Soil Manganese Redox Cycling In Suboxic Zones: Effects On Soil Carbon Stability.

Suboxic soil environments contain a disproportionately higher concentration of highly reactive free radicals relative to the surrounding soil matrix, which may have significant implications for soil organic matter cycling and stabilization. This study investigated fungal-mediated Mn-cycling along a redox gradient and its subsequent effect on organic matter transformation. A sterilized surface forest soil was mixed with ground native pine needles (20%) and synthesized birnessite/hausmanite (0.2%), inoculated with a Mn-oxidizing fungi (Pleurotus ostreatus), and incubated at 24° for 21 days in a saturated soil microcosm. A prolene film, attached to the microcosm face, allowed for vertical imaging. Following incubation, vertical oxygen profiles were quantified at 0.5mm increments using oxygen microelectrodes. Synchrotron-based XRF and XANES were then used to characterize Mn distribution and speciation within different redox environments, followed by µ-FTIR to provide information on the C functional groups associated with Mn in the different redox zones. Results from oxygen micro-profiling showed a rapid decrease in O₂ with depth below the mineral surface, with suboxic/anoxic conditions present below 7.5mm. Optical imaging showed intense fungal growth in this O₂-free environment. Preliminary results from µ-XRF and XANES showed a high level of oxidized Mn (MnIII/IV) along the region of active fungal growth, with less oxidized Mn forms (MnII/III) in the inner regions of the inoculated areas as well as the un-inoculated zones. Synchrotron-based FTIR microscopy showed differences in carbon (C) forms among the different redox and biotic/abiotic environments. Fungal inoculated regions displayed higher polysaccharide C (C-O bonds at 1035 cm^-1) and lower aromatic C (C=C bonds at 1589 cm^-1) relative to the un-inoculated regions. These preliminary results indicate that fungal-mediated Mn oxidation in suboxic/anoxic environments results in the degradation of aromatic functional groups and the subsequent production of fungal polysaccharides.  Further analysis is needed to differentiate the molecular changes carbon experiences in association with fungal-mediated Mn redox cycling.