Ferrihydrite transformations in flooded paddy soils: rates, pathways, and product spatial distributions

Abstract

The rate and pathway of ferrihydrite transformation in soil depends on the properties of the soil pore water and diffusion processes.

A New Approach for Investigating Iron Mineral Transformations in Soils and Sediments Using 57Fe-Labeled Minerals and 57Fe Mössbauer Spectroscopy

“…Thus, our results indicate that the influence of close contact with the soil matrix can result in different mineral transformation processes. For example, the decrease in DOC and Si aqueous concentration in the incubations with the added ferrihydrite (Figures S6 and S7) suggests that the ferrihydrite sorbed organic matter and silicate, which have been shown to hinder the transformation of ferrihydrite into more crystalline Fe­(III) (oxyhydr)­oxides. ,,, A similar phenomenon was recently observed in a study with ferrihydrite filled into mesh bags and incubated flooded paddy soil microcosms, leading to an outer rim of apparently unreacted ferrihydrite, which was attributed to the contact with the dissolved components of pore water such as phosphorus, silicate, and/or organic matter …”

“…Investigating the transformation of ferrihydrite in the Paddy Soil using our approach with 57 Fe-labeled minerals and Mössbauer spectroscopy revealed no (or minimal) formation of crystalline Fe­(III) (oxyhydr)­oxides. Such results contrast investigations that employed mineral suspensions spiked with Fe­(II), ,,− dissimilatory Fe reduction of ferrihydrite under advective flow, or incubations in paddy soils employing mesh bags in which the formation of lepidocrocite, goethite, and/or magnetite was observed as major fractions. Thus, our results indicate that the influence of close contact with the soil matrix can result in different mineral transformation processes.…”

“…So far, such studies were limited to soils containing fairly high total Fe contents (>70 g kg –1 Fe), in part due to analytical constraints. Laboratory soil incubation experiments were mostly conducted using mixed soil slurries with lower soil-to-solution ratio than in soils under field conditions, which allows purging of the reactors with N 2 and O 2 gases to control the redox conditions. , Other studies have employed mesh bags filled with Fe minerals and incubated in soil microcosms or in the field. − Incubation of mesh bags filled with ferrihydrite in flooded soils led to extensive transformation of ferrihydrite to goethite and lepidocrocite. − Recently, one study used gel-based diffusive samplers in the field and demonstrated ferrihydrite sulfidation in a sulfidic tidal flat sediment . While the studies using mesh bags or diffusive samplers analyzed the transformation of added Fe minerals in conditions that more closely resemble natural systems than in mixed mineral suspensions, they do not facilitate contact of the Fe minerals with microorganisms and other soil components and create local environments at the microscale in which the Fe minerals comprise a much higher fraction of the solids than in the surrounding soil.…”

“…While the studies using mesh bags or diffusive samplers analyzed the transformation of added Fe minerals in conditions that more closely resemble natural systems than in mixed mineral suspensions, they do not facilitate contact of the Fe minerals with microorganisms and other soil components and create local environments at the microscale in which the Fe minerals comprise a much higher fraction of the solids than in the surrounding soil. For example, a recent micro-Raman spectroscopic study demonstrated that the lack of soil contact and diffusion of ferrous Fe from the surrounding soil into the mesh bags filled with ferrihydrite can lead to diffusion processes influencing the mineral transformation pathways …”

Iron (hydr)oxide formation in Andosols under extreme climate conditions

“…3 and 4). These bands typically serve as diagnostic criteria for the occurrence of ferrihydrite 30,31 . In addition to Fe-(hydr)oxides, few siderite crystals (Fe 2+ CO 3 ; ~ 10 μm in diameter) were discovered in the OM-rich matrix close to residual MB 2 glass (Fig.…”