Elevated in-stream CO2 concentration stimulates net-N2O production from global fluvial ecosystems.

Riverine CO2 and N2O concentrations have risen since pre-industrial times, yet their biogeochemical interactions remain unclear. This study found a positive correlation between N2O and CO2 saturation in fluvial ecosystems, which was absent at high DOC:NO3 ratios. Low DOC:NO3 ratios and high CO2 saturation promote chemoautotrophic nitrification, suggesting its key role in riverine net N2O production. In vitro experiments confirmed that elevated CO2 enhances nitrification rates, nitrifier gene abundance, and N2O fluxes, indicating that the CO2 fertilization effect on N2O production is a potential climate feedback. Under this effect, current global N2O emissions may be underestimated by 12 % (interquartile range: 8 - 15) due to unaccounted nocturnal CO2 increases. As land use change projections suggest the conversion of natural lands into croplands and urban areas, this CO2-driven rise in riverine N2O emissions could increase, amplifying the global impacts of land use on riverine greenhouse gas emissions.