Abstract
We studied the mechanisms of changes in the microstructure of anaerobic rice paddy soil when it is converted to upland field crops or restored from upland field to a rice paddy, by comparing the paddy soils with a model substance consisting of smectite-Fe oxide complexes. For the paddy soil (Epiaquepts), the sediment volume (SV) of the soil decreased when the matric potential of the soil was lower than —1.5 MPa and increased again with the flooding following drying. The amount of reduced iron with flooding suggested that the increase in the SV depended on the history of soil reduction and not on the state of reduction. The effect of the reduction of iron oxide on the SV was evaluated using the smectite-Fe oxide complex. The SV of the smectite with iron oxide decreased more than that of the smectite without iron oxide when the matric potential of the sample was lower than -1.5 MPa. When samples were reduced by adding sodium ascorbate, only the SV of the smectite that contained iron oxide increased. SEM micrographs showed that layersilicates were not aligned in smectite containing 0.085 kg kg-1 iron oxide, and it seemed that iron oxides bound to the layersilicates randomly. We concluded that the aggregation of layersilicates in the presence of iron oxide with drying was one of the factors that decrease the SV, and that the decreased volume could be restored by the reductive dissolution of the iron oxides in the soil.
