Point source from domestic sewage and nonpoint source from residual nutrients in soil: The dual contributions of nitrogen and phosphorus to river pollution in intensive agricultural watersheds

Accurately identifying nitrogen (N) and phosphorus (P) pollution sources is key to managing water pollution in intensive agricultural watersheds, but the difficulty of tracing nonpoint source pollutants in watersheds is increased due to the combined effect of multiple sources. The typical intensive agricultural watershed of Xingyun Lake was chosen as an example, collected multiple pollution sources in the watershed, multiple isotopes (δ15N-NO3, δ18O-NO3, δ15N-NH4 and δ18O-PO4) and analysis methods (SIAR model, probability statistical analysis and perturbation method) were employed to quantify source contributions to nitrate (NO3-), ammonium (NH4+) and phosphate (PO43-) in the main rivers entering the lake under seasonal changes, and source apportionment accuracy uncertainty and sensitivity were evaluated. The results revealed that domestic sewage (CE) was the main N source for NO3- (mean proportion: 31 ± 9%) and NH4+ (28 ± 7%) in rivers during the rainy season (RS) and dry season (DS), while PO43- originated from P fertilizer (24 ± 9%), livestock manure (22 ± 6%), and soil P (28 ± 6%), CE (26 ± 8%). The nitrification of NH4+ during the DS sewage provided a "new" NO3- source for the watershed. The CE and soil N and P contributions to NO3-, NH4+ and PO43- in rivers exhibited high uncertainty, especially during the DS (mean UI90 value of CE: 0.60). Sensitivity analysis showed that the relative changes in the contributions of CE to NO3-, NH4+ and PO43- in river water were largest. The results provide new insights for precise N and P pollution control in intensive agricultural watersheds.