Lazareva, O., J. Kan, C. Chen, and D.L. Sparks. 2022. Applied Geochemistry 145: 105413.
This study contributes to our evolving understanding of how coupled biogeochemical dynamics of Fe and Mn along the redox gradients affect C cycling and sequestration at Critical Zones. A 12-month monitoring of the in-situ soil pore-water biogeochemistry and redox gradients across the floodplain at the White Clay Creek Watershed in the Christina River Basin – Critical Zone Observatory was investigated. Topographically, eastern floodplain was narrow with a steep hillslope connected to the agricultural field while western floodplain was flat and wide. Soil profile consisted of post-colonial oxic deposits, pre-colonial anoxic buried wetland soils underlain by suboxic valley-bottom gravel. Contrasting soil pore-water chemistry including DOM quality, DOC, Fe and Mn, δ18O and δD profiles, and redox potentials were observed between the eastern and western floodplains due to seasonal fluctuations, divergent topographic and hydrologic settings. We observed diverse types of C pools across the floodplain from short (labile) to long term (recalcitrant) C forms: microbial origin DOM at eastern versus terrestrial or plant-derived DOM at western buried wetland soil pore-waters. Gravel was linked to the soluble microbial byproduct-like fractions indicating bioavailable DOM and microbial activity. Gravel beds may act as a natural filter (or C sink) of terrestrial DOM via adsorption and precipitation with Mn- and Fe (hydr)oxides, stimulating the long-term sequestration of sedimentary OC while retaining the microbially-derived DOM in the pore-water. In contrast, reductive dissolution of Mn- and Fe (hydr)oxides may result in re-mobilization of aromatic DOM and increase of DOC flux (C source) to the stream. Future studies are needed to improve our understanding of how in-situ redox gradients and/or microbial activity transform OC-mineral complexes and the coupled C dynamics at Critical Zones.