Anthropogenic production of reactive nitrogen (N) has been rapidly increasing due to the growing demand for food production. Rivers are the receptors of N, particularly 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 remediating river NO₃^– pollution.

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

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

Appling the protocol in two catchments on the Qinghai-Tibet Plateau representing varying environment conditions, 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 work was supported by the National Natural Science Foundation of China. The research has been published in Science of the Total Environment entitled “Coupling geochemical and microbial molecular techniques to reveal catchment-scale nitrate yield and fluvial export dynamics” (Available online 8 May 2023).

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. ¹⁵N pairing and molecular techniques (e.g., qPCR, high-throughput sequencing, and metagenomic sequencing) can reveal the co-occurring NO₃^– -related transformation processes (e.g., nitrification, denitrification, anammox, and DNRA) and their driving mechanisms from a microscopic perspective. (b) Coupling potentials of natural abundance isotopes, microbial molecular techniques, and ¹⁵N pairing techniques for revealing catchment-scale NO₃^– dynamics. (Image by WBG)