It has been widely proved that nitrogen is vital to maintain terrestrial ecosystem balance and world food safety at global scale.However,with the increasing development of social economy and population growth,excessive synthetic nitrogen has been discharged into water bodies in the world,which is recognized as one of the most important causes of water environment deterioration and eutrophication.Within the numerous nitrogen sources,diffuse pollution from agricultural activities has been identified as the most important contributor to nitrogen loss in more and more areas.Therefore,it is more crucial and meaningful to assess precisely nitrogen loss potential and to identify its impact factors for the effectively integrated watershed management.In this study,a modified nitrogen export coefficient model was developed considering the nitrogen production,surface runoff generation,leaching potential of soil moisture,and landscape interception in the given watershed.To examine the performance of the modified total nitrogen export coefficient model,this modified result was validated by the monitoring data of water quality at the outlet of Chao River watershed,which is one of the 2 major tributaries into Miyun Reservoir in the northeast of Beijing.Then,the critical source areas (CSAs) of nitrogen loss were identified,and the impact factors of nitrogen loss were determined by the boosted regression trees algorithm.The major results were illustrated as follows:1) For monitoring loading of total nitrogen at outlet of Chao River,the modified export coefficient model was characterized by a higher accuracy with a relative error of 8.23％ compared with the traditional export coefficient model with a relative error of 18.94％.2) The CSAs of total nitrogen loss were mainly distributed in the western areas of upper and middle stream,as well as the downstream riparian region in this watershed,where existed higher nitrogen load for intensive agricultural economic activities.Regarding to administrative scale,Fengning County (Dage and Heishanzui Town),Luanping County (Hushiha and Bakeshiying Town),Miyun District (Gaoling,Gubeikou and Taishitun Town) were identified as the CSAs of total nitrogen loss for their higher population and livestock density,and the relative lower capability of landscape interception.In particular,the high potential of total nitrogen loss was also identified in the south area and riparian zone in the Chao River watershed.3) Based on the boosted regression trees model,by extracting 5 000 samples in ArcGIS 10.1 platform,the anthropological factors were determined as the most important impacting factors for total nitrogen loss.The contributions of the amount of total nitrogen fertilizer application,the livestock breeding scale and the topography slope to total nitrogen loss in the watershed were 54.74％,17.48％ and 16.53％,respectively.Therefore,control on the unreasonable scale of total nitrogen and livestock was necessary and recommended to reduce the severe nitrogen pollution and to protect the drinking water safety of Beijing City.In summary,the proposed total nitrogen export coefficient model can be applied to provide available information for the environmentalists,farmers and watershed managers to prevent and alleviate diffuse total nitrogen loss especially in the area lacking effective data.%非点源态氮流失是地表水体水质恶化的重要因素,引发水体富营养化等一系列水环境问题.而准确评估流域内非点源总氮的负荷及分布和影响因素是流域综合管理的必要前提.该研究提出了一个综合考虑产污强度、降雨径流、土壤水分下渗和流域下垫面植被景观截留等作用的氮输出系数模型,将其应用于密云水库潮河流域,并结合流域实测水质数据对构建的模型结果进行验证.在此基础上,识别了流域总氮关键源区,利用增强回归树模型确定了总氮流失的关键影响因子.主要结论:1)与实测总氮负荷相比,改进输出系数模型模拟精度(相对误差8.23％)明显高于传统输出系数模型(相对误差18.94％);2)总氮关键源区主要分布于潮河中上游干流西侧和下游干流两侧区域,从行政区划来看主要位于丰宁满族自治县(大阁镇、黑山嘴镇)与滦平县(虎什哈镇和巴克什营)与密云区(高岭镇、古北口镇和太师屯镇);总氮关键源区具有明显的南高北低,沿河分布的特征;3)人为因素是潮河流域总氮流失的主要影响因素,其中,氮肥施用(54.74％)、畜禽养殖(17.48％)和坡度(16.35％)此3个对潮河流域总氮流失影响最大.该研究可为潮河流域水环境综合调控和氮污染精准管控提供科学依据.
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