Iron speciation in fine sand and fine silt and clay fractions across different land uses

Iron (Fe) (hydr)oxide minerals have been suggested as an important phase for the stabilization of soil organic matter (SOM). Although Fe mineral recrystallization and transformation processes observed in the laboratory are well established, their occurrence in soils, the environmental factors that control their transformation rates and products, and the degree to which they promote long-term SOM stabilization remain to be investigated. In particular, little information is available from a coupled characterization of Fe and SOM in different soil size fractions. We applied Fe Extended X-ray Absorption Fine Structure (EXAFS), X-ray diffraction, and Mössbauer spectroscopy, paired with elemental analysis and thermogravimetric techniques, to two soil size fractions (fine sand, FSa, and fine silt and clay, FSi+Cl) isolated from broadleaved and coniferous forest soils, grasslands, and technosols to examine Fe speciation and SOM composition. Soil organic carbon (OC) and total nitrogen contents differed markedly among land uses, but their relative distribution among soil size fractions was quite consistent. With the exception of technosols, the FSa fractions were dominated by goethite, whereas the FSi+Cl fractions consisted mainly of less crystalline ferrihydrite, while Fe(III)-SOM accounted for 11-42 and 32-47% of total Fe, respectively. In both fractions, goethite and ferrihydrite percentages decreased with increasing OC content. Overall, the proportion of Fe(III)-SOM complexes in FSa was positively correlated to the OC content, whereas no significant relationships were found for the FSi+Cl fraction, independent of the land use. It is worthy to note that technosols showed a completely different composition in terms of Fe phases; in particular, in the FSa fractions ferrihydrite represented the main Fe oxide, while the content of Fe(III)-SOM complexes was the lowest among all land uses. The different behaviour of technosols was confirmed by the thermal analysis: whereas no significant differences in TG-T50 were observed among SOM pools in all the land uses, technosols showed significantly higher values of this stability index in both fractions. Moreover, thermal analysis suggests that SOM stability was greater in the FSa than the FSi+Cl fractions for all land uses, except in the grassland soils. These preliminary data show that different soil size fractions contain diverse Fe phases, but not a clear association between Fe minerals and SOM stability.