Using dual isotopes to identify sources and transformations of nitrogen in water catchments with different land uses, Loess Plateau of China.

Nitrogen pollution in rivers is a research hotspot in the field of biogeochemistry. However, the types and sources of pollution have historically been poorly understood in the water catchments of the Loess Plateau in China. In this study, we have chosen the main waterway and four lesser branches of the Jinghe River that vary by land use. We investigated the concentrations and isotopic signatures of nitrogen in river water. Our results revealed that nitrate was the dominant nitrogen type in river catchments of the Loess Plateau. The δ(15)N and δ(18)O values showed that NO3 (-) ions in the studied river samples were derived from precipitation, manure, sewage, soil organic nitrogen, and synthetic NO3 (-) fertilizer. The δ(18)O-NO3 (-) values during July 2012 (mean ± SD = +18.1 ± 1.5‰) were higher than those during the September 2013 (mean ± SD = +7.8 ± 3.7‰), which indicated that mixing with atmospheric NO3 (-) resulted in the high δ(18)O values during July 2012. It appears that no intense nitrification and denitrification occurred in all five rivers according to the isotopic and chemical data. A Bayesian model was used to determine the contributions of four NO3 (-) sources to all five rivers. Results showed that source contributions differ significantly between July and September, and the four potential NO3 (-) sources also showed high variability between the different land use areas.