Agricultural nitrogen flow in a reservoir watershed and its implications for water pollution mitigation

Miyun Reservoir plays a fundamental role in providing drinking water for Beijing Municipality. In this study, agricultural nitrogen (N) flow was analysed for water pollution mitigation in Miyun Reservoir watershed. A partial substance flow analysis (SFA) method was applied to evaluate the historic changes in N flow from 2001 to 2017. Based on this method, N flow, N losses, N accumulation, and nitrogen use efficiency (NUE) over 17 years were identified and quantified. The results showed that N input, N output, and N stock gradually decreased in this period. The decline of N input mostly resulted from the considerable decrease in chemical fertilizer use and livestock excrement application. With decreasing N input, the N stock also decreased over these years. The N losses generally decreased, except for 2009–2012, as a result of the substantial increase in harvested grains which generated even more straw for feeding or burning. In addition, the average NUE was as low as approximately 15% which may due to the huge straw storage and N losses. It is indicated that there was a great potential to decrease N input especially chemical fertilizer for mitigation of water pollution through analysis. Therefore, combined technology innovation with policy impact was proposed in Miyun County. Reducing chemical fertilizer, improving waste cycling, reducing ammonia volatilization, preventing erosion and runoff and improving peasant awareness of environmental protection were proposed as recommendations. The historic analysis of N flow could be helpful to understand the challenges of N pollution. The integrated policy and technology measures will be effective to achieve water quality control. Both will provide a reference for other countries or regions with the similar issues.