The manner in which soil organic carbon (SOC) binds to and reacts with mineral surfaces affects SOC (bio)degradation, SOC mobility, soil wettability, mineral dissolution, and the bioavailability of essential elements and contaminants. While SOC-mineral interactions in bulk aqueous systems have been extensively studied, little is known about the molecular-level controls on SOC binding and reactivity in low water environments, such as between rainfall events and in the vadose zone. This proposal requests EMSL access and support to investigate the interdependence of SOC-mineral reactivity and concentrations of H2O adsorbed on minerals that are under dry to variable relative humidity conditions. Two hypotheses, which are not mutually exclusive, will drive our research: (1) The manner in which organics complex at and react with mineral surfaces depends on the amount of adsorbed H2O; and (2) The amount of H2O adsorbed in a specific organic-mineral system will depend on organic complexation mode and total organic coverage. We will focus on two classes of model SOC compounds (small carboxylic acids and organic reductants), which are either biological exudates themselves or analogues to major functional groups that are present in natural organic matter. Model minerals will be synthetic nanometer sized aluminum and iron(II,III) (hydr)oxides. In situ infrared (IR) spectroscopy will be used both to investigate how organics complex at and react with mineral surfaces (addressing Hypothesis 1) and to measure adsorbed H2O concentrations on organic coated mineral surfaces (addressing Hypothesis 2) as a function of relative humidity. In situ magic angle spinning nuclear magnetic resonance (NMR) spectroscopy on Fe free minerals will provide chemical and structural information about organic coordination geometries, both from the perspective of the organic and the coordinated metal (addressing Hypothesis 1). In situ 57Fe transmission Mossbauer spectroscopy will be used to investigate redox processes between organic reductants and Fe minerals (addressing Hypothesis 1). Ab initio molecular dynamics/density functional theory simulations will be carried out to calculate IR and NMR spectra of most probable adsorption configurations of organic molecules on mineral surfaces with variable amounts of adsorbed H2O to aid with experimental interpretation. These calculations will also probe the bonding and charge transfer between organics and mineral surface and the reactivity of the minerals towards dissolution (addressing Hypothesis 1). This research will fill an important knowledge gap about SOC-mineral interactions under low water conditions, leading specifically to molecular-level and mechanistic models that explain how SOC adsorption and reactivity varies as a function of adsorbed H2O concentration/RH, and broadly to a better understanding of the cycling, transformation, and transport of critical biogeochemical elements.