Nitrate runoff loss and source apportionment in a typical subtropical agricultural watershed

Nitrate (NO₃^-) loss and enrichment in water bodies caused by fertilization are a major environmental problem in agricultural areas. However, the quantitative contribution of different NO₃^- sources, especially chemical fertilizers (CF) and soil organic nitrogen (SON), to NO₃^- runoff loss remains unclear. In this study, a systematic investigation of NO₃^- runoff and its sources was conducted in a subtropical agricultural watershed located in Yujiang County, Jiangxi Province, China. A semi-monthly sampling was performed at the inlet and outlet from March 2018 to February 2019. Hydrochemical and dual NO₃^- isotope (¹⁵ N and ¹⁸O) approaches were combined to estimate the NO₃^- runoff loss and quantify the contribution of different sources with a Bayesian isotope mixing model. Source apportionment by Stable Isotope Analysis in R (SIAR) suggested that NO₃^- in runoff was mainly derived from nitrification of ammonium (NH₄⁺) mineralized from SON (37-52%) and manure/sewage (M&S) (25-47%), while the contribution of CF was relatively small (14-25%). The contribution of various sources showed seasonal variations, with a greater contribution of CF in the wet growing season (March to August). Compared with the inlet which contributed 37-40% to runoff NO₃^-, SON contributed more at the outlet (49-52%). Denitrification in the runoff was small and appeared to be confined to the dry season (September to February), with an estimated NO₃^- loss of 2.73 kg N ha^-1. The net NO₃^- runoff loss of the watershed was 34.5 kg N ha^-1 yr^-1, accounting for 15% of the annual fertilization rate (229 kg N ha^-1 yr^-1). Besides M&S (22%), fertilization and remineralization of SON (CF + SON) were the main sources for the NO₃^- runoff loss (78%), suggesting accelerated nitrification of NH₄⁺ from CF (24%) and SON mineralization (54%). Our study indicates that NO₃^- runoff loss in subtropical agricultural watersheds is dominated by nonpoint source pollution from fertilization. SON played a more important role than CF. Besides, the contribution of sewage should not be neglected. Our data suggest that a combination of more rational fertilizer N application (CF), better management of SON, and better treatment of domestic sewage could alleviate NO₃^- pollution in subtropical China.