The excess nitrate (NO₃^- ) has lea to eutrophication and cascading environmental implications, which are now a great threat to aquatic ecosystems worldwide. Although high riverine NO₃^- levels in rivers have commonly been attributed to anthropogenic activities, there have been evidences of high NO₃^-  concentrations observed in certain unspoiled or minimally disturbed rivers. Understanding drivers of these unexpectedly high NO₃^-  levels is imperative for protecting river water resources and ecosystems.

Supervised by Prof. ZHANG Quanfa and Associate Prof. JIANG Hao from Wuhan Botanical Garden, LI Shen conducted a study integrating natural-abundance isotopic, ¹⁵N pairing and molecular techniques(qPCR) to determine the processes regulating the high NO₃^-  levels and their driving mechanisms in minimally disturbed rivers.

The natural abundance isotopes indicated that soil sources were the major NO₃^- sources, and the processes responsible for NO₃^-  removal were negligible. The ¹⁵N labeling experiments additionally demonstrated that biological removal processes for NO₃^-  in the soils and sediments were relatively weak compared to nitrification during the summer season. In contrast, nitrification was less significant during the winter season, and NO₃^-  removal was insignificant due to the large quantities of NO₃^-  present in the catchment.

Stepwise multiple regression analyses and structural equation models provided further insights, revealing that the nitrification rates were significantly regulated by amoA-AOB gene and NH₄⁺-N contents in summer. In winter, the freezing temperatures limited microbial nitrification. Furthermore, soil denitrification rates in both summer and winter were predominantly controlled by moisture content, while anammox and dissimilatory NO₃^-  reduction to ammonium (DNRA) processes were influenced by the competition with nitrification and denitrification for their substrate (nitrite-NO₂^-).

These findings are important for understanding the high NO₃^-  levels in pristine or minimally distributed rivers worldwide. In addition, this study shows that coupling the isotopic and molecular techniques can effectively link catchment-scale biogeochemistry with microscopic processes, thus generating an integrated picture of NO₃^-  processes at a catchment scale.

This work was supported by the National Natural Science Foundation of China. The relevant research results have been published in Water Research entitled “From Soil to River: Revealing the Mechanisms Underlying the High Riverine Nitrate Levels in a Forest Dominated Catchment”.

 Integrating natural-abundance isotopic, ¹⁵N pairing, and microbial molecular techniques for revealing the processes driving of the high NO₃^-  levels in a sparsely populated forest river (Image by WBG)