Nitrogen source track and associated isotopic dynamic characteristic in a complex ecosystem: A case study of a subtropical watershed, China.

By identifying the main sources of nitrate (NO) can obtain useful information to support the management of NO pollution, particularly in subtropical catchments with shallow drinking water wells. This study used water chemistry and dual stable isotopes δN and δO methods to assess seasonal and spatial variations of NO in precipitation, surface water, and groundwater in an agricultural and forest subtropical catchment in Jiangxi Province, China. The maximum concentrations of nitrate-nitrogen (NO-N) and ammonium-nitrogen (NH-N) were 10.4 and 10.8 mg Lin samples collected from 221 rainfall events from 2011 to 2013. About 4.4% and 12.3% NH-N concentrations of surface water and groundwater exceeded the thresholds of 1.0 and 0.2 mg L. The NO-N concentrations in surface water were closely correlated with NH-N concentrations in surface water and groundwater (r = -0.71 and r = -0.71, P < 0.05). The concentrations of NH-N and NO-N were significantly higher in a fishery pond and nearby drinking wells than in other monitoring points. Annual exports of NO-N and NH-N were 4.06 × 10 and 8.14 × 10 kg yr, respectively and NO-N is the main form of N loss. The δN values ranged from 0‰ to 20‰ in surface water and groundwater, and the δO values ranged from 0‰ to 15‰ and 1‰-13‰, respectively. Dual stable isotope natural abundance distribution and water chemistry [NO]/[Cl] molar ratio information suggested that manure and sewage and soil N were the main sources of NO in surface water and manure and sewage in groundwater in summer and winter. In spring, water occurred denitrification and ammonium fertilizer, manure and sewage were the main sources of NO in surface water and groundwater which sampling points were closer residential area and fish ponds than paddy field and local farmers used more Manure. Manure applications should be reasonable around drinking water wells to protect the drinking water quality.