From Soil to River: Revealing the Mechanisms Underlying the High Riverine Nitrate Levels in a Forest Dominated Catchment

Elevated riverine nitrate (NO 3 − ) levels have led to increased eutrophication and other ecological implications. While high riverine NO 3 − levels were generally ascribed to anthropogenic activities, high NO 3 − levels in some pristine or minimally disturbed rivers were reported. The drivers of these unexpectedly high NO 3 − levels remain unclear. This study combined natural abundance isotopes, 15 N-labeling techniques, and molecular techniques to reveal the processes driving the high NO 3 − levels in a sparsely populated forest river. The natural abundance isotopes revealed that the NO 3 − was mainly from soil sources and that NO 3 − removal processes were insignificant. The 15 N-labeling experiments also quantitatively showed that the biological NO 3 − removal processes, i.e., denitrification, dissimilatory NO 3 − reduction to ammonium (DNRA), and anaerobic ammonia oxidation (anammox), in the soils and sediments were weak relative to nitrification in summer. While nitrification was minor in winter, the NO 3 − removal was insignificant relative to the large NO 3 − stock in the catchment. Stepwise multiple regression analyses and structural equation models revealed that in summer, nitrification in the soils was regulated by the amoA-AOB gene abundances and NH 4 + -N contents. Low temperature constrained nitrification in winter. Denitrification was largely controlled by moisture content in both seasons, and anammox and DNRA could be explained by the competition with nitrification and denitrification on their substrate (nitrite-NO 2 − ). We also revealed the strong hydrological control on the transport of soil NO 3 − to the river. This study effectively revealed the mechanisms underlying the high NO 3 − levels in a nearly pristine river, which has implications for the understanding of riverine NO 3 − levels worldwide.