Hydrologic drivers of soil organic carbon stabilization in seasonally-saturated wetlands

Recent research emphasizes physiochemical protection of soil organic carbon as a dominant control on carbon residence time in upland soils, but the role of stabilization mechanisms in wetland soils is relatively understudied. Seasonally saturated mineral wetlands experience annual drying and may experience conditions that support carbon stabilization similar to upland soils; as such they are an ideal system to explore the importance of physiochemical stabilization mechanisms in wetland soils. Here, we investigated the relationship between carbon stabilization and hydrologic characteristics across the aquatic-terrestrial interface of five depressional freshwater wetlands on the Delmarva Peninsula (Maryland, USA). At each wetland, we collected samples along a 20-25 m transect spanning from the wetland edge to the upland. For each sample, we quantified carbon stabilization by measuring aggregate formation, clay sorption, and organo-metal associations by horizon to a depth of 50 cm. We also monitored soil moisture, inundation, and reducing conditions at each of five transect points. To understand the effects of hydrology at longer time scales, we simulated hydrologic variability and patterns of inundation in each wetland using a simple water balance model. Typical of hydric soils, we found an increase in soil organic carbon content and a decrease in soil iron content from the upland to the wetland edge. Preliminary results from the fractionation and hydrologic modeling highlight differences in the relative importance of stabilization mechanisms across a moisture gradient, with implications for the vulnerability of wetland soil carbon to microbial respiration if hydrologic regime is altered by climate or land use change.