科研之友
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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 (NO3-) sources, with the nitrification of N-containing organic materials serving as another important source of NO3- In the downstream, with minor variations in hydrological conditions, nitrification played a more important role than assimilation for the decreasing ammonium (NH4+-N) concentrations. The N isotopic enrichment factors (epsilon) during NH4+ utilization ranged from -13.88%. in March to -29.00.parts per thousand in July. The ratio of the increase in delta O-18 and delta N-15 of river NO3- in the downstream was 1.04 in January and 0.92 in March. This ratio indicated that NO3- assimilation by phytoplankton was responsible for the increasing delta N-15 and delta O-18 values of NO3- in winter. The relationships between delta N-15 of particulate organic nitrogen and isotopic compositions of dissolved inorganic nitrogen indicated that the phytoplankton in the Taige River probably utilized NH4+ a preferentially and mainly in summer, while in winter, NO3- assimilation by phytoplankton was dominant.
