Nitrous Oxide (N₂O) Emissions Decrease Significantly under Stronger Light Irradiance in Riverine Water Columns with...

Nitrous oxide (N₂O) emissions from riverine water columns with suspended particles are important for the global N₂O budget. Although sunlight is known to influence the activity of nitrogen-cycling microorganisms, its specific influence on N₂O emissions in river systems remains unknown. This study analyzed the influences of light irradiance on N₂O emissions in simulated oxic water columns with ¹⁵N-labeling and biological molecular techniques. Our results showed that N₂O emissions were inhibited by light in the ammonium system (only ¹⁵NH₄⁺ was added) and significantly decreased with increasing light irradiance in the nitrate system (only ¹⁵NO₃^- was added), despite contrasting variations in N₂ emissions between these two systems. Lower N₂O emission rates in the nitrate system under higher light conditions resulted from higher promotion levels of N₂O reduction than N₂O production. Increased N₂O reduction was correlated to higher organic carbon bioavailability caused by photodegradation and greater potential for complete denitrification. Lower N₂O production and higher N₂O reduction were responsible for the lower N₂O emissions observed in the ammonium system under light conditions. Our findings highlight the importance of sunlight in regulating N₂O dynamics in riverine water columns, which should be considered in developing large-scale models for N₂O processing and emissions in rivers.