soil type, topography and tillage management and similar fertilization rate per unit of land and area proportion of rice paddy fields, but had strikingly different spatial distribution patterns of rice paddy fields, the differences in nitrogen concentration and export fluxes in the runoff of the two catchments were obviously caused by the better N interception and purification in runoff of B than of A.