Pollution abatement reducing the river N2O emissions although it is partially offset by a warming climate: Insights from an urbanized watershed study

Abstract

Global nitrogen (N) pollution has resulted in increased river nitrous oxide (N2O) emissions, which contribute to climate change. However, little is known about how pollution abatement conversely reduces river N2O production in a warming climate. Here, field observations and microcosmic experiments were conducted in a coastal urbanized watershed (S.E. China) to explore the interactive effect of changing nitrate and temperature on river sediment denitrification (DNF) and N2O production. The results showed that urban river reaches (UR) with higher organic carbon content and denitrifying gene abundance in sediments have a greater DNF rate, nitrate removal efficiency (NRE), and N2O concentration than agricultural river reaches (AR). Microcosmic incubation suggested that the DNF rate and associated N2O production decreased under low nitrate addition, wherein the NRE increased. The scenario simulation illustrated a nonlinear response of N2O production to nitrate removal (i.e., ΔN2O/ΔNO3-N) from both UR and AR sediments at a given temperature, and the DNF rate and N2O production increased with increasing temperature. An increase in temperature by 1 degree Celsius would offset 18.75% of the N2O reduction by nitrate removal via DNF. These findings implied that watershed pollution abatement undoubtedly contributes to the reduction in global river N2O emissions although it is partially offset by extra N2O production caused by global warming.