Imaging Soil Litter Decomposition: Integrating Synchrotron-Based Elemental and Molecular Imaging Techniques.

Forest ecosystems contain approximately one-half of the Earths terrestrial carbon (C) (1146 Pg), with two-thirds (787 Pg) of this pool residing in forest soils. Given the magnitude of the forest soil C pool, it is critical to understand the mechanisms that control soil organic matter decomposition. The main objective of this study was to elucidate the effects of Mn redox cycling on the transformation and stability of soil organic carbon within a forest soil litter layer. To accomplish this task, we integrated elemental (i.e., XRF, XAS) and molecular (i.e., µ-FTIR) imaging techniques. Synchrotron-based XRF and XAS were used to characterize Mn distribution and speciation, while µ-FTIR was used to provide information on the type of C functional groups associated with Mn within the Oi soil litter layer. Our results show that Mn plays a central role in the decomposition of soil litter, likely through microbially mediated processes. While fresh plant litter contains predominantly Mn(II), Mn speciation changes in association with C chemistry, showing mixed valance states (Mn[II/III/IV]) in areas with signs of increased OM decomposition (i.e., decreased level of carbohydrates/increased  levels of alkyl C, aromatic C, and carboxyl groups). Areas associated with concentrations of oxidized Mn correspond with higher rates of carbohydrate decomposition, resulting in the accumulation of modified lignin. This approach provides a unique way of understanding the molecular changes that substrate C experience in association with metal-redox cycling and may be used to understand the effects of metal-redox cycling on a range soil biogeochemical processes.