Effects of land use on the concentration and emission of nitrous oxide in nitrogen-enriched rivers

Nitrous oxide (N₂O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Nitrogen-enriched rivers are significant sources of atmospheric N₂O. This study conducted a one-year field campaign in seven N-enriched rivers draining urban, rural, and agricultural land to determine the link between the production, concentrations, and emissions of N₂O and land use. Estimated N₂O fluxes varied between 1.30 and 1164.38 μg N₂O-N m^-2 h^-1 with a mean value of 154.90 μg N₂O-N m^-2 h^-1, indicating that rivers were the net sources of atmospheric N₂O. Concentrations of N₂O ranged between 0.23 and 29.21 μg N₂O-N L^-1 with an overall mean value of 3.81 μg N₂O-N L^-1. Concentrations of ammonium and nitrate in urban and rural rivers were high in the cold season. The concentrations were also high in agricultural rivers in the wet season. N₂O concentrations and emissions in rural and urban rivers followed a similar pattern to ammonium and a similar pattern to nitrate in agricultural rivers. A strong link between the concentrations and emissions of N₂O and land use was observed. N₂O concentrations in and emissions from the rivers draining the urban and rural areas were significantly higher than the rivers draining the agricultural areas (P < 0.01). Stepwise regression analysis indicated that dissolved N₂O were primarily influenced by NH₄⁺ in agricultural rivers and by NO₃^- in rural rivers; while dissolved N₂O in urban rivers was primarily predicted by temperature and reflected the integrated impact of sewage input and river hydrology. Nitrate-N and NO^3--O isotope data and linear regression of N₂O and river water variables strongly indicated that dissolved N₂O was mainly derived from nitrification in agricultural rivers and denitrification in rural and urban rivers.