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

Xing, Meng; Liu, Weiguo

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

2016

Xing, Meng | Liu, Weiguo

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 δ¹⁵N and δ¹⁸O values showed that NO₃ ⁻ ions in the studied river samples were derived from precipitation, manure, sewage, soil organic nitrogen, and synthetic NO₃ ⁻ fertilizer. The δ¹⁸O-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 δ¹⁸O 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.

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