The Role of Mineral Complexation and Metal Redox Coupling in Carbon Cycling and Stabilization

Abstract

The association of carbon with mineral phases has been increasingly recognized as a major stabilizing mechanism for protecting organic matter against microbial degradation in soils. However, direct investigation of organo-mineral interactions has been hampered by a lack of methods that can simultaneously characterize soil organic matter and soil minerals. To elucidate the specific mineral–OM binding mechanisms, scanning transmission X-ray microscopy−near-edge X-ray absorption fine structure spectroscopy (STXM–NEXAFS) was applied to map the major mineral elemental (Si, Al, Ca, Fe, K) composition and to determine the spatial distribution of carbon and carbon functional groups in soil clay fractions from a landscape topographic gradient at the Christina River Basin Critical Zone Observatory (CRB-CZO). Pyrolysis-field ionization mass spectrometry (Py-FIMS) was used to identify organic matter composition in soils. We also investigated Fe speciation to link iron-redox-coupling processes with soil C cycling. Selective chemical extractions, X-ray absorption spectroscopy (XAS), micro-XAS techniques, and Mossbauer spectroscopy were employed to characterize soil Fe speciation. Ferrihydrite, because of its ubiquitous occurrence in the environment and its high surface area, contributes significantly to the sorption of organic matter and protects it against microbial degradation in soils and sediments. In addition, ferrihydrite often forms in the presence of dissolved organic matter in the natural environment, which leads to coprecipitation of organic matter with ferrihydrite. To examine the mechanisms of organo-ferrihydrite complex formation associated with these two processes, C and N NEXAFS spectroscopic techniques were employed. Such studies on mineral complexation and metal redox cycling provide new insights on carbon cycling and stabilization in the Earth’s critical zone.