Rapid Reduction Leads to Changes in Iron Oxide Crystallinity in Humid Tropical Soils

Abstract:

Humid tropical forest soils of Puerto Rico often experience frequent rainfall, which limits O₂ diffusion disproportionately to consumption, thereby creating redox heterogeneity in these upland soils. Multiple studies have shown that redox oscillations in tropical soils change short range-ordered Fe-oxide (ferrihydrite and goethite) crystallinity, but it is unclear how and how fast these changes occur. Our objective was to couple changes in Fe and Al solution chemistry to ⁵⁷Fe-Mössbauer (MBS) spectral features to gain insights into rapid changes in Fe-oxide crystallinity in surface soils from the Luquillo Critical Zone Observatory and Long-Term Ecological Research site in Puerto Rico. We hypothesize that immediately upon switching from oxic to anoxic condition, Al-goethites undergo incongruent dissolution and ferrihydrite transforms to crystalline Fe-oxides, resulting in a redistribution of organic matter on mineral surfaces. While preliminary X-ray diffraction data hint at Al substitution within goethite, MBS data clearly indicated dramatic changes in Al-goethite spectral features within 30 minutes of switching to anoxic conditions along with anticipated ferrihydrite transformation to goethite. The changes in goethite MBS data along with increases in soluble Fe and Al contents unambiguously suggest that incongruent dissolution of Al from goethite is likely a reason for reported changes in SRO mineral crystallinity upon reduction. These data highlight that mineralogical changes happen remarkably quickly following a redox shift, and hint at Fe as the controlling factor behind the underlying molecular-scale processes in variable redox wet tropical soils.