Abstract
Inorganic soil constituents sensitive to varying redox conditions, such as hydrated iron oxide, vivianite, siderite, iron (II) sulfides, and jarosite, are analyzed using optical and electron microscopes, energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Many of these minerals are sourced from paddy field soils, which undergo reducing and oxidizing conditions in the plow layer every year. Iron mottles formed at the soil redox interface in the presence of reducing and oxidizing conditions provide significant visual evidence of varying redox conditions in soil. Polished sections were used to examine the elemental distributions and morphological properties of the mottles. One type of iron mottles is formed around rice roots by oxygen diffusion from the roots. They are cylindrical in form and include soil matrix minerals. Other type of iron mottles is formed on the surfaces of irregular or vesicular pores by oxygen diffusion through soil pores after drainage. These mottles contain few soil matrix minerals. In association with iron, the distribution of phosphate is strongly affected by changes in redox conditions in paddy field soils with low active Al content.
Highlights
Oxidizing conditions are common in surface soils in an oxygen-containing atmosphere
The bulk soil (Fig. 5.9e) contains many transparent sand-sized particles. This observation suggests that the hydrated iron oxide coating the pore surfaces was formed outside the bulk soil. This occurrence of hydrated iron oxide is different from the cylindrical iron mottles that were formed in the bulk soil around plant roots (Figs. 5.6, 5.7, and 5.8), the cylindrical mottles are thicker than the mottles coating the pore surfaces
Considering thermodynamic stability, vivianite is thought to form under reducing conditions and to dissolve after soil oxidation (Lindsay 1979); the vivianite content is most likely affected by the changing redox conditions in paddy field soils between submergence and drainage
Summary
The major factors affecting dissolution and precipitation reactions in soil are the pH, temperature, and elemental composition of the soil. Ca carbonate dissolves in acid soils, whereas it precipitates in alkaline soils. The solubility of iron is very low in neutral and alkaline soils, whereas a part of iron dissolves under reducing conditions. Changes in redox conditions affect the chemical forms of elements, including redox-sensitive elements and related elements (Ponnamperuma 1972). The morphological properties of soil profiles are affected by these redox reactions (Vepraskas and Craft 2016). Reduced soils are typically characterized by the reduction of iron, as included in the diagnostic criteria for redoximorphic features in the United States Department of. In relation to redox reactions, the soil pH changes. A pH–pE diagram can be used as a method to describe the chemical stability of minerals under varying redox conditions in soil, and is recommended for further study (Stumm and Morgan 1996; Kyuma 2004)
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