Nitrous Oxide Production in River Sediment of Highly Urbanized Area and the Effects of Water Quality

Abstract

Nitrous oxide (N2O) is one of the main greenhouse gases, which contributes to the global warming and ozone destruction. Sediment cores and river water were collected from an urban and a suburban river for N2O efflux measurement at the sediment-water interface and the factor-controlled incubations to investigating the N2O effluxes under varying conditions of dissolved oxygen (DO), ammonium (NH4 +), nitrate (NO3 −) and sulfate (SO4 2−). Vertical profiles of dissolved N2O concentrations in pore water were also acquired at eight depths. Results show that N2O effluxes were higher at the urban site (13.01 ± 6.51 μg N m−2 h−1) than the suburban site (4.02 ± 2.01 μg N m−2 h−1). Oxygen consumption rates were optimal under high DO and NH4 + amendment incubation, highlighting the strong nitrification potential in the sediment surface. Although N2O effluxes at the urban site increased with the NO3 − concentration under low DO condition, DO and NH4 + concentrations in overlying water were the principal factors controlling N2O effluxes (R = −0.415, p = 0.000 and R = 0.512, p < 0.05, respectively), indicating that nitrification greatly contributed to N2O production, especially at suburban river. Generally, N2O efflux was substantially higher in the severely polluted urban river, and the sediments were the source of N2O in river water.