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

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 (delta N-15-NO3-, delta O-18-NO3-), ammonium (delta N-15-NH4+) and suspended particulate nitrogen (delta N-15-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 delta N-15 values ( > +5 parts per thousand). Secondly, in the dry season, a significant correlation between the delta N-15-NO3- and delta N-15-NH4+ values, an increasing trend of delta N-15-NO3- with NO3--N concentration, and lower delta O-18-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 delta N-15-NO3- and delta O-18-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 delta N-15-PN was significantly correlated with delta N-15-NH4+ rather than delta N-15-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.