As the largest biogeochemically active terrestrial reserve of carbon (C), soils have the potential to either mitigate or amplify rates of climate change. Ecosystems with large C stocks and high rates of soil C sequestration, in particular, may have outsized impacts on regional and global C cycles. Montane meadows have large soil C stocks relative to surrounding ecosystems. However, anthropogenic disturbances in many meadows may have altered the balance of C inputs and outputs, potentially converting these soils from net C sinks to net sources of C to the atmosphere. Here, we quantified ecosystem-level C inputs and outputs to estimate the annual net soil C flux from 13 montane meadows spanning a range of conditions throughout the California Sierra Nevada. Our results suggest that meadow soils can be either large net C sinks (577.6 ± 250.5 g C m−2 y−1) or sources of C to the atmosphere (− 391.6 ± 154.2 g C m−2 y−1). Variation in the direction and magnitude of net soil C flux appears to be driven by belowground C inputs. Vegetation species and functional group composition were not associated with the direction of net C flux, but climate and watershed characteristics were. Our results demonstrate that, per unit area, montane meadows hold a greater potential for C sequestration than the surrounding forest. However, legacies of disturbance have converted some meadows to strong net C sources. Accurate quantification of ecosystem-level C fluxes is critical for the development of regional C budgets and achieving global emissions goals.