Anthropogenic production of reactive nitrogen (N) is increasing rapidly due to the growing demand for food production. Rivers are the receptors of N, especially nitrate (NO₃^–), produced in their drainage catchments, therefore, quantifying catchment-scale NO₃^– sources and transformations is vital for understanding the global biogeochemical cycles of N and for remediating river NO₃^– pollution.

Historically, natural abundance isotopic compositions of NO₃^– (δ¹⁵N/δ¹⁸O-NO₃^–) in a river have been used to reveal catchment-scale NO₃^– sources and removal, and molecular techniques and ¹⁵N pairing experiments can quantify NO₃^– related processes and their regulators in microenvironments. However, there is a long-standing gap between these techniques because they focus on different aspects of a catchment.

Dr. JIANG Hao, Prof. ZHANG Quanfa, and their colleagues from the Wuhan Botanical Garden of the Chinese Academy of Sciences proposed a novel protocol that comprehensively applies natural abundance isotope tracing, ¹⁵N pairing and molecular techniques to investigate the NO₃^– cycling processes and the regulating mechanisms at catchment scales.

By applying the protocol in two catchments on the Qinghai-Tibet Plateau representing varying environmental conditions, the researchers explicitly described the NO₃^– production and removal processes and their abiotic and biotic driving factors in the catchments. In addition, the spatial variations in the NO₃^– yield rates and fluvial NO₃^– export rates were well explained.

The results successfully demonstrated the effectiveness of the protocol in revealing catchment-scale NO₃^– yield and fluvial NO₃^– export dynamics.

This study, published in Science of the Total Environment, was supported by the National Natural Science Foundation of China.

Coupling geochemical and molecular techniques for catchment-scale NO₃^– dynamics. a. Natural abundance isotopes of river waters contain composite information of multiple NO₃^– sources and various NO₃^– cycling processes at the catchment scale; b. Coupling potentials of natural abundance isotopes, microbial molecular techniques, and ¹⁵N pairing techniques for revealing catchment-scale NO₃^– dynamics. (Image by WBG)