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.