Nitrous Oxide (NO) Emissions Decrease Significantly under Stronger Light Irradiance in Riverine Water Columns with Suspended Particles.

Nitrous oxide (NO) emissions from riverine water columns with suspended particles are important for the global NO budget. Although sunlight is known to influence the activity of nitrogen-cycling microorganisms, its specific influence on NO emissions in river systems remains unknown. This study analyzed the influences of light irradiance on NO emissions in simulated oxic water columns with N-labeling and biological molecular techniques. Our results showed that NO emissions were inhibited by light in the ammonium system (only NH was added) and significantly decreased with increasing light irradiance in the nitrate system (only NO was added), despite contrasting variations in N emissions between these two systems. Lower NO emission rates in the nitrate system under higher light conditions resulted from higher promotion levels of NO reduction than NO production. Increased NO reduction was correlated to higher organic carbon bioavailability caused by photodegradation and greater potential for complete denitrification. Lower NO production and higher NO reduction were responsible for the lower NO emissions observed in the ammonium system under light conditions. Our findings highlight the importance of sunlight in regulating NO dynamics in riverine water columns, which should be considered in developing large-scale models for NO processing and emissions in rivers.