Redox conditions rice soils

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

Some plants can induce a release of O2 at the surface of roots and thus an increase in pO2 in the rhizosphere. This process is known to occur in an adaptation of plants to submerged soil conditions, as in wetland plants, and is well documented for lowland rice Oryza sativa). To cope with anoxic or hypoxic conditions occurring in the soil or sediments, such plants have evolved a specialized structure, the aerenchytna, which conducts O2 to root tissues from the atmosphere and the shoots. The portion of O2 that is not consumed in the roots for respiration leaks through the root apoplasm (cell walls) and ultimately into the rhizosphere. Two pieces of evidence support this phenomenon. First, an increase in redox potential... 

Ponnamperuma (1972) reviewed the chemistry of flooded soils. This research has understandably concentrated on the soil conditions of paddy rice agriculture. Only some generalizations are mentioned here. The behavior of C, N, S, Fe, and Mn generally follows that shown in Table 4.3. When rice paddies are drained before harvest, redox potentials rise, Fe2 1 and Mn2+ concentrations decrease, and C, N, and S oxidize. When the soils are flooded again, the reactions reverse. 

There have been few reports of rice (Oryza sativa L.) responding to Mo application. Because most rice is grown in flooded paddies, the chemical interactions involving Mo are not the same as in dryland agriculture. When acid soils are flooded, there are increases in soil pH and changes in redox potential, thus changing the solubilities of Fe minerals (Ponnamperuma, 1972). Moore and Patrick (1991) reported increases in the amounts of MoO/ in soil solution with increasing pH in flooded soils, and those increases resulted in increased Mo uptake. The conditions in flooded soils may explain why there have been few reports of increased yields due to addition of Mo. 

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