Tracking nitrogen pollution sources in plain watersheds by combining high-frequency water quality monitoring with tracing dual nitrate isotopes

High-frequency water quality measurements at daily resolutions combined with tracing of nitrate δ15N and δ18O isotopes was proposed as a method to quantify nitrogen (N) export from point and nonpoint sources, which is usually difficult to achieve at a watershed scale. The method was applied to two agricultural watersheds located in two nearby plains along the Xifeihe and Nihe Rivers in China. The two watersheds had different crops and planting patterns, in addition to their different urbanization percentages of 25.3% and 14.1%, respectively. Analysis showed that these two watersheds exported approximately 7.8 and 11.2 kg N ha−1y−1 to their receiving rivers. Isotope analysis identified that point sources of sewage/manure contributed majority (53%) of nitrate load to the Xifeihe River, whereas nonpoint sources from soils, fertilizer and atmospheric deposition contributed majority (60%) of nitrate load to the Nihe River. Crop patterns reduced the N export through soybean planting or denitrification in rice paddies despite 200–300 kg N ha−1 fertilizer application to the fields per year. Annual nitrate export patterns of the two watersheds did not show correlations with their corresponding daily flow rates, which had important implications to watershed monitoring design. Nitrogen (N) concentrations were neither chemeostatic nor discharge-correlated except for during high flows in the soybean-wheat agriculture dominant Xifeihe watershed. This indicated that the traditional monitoring protocol for pollution source identification with measurements only conducted during few rainfall events was likely to be insufficient to estimate the total loads of nutrients (e.g., N) from watersheds.