Redox-driven mineralogical changes in Fe (hydr)oxides across particle-size SOM fractions

The kinetics of iron (Fe) recrystallization and transformation processes under changing redox conditions have been mostly studied in reductionist model systems. Despite the well-known influence of redox conditions on Fe crystallinity and speciation, as well as on soil organic matter (SOM) stabilization and C turnover, the link between redox-driven changes in Fe minerals composition and crystallinity, and SOM chemical properties in the field remains elusive. Under reducing conditions, increasing concentrations of Fe(II) released in solution from the reductive dissolution of Fe (hydr)oxides may in turn accelerate ferrihydrite (Fh) transformation, although our understanding of the influence of SOM on these transformations is still lacking. In this work we evaluated Fe(II)-catalyzed mineralogical changes in pedogenetic Fe (hydr)oxides in bulk soils and physically fractionated SOM pools (for comparison, fine silt plus clay, FSi+Cl and fine sand, FSa) of an agricultural soil unamended or amended with biochar, municipal solid waste compost, or both. Slurries containing bulk soils or size fractions were incubated for 7 days under anoxic conditions at neutral pH after addition of Fe(II). Solid phase Fe transformations were evaluated using Fe extended X-ray absorption fine structure (Fe-EXAFS) at the XAFS beamline (Elettra Sincrotrone, Trieste). The proportions of different Fe mineral phases were determined by principle component analysis and linear combination fitting (LCF) using a suite of Fe reference spectra. Statistical methods were used to determine goodness-of-fit parameters for LCF of synchrotron-based XAS data. FSa fractions showed the most significant Fe(II)-catalyzed Fh transformations with the consequent production of well-ordered Fe oxides irrespective of soil amendment, with the only exception being the compost-amended ones. In contrast, poorly crystalline Fh still constituted ca.45% of the FSi+Cl fractions of amended soils only, confirming the that the higher SOM content in this fraction inhibits atom exchange between aqueous Fe(II) and the solid phase. Building on our knowledge of Fe(II)-catalyzed mineralogical changes in simple systems, our results evidence that the mechanisms of Fe mineral transformations in bulk soils may be rather complex and depend on the natural gradients in Fe mineralogy, organic C content and quality, and organo-mineral associations that exist across particle-size SOM pools.