Beavers can convert stream corridors to persistent carbon sinks

Recent reintroductions of the Eurasian beaver (Castor fiber) across Europe represents an ecological shift with potential implications for carbon cycling in stream corridors. However, the capacity of beaver impacts to influence short- and long-term carbon fluxes, and the mechanisms that govern these changes, remains poorly understood. We present a comprehensive carbon budget of a beaver-influenced stream corridor, covering all major aqueous and atmospheric exchanges, as well as biomass and sedimentary carbon storages, from a beaver wetland in Switzerland. By integrating carbon flux measurements with hydrology and bathymetry, we identify dominant pathways and dynamics in gaseous carbon emissions. Annually, the beaver wetland was a net carbon sink (98.3 ± 34.4 t yr^-1), driven by subsurface removal of dissolved inorganic carbon. Carbon dioxide emissions were the dominant source of carbon loss, seasonally shifting the system to a net carbon source during summer water recession. Projecting the long-term sediment and deadwood storage following wetland infilling, our upper estimate of sequestered carbon was 1194 t (10.1 t ha-1 yr^-1), nearly an order of magnitude greater than without beaver modification. Our findings demonstrate that beaver-induced hydrological change is fundamentally reshaping carbon cycling and reinforces the relevance of headwater catchments in climate change mitigation strategies.