The controlling mechanism of nitrogen dynamics across a large river basin

Investigating the dynamics and distribution of nitrogen (N) in river networks is essential for environmental management and pollution control. However, the controlling mechanisms of N dynamics across large watersheds are not well understood. In this study, we examined N concentration and stable isotopes (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in river water and groundwater through field sampling from 284 sites across the Pearl River Basin, China. Preliminary results show that nitrate (NO₃^-) is the primary form of riverine dissolved inorganic nitrogen (DIN), and NO₃^- concentration is three times higher in the groundwater than in river water (mean of  330.5 ± 480.1 μmol/L v.s. 93.2 ± 65 μmol/L). The signature of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- indicates that riverine nitrogen is primarily fromsoil organic N. The δ¹⁸O-NO₃^- values ranged from 2.76‰ to 7.52‰, indicating that nitrification is the dominant process in the N cycle of river water across the basin. Denitrification is not apparent in the water column because δ¹⁵N-NO₃^- does not show a negative correlation with NO₃^- concentration. We find that the source region has the highest NO₃^- concentration (187.1 ± 16 μmol/L) in river waters. The high cropland proportion (36.5% ± 5%) leads to higher soil N accumulation due to fertilization, and the highest oxidation-reduction potential (222.3 ± 7 mV) indicates the strongest oxidation environment for nitrification. As the nitrification process produces H⁺, which can consume carbonate and increase dissolved inorganic carbon (DIC), the highest DIC concentration (3139.3 ± 777.5 μmol/L) further proves the most robust nitrification process in the source regions. In conclusion, nitrification can control N dynamics and dominate NO₃^- distribution in river water in large watersheds.