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

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.