环境地球化学国家重点实验室知识库

Surface waters impacted by urbanization and anthropogenic activities, especially streams and rivers, can be a significant source of nitrous oxide (N₂O), but little is known about how the nitrogen enrichment in urban and rural rivers will increase the N₂O emission. Tianjin surface waters including urban and rural river network and lakes were sampled and analyzed for N loadings and N₂O concentrations. The study showed that eutrophied urban rivers in Tianjin were oversaturated with respect to N₂O with a saturation ranging from 252% to 3116% and acted as source of N₂O whereas rural rivers were generally undersaturated, with the a saturation ranging from 3% to 354% and acted as a sink of N₂O. Mean values of estimated N₂O fluxes in urban and rural rivers were 2.10 μmol m^− 2 d^− 1 and − 0.24 μmol m^− 2 d^− 1, respectively. Anthropogenic inorganic N load plays primary control on the distribution of N₂O saturation in heavily eutrophied rivers and estuaries, but N₂O emission did not increase as much as expected in response to high N inputs, especially in rural rivers. Whereas nitrification might be the main process responsible for N₂O production in urban lakes and rivers in Tianjin, both nitrification and denitrification could contribute to N₂O production in nearby estuaries. In comparison, denitrification is a N₂O sink in rural rivers of Tianjin. The annual emission from the river network of Tianjin was estimated to be 5.78 × 10³ kg N-N₂O, a moderate emission when compared with worldwide rivers, but N₂O emission in urban rivers and lakes indicated that globally aquatic N₂O emission rate and fluxes were probably underestimated in the 2007 IPCC report. More efforts should be paid to globally quantified N₂O in eutrophied urban rivers for better assessment of N₂O emission in riverine systems.