中文摘要：

选择天津市水源地于桥水库周边2个典型的农业小流域进行了长期的氮素流失研究，结果表明，地表径流的总氮浓度以村庄和果园为最高，平均浓度在20mg·L^-1左右；农田次之，山坡最低，平均浓度低于10mg·L^-1，亚表层流中氮素流失在不同地点和不同降雨事件中浓度有较大差异，总体趋势以果园最高，村庄中菜园次之，农田最低，在流域出口处，曹各庄流域地表径流总氮浓度达到18.5mg·L^-1，桃花寺流域出口处浓度为5.9mg·L^-1，流域内不同景观格局和迁移廊道地貌的差异是造成2个流域氮素流失浓度差异的主要原因，曹各庄流域中主要污染源村庄离水库较近（约200m），且道路型季节性河流等性质特征对污染物的滞留能力较弱，增加了氮素流失到受纳水体的风险；桃花寺流域的污染源村庄和果园离受纳水体较远（约1500m），氮素在季节性河流迁移过程中被小石坝、植被过滤带和干塘等“汇”型结构所滞留，从而降低了向水体迁移的风险。

英文摘要：

A field study on nitrogen losses was undertaken for five years in the agricultural catchments by the Yuqiao Reservoir, the drinking water source of Tianjin, the third largest city of China. Total nitrogen concentrations （TN） in surface runoff of village and orchard were the highest （mean value around 20 mg·L^-1） among five kinds of land uses. That of cropland was the second and that of hill land was the lowest. The ranges of nitrogen concentration in subsurface flow were wider depending upon sampling sites and runoff. During runoff events, the general trend was orehard 〉 farmyard 〉 cropland. The TN concentrations of surface runoff at the outlet of the Taohuasi catchment amounted to 18.5 mg·L^-1, while that of the Caogezhuang catchment was 5.9 mg·L^- 1. The spatial pattern of different landscapes and transport pathways may be responsible for these differences. The short distance of about 200 m from the village to the reservoir in the Caogezhuang catchment, and a ＂road＂ type pathway with packed soil increased the risk of nitrogen loss. The longer distance about 1500 m between village and orchard in the Taohuasi catchment away from the reservoir increased the nitrogen transport pathway. Also, small stone dams, vegetated filter strips and dry ponds within the ephemeral stream could detain nitrogen transport in the catchment.