Nitrous Oxide (N 2 O) Emissions Decrease Significantly under Stronger Light Irradiance in Riverine Water Columns with Suspended Particles.

Nitrous oxide (N 2 O) emissions from riverine water columns with suspended particles are important for the global N 2 O budget. Although sunlight is known to influence the activity of nitrogen-cycling microorganisms, its specific influence on N 2 O emissions in river systems remains unknown. This study analyzed the influences of light irradiance on N 2 O emissions in simulated oxic water columns with 15 N-labeling and biological molecular techniques. Our results showed that N 2 O emissions were inhibited by light in the ammonium system (only 15 NH 4 + was added) and significantly decreased with increasing light irradiance in the nitrate system (only 15 NO 3 - was added), despite contrasting variations in N 2 emissions between these two systems. Lower N 2 O emission rates in the nitrate system under higher light conditions resulted from higher promotion levels of N 2 O reduction than N 2 O production. Increased N 2 O reduction was correlated to higher organic carbon bioavailability caused by photodegradation and greater potential for complete denitrification. Lower N 2 O production and higher N 2 O reduction were responsible for the lower N 2 O emissions observed in the ammonium system under light conditions. Our findings highlight the importance of sunlight in regulating N 2 O dynamics in riverine water columns, which should be considered in developing large-scale models for N 2 O processing and emissions in rivers.