Decomposition of distinct organic matter pools is regulated by moisture status in structured wetland soils

• Drying of OM rich high elevation soils induces porosity loss and soil consolidation.
• Consolidation causes comparable OM decay rates at high and low water potentials.
• Hydrological and biogeochemical responses of OM rich soils to drying are coupled.

Peatlands are garnering much attention for their greenhouse gas feedback potential in a warming climate. As of yet, the coupled biogeochemical and hydrological processes that control the amount and timing of soil organic matter (SOM) mineralization and, ultimately, whether peatlands will be sinks or sources of atmospheric CO2 are not fully understood. Soil structure is a key feature of soils that mediates the coupling between biogeochemical and hydrological processes. However, we know very little about how soil structure responds when soils are exposed to wetting–drying cycles outside their normal range. In order to better understand how high elevation peatlands will respond to increasingly dry years, we incubated soils from high elevation meadows in the Sierra Nevada at 5 different water potentials and measured the CO2 flux for over one year. We found that the cumulative carbon mineralization had a U-shaped pattern, with the greatest mineralization at the wettest (−0.1 bar) and driest (−4 bar) water potentials, across all hydrologic regions of the meadow. We propose a conceptual model that reproduces a similar pattern by incorporating the concept of dual porosity medium, with two distinct pore-size populations representing inter- and intra-aggregate porosity. Availability of water and oxygen to the two pore-size populations depends on the soil's equilibrium water potential. The model and the data suggest that the decomposition rates of intra-aggregate SOM may increase due to prolonged drought events that lead to accelerated release of C from previously untapped pool.