Mechanisms of nitrate accumulation in highly urbanized rivers: evidence from multi-isotopes in the Pearl River Delta, China

Abstract Nitrate accumulation due to anthropogenic input in highly urbanized rivers is a general recognition. The excess anthropogenic nitrogen (N) can alter N transformations and would further accelerate nitrate accumulation. However, there is lack of direct evidence to reveal the mechanisms of nitrate accumulation in the urbanized river. In this study, multi isotopes of nitrate (δ15N-NO3−, δ18O-NO3−), ammonium (δ15N-NH4+) and suspended particulate nitrogen (δ15N-PN) were obtained to investigate mechanisms of nitrate accumulation in typical highly urbanization rivers in the Pearl River Delta (PRD). Firstly, source apportionment revealed sewage was the dominant source of nitrate and ammonium, evidenced by higher δ15N values (>+5‰). Secondly, in the dry season, a significant correlation between the δ15N-NO3− and δ15N-NH4+ values, an increasing trend of δ15N-NO3− with NO3−-N concentration, and lower δ18O-NO3− values provided isotopic evidence of nitrate accumulation that NO3− was increased by enhanced nitrification of sewage effluent NH4+. Thirdly, in the wet season, the relationships between the δ15N-NO3− and δ18O-NO3−, and lower isotope enrichment factor of denitrification indicated that denitrification in soils or riparian zones was weakened due to urbanization, which increased NO3− fluxes to the river water. Fourthly, the δ15N-PN was significantly correlated with δ15N-NH4+ rather than δ15N-NO3−, revealing the preference of NH4+ uptake by phytoplankton. The NH4+ preferred assimilation repressed NO3− removal and consequently result in nitrate accumulation. This study provided direct lines of evidence by multi isotopes to understand the mechanisms of nitrate accumulation in the urbanized rivers, which deepens our understanding of roles of isotopes in the study of N biogeochemical processes and has important implication for regional and global N cycling.