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

Abstract

By identifying the main sources of nitrate (NO₃^-) can obtain useful information to support the management of NO₃^- pollution, particularly in subtropical catchments with shallow drinking water wells. This study used water chemistry and dual stable isotopes δ¹⁵N and δ¹⁸O methods to assess seasonal and spatial variations of NO₃^- in precipitation, surface water, and groundwater in an agricultural and forest subtropical catchment in Jiangxi Province, China. The maximum concentrations of nitrate-nitrogen (NO₃^--N) and ammonium-nitrogen (NH₄⁺-N) were 10.4 and 10.8 mg L^-1in samples collected from 221 rainfall events from 2011 to 2013. About 4.4% and 12.3% NH₄⁺-N concentrations of surface water and groundwater exceeded the thresholds of 1.0 and 0.2 mg L^-1. The NO₃^--N concentrations in surface water were closely correlated with NH₄⁺-N concentrations in surface water and groundwater (r = -0.71 and r = -0.71, P < 0.05). The concentrations of NH₄⁺-N and NO₃^--N were significantly higher in a fishery pond and nearby drinking wells than in other monitoring points. Annual exports of NO₃^--N and NH₄⁺-N were 4.06 × 10⁴ and 8.14 × 10³ kg yr^-1, respectively and NO₃^--N is the main form of N loss. The δ¹⁵N values ranged from 0‰ to 20‰ in surface water and groundwater, and the δ¹⁸O values ranged from 0‰ to 15‰ and 1‰-13‰, respectively. Dual stable isotope natural abundance distribution and water chemistry [NO₃^-]/[Cl^-] molar ratio information suggested that manure and sewage and soil N were the main sources of NO₃^- 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 NO₃^- 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.

Keywords: Ammonium; Nitrate; Subtropical watershed; δ(15)N; δ(18)O.