Hydrological transport pathways of dissolved organic nitrogen and their seasonal changes in an agricultural watershed

[Display omitted] •DON export load was quantified in an agricultural watershed from 2011 to 2016.•The DON loss is concentrated in the wet season.•Baseflow is the main transport pathway of the DON in the watershed.•2.03 m3/s to 2.15 m3/s is the critical threshold of the streamflow that causes abrupt changes in DON concentration. The role of dissolved organic nitrogen (DON) in the eutrophication of surface water and the potential risk to drinking water safety as a precursor of disinfection by-products is gradually emphasized. Therefore, accurate estimates of watershed DON load fluxes and identifying thresholds of streamflow that cause abrupt changes in riverine DON concentration are important. Here, we adopted the load estimator model (LOADEST) and nonparametric change-point analysis (nCPA) to comprehensively investigate the variation of DON load from 2011 to 2016 in different seasons in the Fengyu River watershed, Southwest China. Results indicated that the wet season is the hot moment of DON loss, and the average cumulative load reaches 14.1 t, accounting for 72.5 % of the annual load. Meanwhile, baseflow is the main pathway of DON export, accounting for about 74.4 % of the stream export load. Furthermore, the abrupt threshold of streamflow on DON concentration is 2.03 m3/s to 2.15 m3/s. When the stream flow is below 2.15 m3/s, the concentration of river DON increases rapidly and poses a great threat to human drinking water safety. This study will provide a scientific basis for regulating DON at the watershed scale and water quality protection in drinking water source areas.