Soil nitrogen supply capacity as an indicator of sustainable watershed management in the upper basin of Miyun Reservoir

Comprehensive evaluation of soil N supply capacity is a fundamental approach to reduce N pollution. In this research, we developed a novel framework to quantify soil N supply capacity. This was accomplished by integrating a multivariate regression analysis and a path analysis to establish the relationship between the amount of soil supplied N and six main processes (i.e., organic N mineralization, atmospheric N deposition, litter decomposition, nitrification, denitrification and surface runoff) related to soil N cycle, with exclusion of the multi-collinearity among these six main processes. Soil N supply capacity was measured by the ratio of soil supplied N and plant required N. The results revealed that (1) organic N mineralization was the dominant process that sustained the amount of soil supplied N, contributing 81.51-121.54 kg N/hm2 a under different land utilization patterns; (2) processes such as atmospheric N deposition, litter decomposition and surface runoff could affect the amount of soil supplied N as well. In detail, atmospheric N deposition contributed 11.88-27.79 kg N/hm2 a to soil supplied N. Litter decomposition in coniferous, broadleaf and mixed forests provided 57.31-59.26 kg N/hm2 a to soil supplied N, which accounted for over half of the N provided by organic N mineralization. Surface runoff reduced soil supplied N by about 14.78% (73.57 kg N/hm2 a) in the shrub forest; (3) soil N supply capacity under different land use types ranged from 1.43 to 8.30, indicating sufficient fertility for plant growth and an insistent demand for soil N management.