Deciphering the effects of hydrological, biogeochemical and anthropogenic factors on nitrogen species in a subtropical watershed using multiple isotopes

The sources and transformations of various nitrogen (N) species in rivers under the influence of anthropogenic inputs are the main concern of eutrophication research, especially in intermediate- and small-sized rivers, and are essential to understand for practical management. A key challenge lies in determining how N species can be altered by diverse biogeochemical processes along flow paths under different hydrological controls (e.g. river discharge). To better understand the response of in-stream N dynamics, we examined N transformation processes and identified the sources of various N species in the Jiulong River (JLR), a eutrophic river in China composed of two main tributaries: the North Stream (NS) and the West Stream (WS), using multiple isotopes (δ15NNO3, δ18ONO3, δ15NNO2, δ15NNH4, δ15NPN, δDH2O and δ18OH2O). Our results reveal that hydrological and biogeochemical processes exert a joint control on river N dynamics. Key processes, such as nitrification and ammonium (NH4+) uptake, predominated in both tributaries during non-rainfall periods, while during rain events, nitrification remained significant. However, denitrification occurred differently in the NS (mainly on particles) and the WS (primarily in sediment), and was influenced by factors such as sediment characteristics and water flow patterns. We also observed differences in the uptake velocity of denitrification and NH4+ between the two tributaries, highlighting the variable importance of these two processes. The main sources of N species varied based on hydrological conditions, with different contributions in the NS and WS. During non-rainfall periods, point sources significantly controlled N species compositions in both tributaries, with manure & sewage contributing 46 % and 42 % of the NH4+, 30 % and 50 % of the nitrate (NO3−), and sewage accounting for 62 % and 56 % of the particulate N (PN) in the NS and WS, respectively. During rainstorms, non-point sources became the predominant contributors of NO3− in both tributaries (soil organic N: NS = 48 %, WS = 38 %; chemical fertilizers: NS = 29 %, WS = 26 %) and PN (soil: NS = 52 %, WS = 57 %). The insights revealed using multiple isotopes serve as a foundational basis for advanced modeling and scenario development when managing subtropical rivers under complex human interferences and climate change.