Sources and fate of nitrate in the Haicheng River basin in Northeast China using stable isotopes of nitrate

Abstract

Due to the serious nitrate pollution in Northeast China, the in-stream nitrate and its isotopic composition (δ15N-NO3 and δ18O-NO3) were analyzed to explore the seasonal nitrate sources and its transformation processes combining with chemical indicators in the Haicheng River during the spring, early summer, late summer, and autumn seasons throughout the year of 2010. The nitrate nitrogen (NO3-N) concentrations in riverine water co-varied with the discharge data of the river, showing significant seasonal variations (p<0.001) based on the one-way ANOVA. The riverine δ15N-NO3 values and chloride (Cl−) variations suggested that NO3-N was mainly contributed by sewage/manure in spring. The NO3-N sources in early summer were largely induced by nitrification process from synthetic fertilizers. The combined application of δ15N-NO3 and δ18O-NO3 explained that the NO3-N sources in late summer and autumn were derived largely from soil organic matter and sewage/manure. Denitrification process of soil organic matter and sewage/manure in autumn and their nitrification process in late summer mostly controlled the NO3-N concentrations in river water. The results will provide useful information on nitrate sources of seasonal variations in isotope compositions of river systems for controlling NO3-N concentrations and improving the ecological quality near the Liaodong Bay and Bohai Sea in the near future.