Inorganic carbon unexpected driver of carbon sink response in an established beaver wetland

Riparian zones are critical links between terrestrial and aquatic ecosystems, controlling the biogeochemical fluxes and thus the fate of carbon (C) in stream networks. However, long-standing anthropogenic modifications of waterways have resulted in significant losses of riparian connectivity. Following re-introduction of beavers across Europe, the resulting reconnection of riparian interfaces shows a high potential for improving water quality and C sequestration. Beaver dam construction gives rise to sequential shifts in lotic and lentic conditions that support high capacities for C deposition and increase the C produced by aquatic primary producers. However, due to inconsistent system boundaries and the overlooking of certain C pathways, our current understanding of C budget dynamics in beaver wetlands remains incomplete.

In this study, we quantified the annual C budget in an established beaver-impacted reach in Switzerland. Inputs and outputs of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) loads were modelled from biweekly water sampling and flow monitoring, in conjunction with measurements of gaseous C fluxes from soil, water and dead trees. Sediment storage of deposited C fractions was quantified in soil samples that were subsequently analysed with Rock-Eval pyrolysis. Biomass C storage was estimated at a plant species level by combining biomass surveys in field with multispectral imagery from drone remote sensing. Following hydrology and bathymetry measurements, the reach water balance was established by quantifying in- and outflow, wetland storage, subsurface storage and infiltration, and evapotranspiration.

We found large reductions in DIC loads along the reach, representing the main driver of the wetland's overall C sink response. The water balance partitioning further demonstrated that subsurface pathways were the primary sink of DIC, which was removed through transient and permanent storage, and deeper infiltration. Carbon dioxide (CO₂) mineralisation in non-inundated soils was the dominant source of C emissions from the system. However, the limited release of CO₂ from water surfaces showed that only a negligible fraction of DIC was released via this pathway. Instead, the annual accumulation of inorganic C in sediments suggests that DIC immobilisation in sediments, in conjunction with deeper infiltration, can be a significant C sink.

These results show that established, semi-confined beaver wetlands primarily regulate C dynamics via hydrological processes, overriding biogeochemistry and riparian feedbacks from primary productivity. It further stresses their high sensitivity to shifts in the C sink-source balance, and the importance of including inorganic C to elucidate their full impact on C sequestration in stream networks.