Sub-micrometer distribution of Fe oxides and organic matter in Podzol horizons

Abstract

The spatial distribution of soil constituents at the micrometer scale is of great importance to understand processes controlling the formation of micro-aggregates and the stabilization of organic carbon. Here, the spatial distribution of organic and mineral constituents in Podzol horizons is studied by concerted measurements of (i) the content of various forms of Fe, Al, Si and C determined by selective extraction in the fine earth fraction of soil (f < 2 mm); (ii) the elemental composition of the clay fraction (f < 2 μm) with lateral resolution using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and with surface selectivity using X-ray photoelectron spectroscopy (XPS); (iii) the specific surface area (SSA) of fine earth and clay fractions by krypton physisorption. The SSA of the fine earth in illuvial horizons is predominantly due to finely divided Fe oxides, including goethite, characterized by an equivalent particle size of about 10 nm. Kaolinite platelets of about 2 μm size account for a large volume proportion in the clay fraction but have a minor contribution to SSA. Fe oxides and organic matter (OM) are intimately associated. Heterogeneity at the μm scale is created by local variations in the relative amounts of kaolinite and Fe-OM associations. These two kinds of physical entities are in random mixture. Moreover, variation of C/Fe atomic ratios reveals sub-μm scale heterogeneity. The latter is due to variation in the relative proportion of organic compounds and Fe oxides, indicating that aggregation of nanoparticles, and not only mere adsorption or pore filling, plays a role in these associations. In this regard, our results highlight that OM associated with Fe protects Fe oxides against physical displacement and that part of this associated OM is oxidizable by NaOCl treatment. These findings demonstrate that the concept of OM stabilization through association with Fe must be revisited when considering the sub-μm scale level because fine Fe oxide particles can be easily dispersed during oxidation of associated carbon. Combination of physical fractionation and microanalysis (e.g. SEM-EDS, vibrational spectroscopy) offer promising perspectives to clarify the relationship between chemical composition and sub-μm scale architecture, and to better understand soil processes.

Is microbial reduction of Fe (III) in podzolic soils influencing C release?

“…Fe is systematically closely associated with the organic crust at the surface of quartz grains as revealed by the concomitant presence of Fe and C (and also Al) in EDX elemental maps (Fig. 2), and as previously shown (Cornelis et al, 2018). The reductive dissolution of Fe(III) in these Fe coatings may thus affect associated OC.…”

“…The inaccessibility of Fe(III) phases to Fe reducing bacteria has been invoked in riverine sediment settings to explain the discrepancy between the extent of (i) microbial Fe(III) reduction and (ii) reductive extraction by ascorbate (Hyacinthe et al, 2006). In this respect, Cornelis et al (2018) highlighted a mutual protection mechanism involving Fe protection within Fe-OMA as well as OM protection through coprecipitation and/or adsorption. DOC oxidation to CO 2 or HCO 3 − during DIR reactions might also explain the limited DOC release after S. putrefaciens addition.…”

Podzolisation affects the spatial allocation and chemical composition of soil organic matter fractions

“…This is in contrast to Vermeire et al (2019), who reported increasing accumulation of Fe d in subsoils of a Podzol chronosequence. Overall, Fe d contents were at least one order of magnitude lower than in other sandy Podzols (Egli et al 2001;Mokma et al 2004;Cornelis et al 2018), indicating that for our case the parent material was generally very low in Fe.…”

What do relationships between extractable metals and soil organic carbon concentrations mean?

“…Re‐oxidation of this Fe II is often rapid when it encounters molecular oxygen, generating Fe‐OC associations when DOC is present (Chen & Thompson, 2018). A large fraction of Fe‐associated C appears to consist of organo‐Fe coprecipitates with high molar C/Fe ratios (Wagai & Mayer, 2007; Zhao et al., 2016), but in many cases these contain Fe (oxyhydr)oxide phases buried within an organic matrix, which may not necessarily provide protection against OM oxidation (Cornelis et al., 2018; Kleber et al., 2015). Furthermore, Mössbauer spectroscopic analyses suggest that only a small fraction of the Fe III formed during redox cycling is likely to occur strictly in organo‐Fe III complexes in upland soils (Chen et al., 2020; Hall et al., 2018), especially given that much of the Fe II oxidation is catalyzed by mineral surfaces (Chen & Thompson, 2021).…”