Physical Protection in Aggregates and Organo-Mineral Associations Contribute to Carbon Stabilization at the Transition Zone of Seasonally Saturated Wetlands

Abstract

Wetlands store significant soil organic carbon (SOC) globally due to anoxic conditions that suppress SOC loss, yet this SOC is sensitive to climate and land use change. Seasonally saturated wetlands experience fluctuating hydrologic conditions that may also promote mechanisms known to control SOC stabilization in upland soils; these wetlands are therefore likely to be important for SOC storage at the landscape-scale. We investigated the role of physicochemical mechanisms of SOC stabilization in five seasonally saturated wetlands to test the hypothesis that these mechanisms are present, particularly in the transition between wetland and upland where soil saturation is most variable. At each wetland, we monitored water level and collected soil samples at five points along a transect from frequently saturated basin edge to rarely saturated upland. We quantified physical protection of SOC in aggregates and organo-mineral associations in mineral horizons to 0.5 m depth. As expected, SOC decreased from basin edge to upland. In the basin edge and transition zone, the majority of SOC was physically protected in macroaggregates. By contrast, overall organo-mineral associations were low, with the highest Fe concentrations (5 mg Fe g -1 soil) in the transition zone. While both stabilization mechanisms were present in the transition zone, physical protection is more likely to influence SOC stabilization during dry periods in seasonally saturated wetlands. As future climate scenarios predict changes in wetland wet and dry cycles, understanding the mechanisms by which SOC is stabilized in wetland soils is critical for predicting the vulnerability of SOC to future change.