High-elevation wet meadows are hot spots of soil carbon (C) sequestration that contribute disproportionately to regional C cycles, but are also highly sensitive to disturbance. Up to 70% of Sierra Nevada meadows in California and Nevada have been degraded as a result of anthropogenic activities, leading to decreased primary productivity, disconnected floodplain hydrology, and aerobic soil conditions that may convert meadows from net C sinks to net C sources to the atmosphere. High spatial and temporal variability in depth to groundwater also lead to complex biogeochemical processes that do not follow neat paradigms of either upland or wetland ecosystems.
As interest in voluntary and compliance C markets increases, and widespread tree mortality highlights the vulnerability of aboveground C stocks, the need to better quantify belowground C sequestration is apparent. Here, we quantify ecosystem-level C inputs and outputs to construct net C budgets from 13 meadows across the Sierra Nevada range. We combined high resolution field measurements with a 13C pulse-chase experiment, allowing us to determine the magnitude and direction of annual C fluxes from meadows ranging from high functioning to severely degraded. We also measured watershed and meadow characteristics to determine which biotic and abiotic factors were most strongly correlated with differences in soil C loss from a range of degraded meadows.
Results/Conclusions
The three highest functioning meadows we examined sequestered large amounts of soil C, with rates ranging from 9.5 to 13.5 Mg C ha-1 yr-1 - equivalent to the amount sequestered by 16.6 to 23.6 ha of temperate forest in one year. The ten more degraded meadows were either C neutral or net sources of C to the atmosphere (0.25 to -4.6 Mg C ha-1 yr-1). When normalized for soil C stocks, annual C fluxes ranged from gains of 7.9% to losses of 4.9% per year. Generalized linear models validated by path analysis revealed that elevation, % wetland plant species, % bareground in the meadow, and % forest in the surrounding upland explained the greatest variation in C losses from the 10 more degraded meadows (deviance explained > 0.90). These results demonstrate the potential of high-elevation wet meadows to sequester large amounts of C from the atmosphere. They also demonstrate that, in their current condition, most meadows in the Sierra Nevada represent a previously unquantified C source to the atmosphere - but one that might be mitigated with proper meadow management.