PNNL

Abstract

Humid tropical forest soils experience frequent rainfall, which limits oxygen diffusion disproportionately to consumption, thereby creating redox heterogeneity in upland soils, but it is unclear how these conditions affect short-range ordered (SRO) crystallinity changes. In this study, oxic and laboratory-reduced soils from the Luquillo Critical Zone Observatory and Long-Term Ecological Research site in Puerto Rico were characterized by variable 57Fe-Mössbauer spectroscopy to gain insights into reduction-induced sequential Fe-SRO mineralogical changes. Mössbauer spectra generated at multiple temperatures (room temperature, 225 K, 77 K and 8 K) suggest: i) SRO minerals of oxic soil is primarily a mixture of Al-goethites of varying Al-content (83% of the total Fe) with minor contributions from ferrihydrite (~4%) and Fe(III)-organic phases (~13%); ii) Fe/Al-SRO undergoes significant rapid changes in mineralogy within 30 minutes, likely due to incongruent removal of Al and Fe from SRO (based on 225 K and 77 K spectra); and iii) relatively Al-poor Fe-SRO is remarkably stable towards further mineralogical changes. More importantly, Mössbauer spectroscopy measurements unambiguously suggest rapid increase in crystallinity of Fe-SRO with reduction given the inverse relationship between Al-content and crystallinity. In contrast, incongruent removal of Fe and Al from the soils was not evident from ammonium-oxalate (AO) and citrate-dithionite (CD) extractions employed to quantify amorphous and crystalline Fe-oxide contents, respectively. Adsorption of biogenic Al to residual SRO’s appeared to be responsible for such an inconsistency. Pyrophosphate (PP) extractions employed to estimate sorbed OM contents mimicked AO data suggesting likely association of C with AO soluble Fe/Al-oxide phase. Lack of correlation between Mössbauer spectroscopy measurements and selective chemical extraction results suggest soil SRO transformation mechanisms are complex and cautions overemphasis of operationally defined selective chemical extraction data to interpret mineralogical changes. Additionally, coupled Mössbauer spectroscopy and selective chemical extraction data measurements clearly hinted association of reactive Fe with C.