Diffuse nitrogen pollution in a forest-dominated watershed: Source, transport and removal

Abstract

Along with gradually controlled point source discharges, diffuse nitrogen pollution has become an important cause of eutrophication. Diffuse nitrogen export from forests is an inherent characteristic, and increasing human disturbance aggravates this process. To control diffuse nitrogen pollution effectively, it is of great significance to determine the source, transport path and removal method of diffuse nitrogen export in forest-dominated watersheds. Long-term variations in the nitrogen concentrations indicate that nitrate and dissolved organic nitrogen (DON) are the main diffuse nitrogen export forms in the Buerhatong River watershed (BRW). Thus, the Soil and Water Assessment Tool (SWAT) hydrological model is conducted to simulate diffuse nitrate and DON export. First, main source of diffuse nitrate and DON export is determined by the fitting results of nitrogen contribution and forest/farmland cover. Then, the relationship between nitrogen export from forest/farmland and main hydrologic streamflow is explored. Finally, the role of wetlands in the diffuse nitrogen load removal is analyzed, and sensitivity results by Monte Carlo simulation of nitrogen export from wetlands, forests and farmlands are compared. The average annual diffuse nitrate and DON export values are 1.19 and 0.43 kg ha−1 yr−1, respectively, from 2014 to 2017. When forest cover and farmland cover are higher than 70% and 5%, farmlands and forests are the main sources of diffuse nitrate and DON export, respectively. Lateral flow is the dominant transport path of DON export from forests and nitrate export from farmlands, with correlation coefficient of 0.73 and proportion of 98%, respectively. Wetlands have the ability to reduce the diffuse nitrogen load, and their efficiency is related to the relative geographic location. The in situ experimental results show that the nitrate and DON removal rates by wetlands are 24.46% and 11.71%, respectively. The sensitivity of forest and wetland for diffuse nitrate and DON load removal is 0.31, −0.12 and 0.2, −0.21, respectively. Returning forests to wetlands is a good method for diffuse nitrogen removal with food production taken into consideration.