Assessment of the sources and transformations of nitrogen in a plain river network region using a stable isotope approach

摘要

The great spatial and temporal variability in hydrological conditions and nitrogen(N)processing introduces large uncertainties to the identification of N sources and quantifying N cycles in plain river network regions. By combining isotopic data with chemical and hydrologic measurements, we determined the relative importance of N sources and biogeochemical N processes in the Taige River in the East Plain Region of China. The river was polluted more seriously by anthropogenic inputs in winter than in summer. Manure and urban sewage effluent were the main nitrate(NO-3) sources, with the nitrification of N-containing organic materials serving as another important source of NO-3. In the downstream, with minor variations in hydrological conditions, nitrification played a more important role than assimilation for the decreasing ammonium(NH+4-N) concentrations.The N isotopic enrichment factors(ε) during NH+4utilization ranged from- 13.88‰ in March to- 29.00‰ in July. The ratio of the increase in δ18O and δ15N of river NO-3in the downstream was 1.04 in January and 0.92 in March. This ratio indicated that NO-3assimilation by phytoplankton was responsible for the increasing δ15N and δ18O values of NO-3in winter. The relationships between δ15N of particulate organic nitrogen and isotopic compositions of dissolved inorganic nitrogen indicated that the phytoplankton in the Taige River probably utilized NH+4preferentially and mainly in summer, while in winter, NO-3assimilation by phytoplankton was dominant.