Biogeochemical transformations of Iron-bearing soil minerals along a redox gradient: Implication for C cycling

Background/Question/Methods

Intimate associations of mineral phases with organic matter has been recognized as a fundamental mechanism of stabilizing organic compounds against biological degradation, and  is therefore a major control for soil carbon storage. Iron (Fe) oxides are of particular importance because of their abundance in soils and their high reactive surface area. Iron is susceptible to redox variability along landscape gradients. Fe (III) minerals predominate in well-drained upland soils, while under poorly-drained conditions at lowland locations such as floodplains and streams, reductive dissolution of Fe (III) oxides occurs. These redox fluctuations drive local mobilization of Fe²⁺(aq), which can be either removed from soil by leaching, re-oxidiation and precipitation as Fe(III)-oxide and –hydroxide coatings on soil mineral surfaces, or incorporated into ferrous-bearing minerals. Additionally, reductive dissolution and transformation of Fe minerals governs the amount, form and transport of sequestered C. At the Christina River Basin-Critical Zone Observatory (CRB-CZO), one of six USA observatories funded by the National Science Foundation, we have investigated  Fe speciation as well as the composition of organic matter (OM) and its molecular interaction with soil minerals across a redox gradient to link iron-redox coupling processes with soil C cycling.

Results/Conclusions

X-ray absorption spectroscopy (XAS), micro-XAS, X-ray diffraction (XRD) and Mössbauer spectroscopy were used to investigate soil solid-phase Fe speciationin a series of field sites under varying redox conditions including uplands and floodplains. By applying scanning transmission X-ray microscopy and near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS), we mapped the spatial distribution of carbon and carbon forms, and imaged the association of carbon functional groups with specific soil minerals. Ferrihydrite, because of its high surface area, contributes significantly to C stabilization 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. The coprecipitation of OM with ferrihydrite may subsequently affect its reactivity. Data on the role of OM coatings on the abiotic transformation of ferrihydrite will be presented. This study addresses the important connection between soil Fe-redox coupling processes and carbon cycling and stabilizaion at soil/sediment-water interfaces.