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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 August 2018, Vol.235, pp.89-102
    Description: Ferric iron (Fe ) solid phases serve many functions in soils and sediments, which include providing sorption sites for soil organic matter, nutrients, and pollutants. The reactivity of Fe solid phases depends on the mineral structure, including the overall crystallinity. In redox-active soils and sediments, repeated reductive dissolution with subsequent exposure to aqueous ferrous iron (Fe ) and oxidative re-precipitation can alter Fe phase crystallinity and reactivity. However, the trajectory of Fe mineral transformation under redox fluctuations is unclear and has been reported to result in both increases and decreases in Fe phase crystallinity. Several factors such as water budget, organic matter input, redox dynamics as well as the initial Fe phase composition might play a role. The objective of our study was to examine if Fe minerals in soils that differ in porosity-dependent water leaching rate and initial Fe phase crystallinity, demonstrate distinct mineral transformations when subjected to redox fluctuations. We sampled paired plots of two soil types under similar management but with different water leaching rates and contrasting Fe oxide crystallinity — an Alisol rich in crystalline Fe phases and an Andosol rich in short-range-ordered (SRO) Fe phases. The two soils were either exposed to several decades of redox fluctuations during rice paddy cultivation (paddy) or to predominantly oxic conditions in neighboring vegetable gardens (non-paddy). Paddy soils are uniquely suited for this type of study because they are regularly submerged and develop regular redox fluctuations. We also incubated the non-paddy soils in the laboratory for one year through eight anoxic/oxic cycles and monitored the aqueous soil geochemistry. Mössbauer spectroscopy was then used to evaluate Fe mineral speciation in field soils (paddy and non-paddy) and laboratory incubations. In the field soils, we found that redox fluctuation had contrasting effects on Fe oxide crystallinity, with crystallinity being lower in the Alisol paddy soil and higher in the Andosol paddy soil than in their corresponding non-paddy controls. In the laboratory incubation experiment, Eh, pH and dissolved Fe responded as anticipated, with elevated Fe concentrations during the anoxic periods as well as low Eh and high pH. Mössbauer measurements suggest the fluctuating redox incubation was beginning to alter Fe oxide crystallinity along the same trajectory as observed in the field, but the changes were within the range of fitting errors. We propose that reductive dissolution of crystalline Fe oxides prevails in the soil rich in crystalline Fe oxides (Alisol) and that re-precipitation as SRO Fe oxides is favored by constrained leaching, which leads to the observed decrease in Fe oxide crystallinity. In the soil rich in SRO Fe phases (Andosol), preferential reductive dissolution of SRO Fe oxides coupled with stronger leaching of dissolved Fe causes the observed relative increase in crystallinity of the remaining Fe oxides. The observed increase in Fe oxide crystallinity may further be a result of Fe(II)-catalyzed re-crystallization of SRO Fe oxides. These findings indicate that, besides other factors, the Fe mineral composition of the initial soil or sediment as well as the leaching rate likely influence the trajectory of Fe oxide evolution under alternating redox-conditions.
    Keywords: Fe Oxides ; Redox Fluctuation ; Paddy Soils ; Mössbauer Spectroscopy ; Alisol ; Andosol ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, 2007, Vol.71(10), pp.2569-2590
    Description: Mineral-associated organic matter (OM) represents a large reservoir of organic carbon (OC) in natural environments. The factors controlling the extent of the mineral-mediated OC stabilization, however, are poorly understood. The protection of OM against biodegradation upon sorption to mineral phases is assumed to result from the formation of strong bonds that limit desorption. To test this, we studied the biodegradation of OM bound to goethite (α-FeOOH), pyrophyllite, and vermiculite via specific mechanisms as estimated from OC uptake in different background electrolytes and operationally defined as ‘ligand exchange’, ‘Ca bridging’, and ‘van der Waals forces’. Organic matter extracted from an Oa forest floor horizon under Norway spruce ( (L.) Karst) was reacted with minerals at dissolved OC concentrations of ∼5–130 mg/L at pH 4. Goethite retained up to 30.1 mg OC/g predominantly by ‘ligand exchange’; pyrophyllite sorbed maximally 12.5 mg OC/g, largely via ‘van der Waals forces’ and ‘Ca bridging’, while sorption of OM to vermiculite was 7.3 mg OC/g, mainly due to the formation of ‘Ca bridges’. Aromatic OM components were selectively sorbed by all minerals (goethite ≫ phyllosilicates). The sorption of OM was strongly hysteretic with the desorption into 0.01 M NaCl being larger for OM held by ‘Ca bridges’ and ‘van der Waals forces’ than by ‘ligand exchange’. Incubation experiments under aerobic conditions (initial pH 4; 90 days) revealed that OM mainly bound to minerals by ‘ligand exchange’ was more resistant against mineralization than OM held by non-columbic interactions (‘van der Waals forces’). Calcium bridges enhanced the stability of sorbed OM, especially for vermiculite, but less than the binding via ‘ligand exchange’. Combined evidence suggests that the extent and rate of mineralization of mineral-associated OM are governed by desorption. The intrinsic stability of sorbed OM as related to the presence of resistant, lignin-derived aromatic components appears less decisive for the sorptive stabilization of OM than the involved binding mechanisms. In a given environment, the type of minerals present and the solution chemistry determine the operating binding mechanisms, thereby the extent of OM sorption and desorption, and thus ultimately the bioavailability of mineral-associated OM.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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