Beavers can convert stream corridors to persistent carbon sinks

Stream corridors are widely recognised as hotspots for carbon (C) cycling, where terrestrial and aquatic processes interact to transform, store and transport C. The recent reintroduction of the Eurasian beaver ( Castor fiber ) across Europe represents a significant ecological shift with potential implications for C dynamics in headwater systems. However, the capacity of beaver-modified stream corridors to influence short- and long-term C turnover, and the mechanisms that govern these changes, remains poorly understood. Here, we present the first annual C budget of a beaver-influenced stream corridor that through coupling with the water balance covers all major aqueous and atmospheric exchanges, as well as biomass and sedimentary storages of C, from a lowland beaver wetland in Switzerland. By integrating C flux measurements with hydrology and bathymetry, we identify dominant transport pathways and quantify how intermittent inundation mediates gaseous C emissions. Additional estimates of sediment deposition and pyrolysis/oxidation analysis allowed us to quantify sediment C fractions and their long­term fate. Annually, the beaver wetland functioned as a net C sink (77.7 ± 33.4 t yr ^-1 ), primarily driven by subsurface removal of dissolved inorganic C. Carbon dioxide emissions were the dominant source of C loss and seasonally shifted the system to a net C source during the summer water recession. Projecting the long-term evolution of sediment and deadwood storage following complete wetland infilling, we estimate a net sequestered C of 1194 t (10.1 t ha ^-1 yr ^-1 ), nearly an order of magnitude greater than the same stream corridor without beaver modification. Our findings demonstrate that temperate beaver wetlands can act as persistent C sinks, driven annually through hydrologically controlled reductions in dissolved inorganic C export, and over the longer-term via burial of predominantly organic, but also inorganic, C. This highlights the transformative role of beaver-induced hydrological change in reshaping C cycling dynamics at terrestrial-aquatic interfaces, and reinforces the relevance of headwater catchments in climate-mitigation strategies.