Research papersModified control strategies for critical source area of nitrogen (CSAN) in a typical freeze-thaw watershed

- The features of N (Nitrogen) loss in typical freeze-thaw watershed were elaborated.
- The identification of critical source area of N (CSAN) was optimized.
- The spatiotemporal variations of the critical source area of N (CSANs) was analyzed.
- The response of BMPs to the changes of precipitation and seasons were studied.

The management of critical source areas of diffuse nitrogen (CSANs) remains challenging in freeze-thaw areas due to the different N loss characteristics in different hydrological conditions and seasons. To address these challenges, a modified strategy was proposed in this study using the Soil and Water Assessment Tool (SWAT) to simulate diffuse N loads in the study catchments. Specifically, the spatial and temporal variations of CSANs caused by differences in precipitation and seasons were considered. In addition, the selection of best management practices (BMPs) was selected according to BMP performance and their seasonal characteristics in diffuse N control. The diffuse N load formed during freeze-thaw seasons accounts for approximately 50% of the annual diffuse N load. The diffuse N load discharged to rivers was higher in wet conditions than dry conditions by 127.4% and 181.5% during freeze-thaw seasons and growing seasons, respectively. The spatial distribution of CSANs was more sensitive to differences between freeze-thaw and growing seasons. Among BMPs, buffer strips (BS), no tillage (NT) and reducing N fertilizer applications (RNFA) all showed differences in their diffuse N removal efficiency under different hydrological conditions and seasons, while reforestation operations were not affected by these factors. The benefit of reforestation operations was lower in flatter areas. When areas with slopes greater than 2 degrees were reforested, the average N removal efficiency of the 1st CSAN could be as high as 82.4%. In the 2nd CSAN, the average N removal efficiency of BS was relatively constant across freeze-thaw seasons. Across growing seasons, the N removal efficiency of BS in wet years was 8%-10% higher than in dry conditions due to the lower percentage of lateral flow. The average N removal efficiency of NT was higher during freeze-thaw seasons and lower during growing seasons with average values of 9.3% and 6.1%, respectively. The N control efficiency of RNFA 10% and RNFA 20% (a 10% or 20% reduction in fertilizer application, respectively) was highest during dry growing seasons with average N control efficiencies of 9.6% and 17.8%, respectively. This study is expected to improve diffuse pollution control in cold areas and to improve the understanding of how N removal efficiency of BMPs responds to variations in hydrological conditions.