Nitrous oxide emissions from river network with variable nitrogen loading in Tianjin, China

Abstract

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 2.10μmolm−2d−1 and −0.24μmolm−2d−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 5.78×103kg 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.