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Manganese Bioavailability Drives Organic Matter Transformations Across Oxic-Anoxic Interfaces via Biotic and Abiotic Pathways

Soil organic matter decomposition is a critical process that affects nutrient cycling, CO2 emissions, and carbon storage in terrestrial environments. Recent evidence suggests reactive manganese (Mn) phases, potent oxidants that depolymerize compounds like lignocellulose in soil organic matter, act as critical drivers of organic matter decomposition in soil and sediment environments. Furthermore, oxic-anoxic interfaces (OAIs) have been shown to be crucial hotspots for the formation of reactive Mn(III) species and associated organic matter degradation. However, the extent to which microbially mediated Mn(III) formation and subsequently Mn(III)-driven organic matter oxidation depends on Mn availability remains largely unknown. Additionally, the relative contributions between abiotic and biotic Mn-mediated organic matter oxidation pathways have been poorly quantified. In this study, we quantified the impact of Mn availability on Mn-mediated particulate organic carbon (POC) oxidation across the redox gradient and the specific contributions of abiotic and biotic reactions. To accomplish this, we established soil redox gradients in diffusion reactors and varied Mn(IV) oxide concentrations in the anoxic zone. The ensuing reductive mobilization of Mn(IV) oxides in the anoxic zone was meant to manipulate Mn(II) supply towards the OAI. The addition or exclusion of microbial inoculum allowed us to examine the abiotic contributions to Mn translocation and POC oxidation. Mn(II) translocation, Mn(III) formation, and C transformations across the redox gradient were quantified over a 12-week incubation period. Wet-chemical extractions combined with Mn XANES indicated that reactive Mn(III) formation at OAIs increased with enhanced Mn availability. Comparison of inoculated and uninoculated treatments revealed microbial Mn oxide reduction to be the critical driver of Mn translocation to oxic-anoxic interfaces. Subsequent enhanced Mn availability at the OAI enhanced POC oxidation and increased CO2 production rates due to enhanced microbial translocation and primarily attributed to microbially mediated Mn(III) formation. Our study emphasizes the importance of Mn(III)-mediated C oxidation across OAIs and its dependence on the provision of Mn(II) through microbial Mn reduction. Combined, our results show Mn–C coupled cycling across redox gradients as a critical biogeochemical process that has profound impacts on ecosystem scale soil C storage and CO2 fluxes.