Three years of belowground carbon flux under a range of grazing intensities in semiarid meadows of the central Great Basin, USA reveal substantial interannual variability.

Montane meadows are ecosystems characterized by productive herbaceous vegetation which is sustained by water tables at or near the soil surface for periods of the year. Meadows are crucial for late-season water supply, wildlife habitat and forage, and function as carbon (C) sinks for atmospheric carbon dioxide (CO2), contributing as much as ~700 g C m-2 yr-1 to soils. Our previous research shows that per unit area, meadows found in semiarid environments of the central Great Basin, USA can contain C stocks that rival those found in more humid montane regions. Meadow C stocks may be even more important to the C cycle of semiarid regions than meadows in more humid montane regions. Per unit area, Great Basin meadow soils contain three times more C than their surrounding upland ecosystems, while meadow soils of more humid regions contain roughly half the C in surrounding upland ecosystems. Meadows in semiarid ecosystems play a disproportionate role (relative to their spatial extent) in regional C cycles. Here we estimated C fluxes among three meadow vegetation communities (wet, mesic, and shrub), and grazing regimes (unmanaged grazing, controlled grazing, and grazing exclusion), over three years with different precipitation totals (ranging from 174 mm to 421 mm). We measured total belowground carbon flux (TBCF), a mass balance approach which uses measurements of gaseous losses of C from soils (CO2 and CH4) and changes in C stored in roots and soils to estimate annual additions of C from microbial growth, root turnover, and root exudation. Across four meadows we observed TBCF ranging from -2.21 kg C m-2 yr-1 to 2.94 kg C m-2 yr-1. The magnitude of belowground C fluxes were largely influenced by annual precipitation, with losses of soil C occurring during wet years and gains of soil C occurring during dry years. Grazing reduction appears to have a smaller, positive impact on belowground C fluxes during this time period. Our results provide further evidence that meadow C fluxes can be substantial on an annual basis, but also highly sensitive to weather. These results indicate that the utility of meadow soils as part of a natural climate solution in the Great Basin may depend as much or more on annual weather than grazing management.