氮是动植物生长必需元素，其在“土壤-作物-畜牧业”生产体系的利用效率不但决定着动植物产品产量，也显著影响环境质量。近年来，随着农牧业集约化程度的提高，农牧分离导致过量的化肥投入和过低的畜禽粪尿还田率，已经成为华北乃至全国氮污染的重要因素。因此，阐明农牧结合与 “土壤-作物-畜牧业”生产系统氮利用效率的关系，探索农牧结合的调控途径已经成为我国农牧业可持续发展的重要科学命题。为此，本研究以农牧业主产区华北为例，从系统观点出发，采用农户调研、跟踪、监测和模型分析的方法，阐明华北不同类型农户和村庄“土壤-作物-畜牧”生产系统氮利用效率、环境损失等特征；揭示生态条件、农场规模、类型和农牧结合程度对农户和村庄“土壤-作物-畜牧”生产系统氮利用效率的影响机理；探索不同农牧结合模式和“土壤-作物-畜牧”生产系统可持续氮管理的实现途径。研究不仅对华北氮优化管理具有指导意义，而且也可为其它地区研究提供借鉴。

Nitrogen (N) is an essential nutrient for the growth of plants and animals. Simultaneously it is also an important contributor for the environmental pollution. The North China Plain (NCP) is an ideal case study because it is a productive area in a crop and intensive livestock production, but decoupling between crop and animal production. This region already faces serious groundwater pollution by nitrate leaching, eutrophication of surface waters, as well as air quality degradation cause by over-fertilization and manure discharge. Current researches of N management focus on N fertilizer experiments and technology developments at field level, and feed management at animal and herd level. However, the linkages of research between crop and animal production are lacked. Meanwhile, the pathways and magnitude of N flows in soil-crop production-animal production system are not well known quantitatively at different levels. Therefore, the objectives of this study are (1) to develop the methods for quantifying N flows, N use efficiencies, losses in soil-crop production-animal production system at farm, village and regional levels by monitoring farmers’ activities in NCP, (2) to quantify the effects of coupling crop and animal production on increasing N use efficiencies and decreasing losses in soil-crop production-animal production system at farm and village level in NCP, (3) to explore strategies for coupling crop and animal production and optimizing N flows in soil-crop production-animal production system at farm and village level in NCP, and (4) to develop options and scenarios for improving N management by coupling crop and animal production at region levels in NCP. This study is not only helpful for the optimization of N manage in soil-crop production-animal production system in NCP, but also provides useful theories and techniques for other regions.