Nitrogen pollution and sources in an aquatic system at an agricultural coastal area of Eastern China based on a dual-isotope approach.

Nitrogen (N) pollution of water courses is a major concern in most coastal watersheds in eastern China with intensive agricultural production. We use hydrogeological and dual-isotopic approaches to analyze the N concentrations, pollution, transformations, and sources of surface water and groundwater in an agricultural watershed of the Jiaozhou Bay (JZB) area. Results showed that dissolved total N (DTN) concentrations in sub-rivers (SRs) ranged from 6.0 to 25.3 mg N L in the dry season and 9.1-26.7 mg N L in the wet season, which indicated a positive relationship with the percentages of agricultural land. Meanwhile, the dominant dissolved N species in SRs changed from nitrate (NO, 64-100%) to dissolved organic N (DON, 52-77%) from the dry season to the wet season and the increased DON concentrations showed a positive relationship with the planted proportions of vegetable production systems. The NO concentrations of groundwaters ranged from 10.6 to 121.4 mg N L, which were over the limit for drinking water by the World Health Organization. Isotopic analysis indicated that most NO originated from the microbiological conversion via nitrification, whereas the deletion of denitrification was insignificant in this area. The results of the stable isotope analysis in R mixing model showed the contributions of potential NO sources which were in order of manure fertilizers (20.6-69.0%) > soil organic matter (19.5-53.2%) > chemical fertilizers (5.5-34.3%) > atmospheric deposition (1.3-18.8%). This study suggests that the management of crop productions and reasonable manure fertilizer application should be implemented to protect the quality of aquatic systems in the JZB area.