Pollution abatement reducing the river NO emissions although it is partially offset by a warming climate: Insights from an urbanized watershed study.

Global nitrogen (N) pollution has resulted in increased river nitrous oxide (NO) emissions, which contribute to climate change. However, little is known about how pollution abatement conversely reduces river NO 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 NO 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 NO concentration than agricultural river reaches (AR). Microcosmic incubation suggested that the DNF rate and associated NO production decreased under low nitrate addition, wherein the NRE increased. The scenario simulation illustrated a nonlinear response of NO production to nitrate removal (i.e., ΔNO/ΔNO-N) from both UR and AR sediments at a given temperature, and the DNF rate and NO production increased with increasing temperature. An increase in temperature by 1 degree Celsius would offset 18.75% of the NO reduction by nitrate removal via DNF. These findings implied that watershed pollution abatement undoubtedly contributes to the reduction in global river NO emissions although it is partially offset by extra NO production caused by global warming.