Influence of water potential on decomposition of soil organic matter in high elevation meadows

High elevation meadows of the Sierra Nevada, California are defined by the presence of a high water table throughout the growing season. This definition takes into account the natural drawdown of most meadow systems during the summer months to within a few feet of the surface. The cyclic drawdown/recharge of the meadow water table, as well as local hydrologic gradient that exists in most meadow systems (ranging from xeric to hydric) impose important controls on organic matter storage and decomposition in these systems. In this study we incubated soils from the xeric, mesic and hydric regions of a subalpine Sierra Nevada meadow at five different water potentials and measured the CO2 flux at intervals over one year. We found that the cumulative carbon mineralization was greatest at the lowest (0.1 bar) and highest (4 bar) water potentials, across all regions of the meadow, suggesting a microbial response threshold that is reached as the soil dries. Soils from the hydric meadow regions showed the largest response to extreme drying of the soil. These results highlight the importance of soil moisture in regulating soil respiration in these systems and different levels of microbial activity when these soils are wet vs. dry. While normal conditions in these systems keep water potentials low (< 0.2 bar), future drought conditions can potentially increase the level of dryness in the meadows to the point where rapid carbon loss can be expected.