Mineral and Redox Controls on Soil Organic Matter Cycling in Seasonal Wetlands.

Soils contain nearly three times the amount of carbon (C) than the atmosphere, with C turnover times ranging from centuries to millennia. Although wetlands are a relatively small portion of the terrestrial landscape, they account for an estimated 20-30% of the global C reservoir. Seasonally flooded wetlands are likely the most vulnerable wetlands to climate change, as changing temperature and precipitation patterns are expected to alter the timing and duration of flooding. Seasonal variations in soil moisture are recognized as a critical control on soil C stocks and greenhouse gas emissions. However, the relative influence of associated changes in soil oxygen availability, root dynamics and the stability of mineral-organic associations are largely unknown. The overarching goal of this study was to determine the relative influence of redox state, root density and mineralogy on C cycling within forested seasonal wetlands. To accomplish this goal, we monitored seasonal dynamics of soil moisture, redox potential, and greenhouse gas emissions with a molecular characterization of organic matter composition and mineralogy along upland to wetland transects. Our first results suggest that with increasingly reducing conditions, mineral protection becomes less important. Meanwhile, the impacts of oxygen limitations increase, concurrent with an increase of C concentrations by nearly 150%. Upland soil horizons showed fourteen times the amount of root biomass as the wetland soils. Initial mass spectrometry and X-ray spectroscopy results indicate that C stored in deeper upland soil horizons consists of chemically oxidized root and microbial residues, while chemically reduced litter- and microbial-derived compounds accumulate in seasonal wetlands. In sum, our results suggest that anaerobically protected soil C in seasonal wetlands is particularly vulnerable to changing moisture regimes in response to climate change. To what extent this expected C loss is compensated by upland plant encroachment, or the neoformation of mineral-organic associations, warrants future research.