Autor:innen:
Katrin Schulz | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
Andrew Grigg | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
Luiza Notini | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
L. Joëlle Kubeneck | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
Worachart Wisawapipat | Department of Soil Science, Faculty of Agriculture, Kasetsart University | Thailand
Kurt Barmettler | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
Laurel K. ThomasArrigo | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
Ruben Kretzschmar | Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich | Switzerland
In sub- or anoxic soil environments, iron (Fe) (oxyhydr-)oxides can undergo microbial reductive dissolution, mineral recrystallization, or transformation, which can lead to the release of associated elements. An important nutrient, which is often associated with Fe (oxyhydr-)oxides and can impact their transformation, is phosphate (PO4). Until now, the transformation of Fe (oxyhydr-)oxides, such as ferrihydrite and lepidocrocite, and the effect of associated PO4, have mainly been studied in laboratory systems. It remains unclear which transformation processes occur if there is direct contact between minerals and the soil matrix, where microbial Fe reduction, diffusion limitations and heterogeneity at the pore- and aggregate scale can impact mineral transformations. Therefore, in this study, we combined the use of 57Fe-enriched (PO4-adsorbed) ferrihydrite and lepidocrocite with 57Fe Mössbauer spectroscopy, which enabled us to track the speciation of 57Fe in mineral-soil mix samples. To understand the effect of soil contact, we additionally studied the transformation of the respective pure mineral phases (with natural abundance Fe isotope composition) in the same soil environment. The pure and soil-mixed mineral samples were incubated in a flooded rice paddy field in Thailand for four months. Porewater conditions were monitored regularly, and the Fe mineral composition was analyzed with X-ray diffraction and/or 57Fe Mössbauer spectroscopy. The incubation of pure minerals resulted in ferrihydrite transformation to goethite (80%), but no transformation of lepidocrocite. When incubated as mineral-soil mixes, both ferrihydrite and lepidocrocite transformed to similar amounts of goethite (~30%). In contrast, PO4-adsorbed ferrihydrite did transform in pure mineral samples, while in mineral-soil mixes, PO4 strongly enhanced reductive dissolution of ferrihydrite, forming adsorbed Fe(II). These results show that direct soil contact enhances the accessibility of Fe (oxyhyr-)oxides for soil microbes and suggest that the initial crystallinity and Fe mineral identity only had a minor impact on the extent and pathway of Fe mineral transformations. The pathways of in-situ Fe (oxyhydr-)oxide transformations, presented in this study, advance the understanding of Fe mineral dynamics in soils, and demonstrate how these dynamics can be impacted by adsorbed PO4.