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

Nitrate (NO) loss and enrichment in water bodies caused by fertilization are a major environmental problem in agricultural areas. However, the quantitative contribution of different NO sources, especially chemical fertilizers (CF) and soil organic nitrogen (SON), to NO runoff loss remains unclear. In this study, a systematic investigation of NO 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 NO isotope ( N and O) approaches were combined to estimate the NO 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 NO in runoff was mainly derived from nitrification of ammonium (NH) 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 NO, 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 NO loss of 2.73 kg N ha. The net NO runoff loss of the watershed was 34.5 kg N ha yr, accounting for 15% of the annual fertilization rate (229 kg N ha yr). Besides M&S (22%), fertilization and remineralization of SON (CF + SON) were the main sources for the NO runoff loss (78%), suggesting accelerated nitrification of NH from CF (24%) and SON mineralization (54%). Our study indicates that NO 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 NO pollution in subtropical China.