Spectroscopic investigation of manganese and carbon transformation during litter decomposition

Soil organic carbon (C) storage and turnover remain key uncertainties in terrestrial C cycle models, limiting their ability to predict C fluxes in response to changing climate. Decomposition of soil organic matter is often parameterized as a function of lignin content in models without consideration for the biogeochemical processes that regulate its degradation. Manganese (Mn) is a redox-sensitive micronutrient that stimulates enzymatic oxidation of lignin by soil microorganisms. In this study, we used synchrotron-source x-ray absorption spectroscopy (XAS) to examine chemical transformation of Mn and C in decomposing leaf litter in response to experimental warming and Mn addition. Surface organic soils were enclosed in nylon mesh bags and decomposed for up to one year in upland soils of the temperate forested Walker Branch watershed in east Tennessee. Decomposing organic matter was either warmed +2-3°C or not and received between zero to 10 mg g-1 aqueous Mn(II) addition. For organic matter receiving no Mn addition, Mn K-edge spectra from the initial soils were dominated by Mn(II) peaks that decreased concurrent with increases in Mn(IV) peaks in the post-decomposition soils. We infer that the initial litter contained primarily Mn(II) that was either leached or partially oxidized during decomposition. Average Mn oxidation state also increased in all but the highest Mn addition treatments, indicating that rates of Mn oxidation exceeded accumulation of added Mn(II). Decomposing organic matter showed decreases in aliphatic C over time, indicating preferential removal of aliphatic groups during decomposition. Our results indicate that Mn contained in leaf litter is partially oxidized during over time, a process which has been shown to retain Mn in surface soils and contribute to litter decomposition. Spectroscopic analyses will be coupled with measurements of litter mass loss and soil respiration to examine whether Mn enrichment enhances C transformation and efflux.