The Relationships among Soil Solution Composition, Saturation, Sedox Processes, and Redoximorphic Features of Soils in Anthraquic Condition:Case Studies in Rice-growing Soils of Chungli Terrace

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The hydrological conditions of the rice-growing soil are major affected by cultivation and soil management. The hydrological conditions directly affect on soil pedology and morphological feature. Three rice-growing soils associated with various elevations and hydrological conditions, as derived from Quarternary aged materials, were selected from Chungli Terrace of northern Taiwan in this study. These soils are Houhu, Hsinwu, and Lungchung series. Water table, saturation condition, and redox potential of three soils at different depths were monitored for two years (from January, 1996 to December, 1997) to identify the anthraquic conditions. This study also attempt to investigate the kinds, distributions, and genesis of redoximorphic features through the profiles. In addition, we also monitor the soil solution composition at different depths, including surface layer (0-20cm), the concretion formed layer, and the gray mottle formed layer, to understand the forms of iron or manganese oxide can be formed in this condition. The result show that Houhu and Lungchung series were satisfied the criteria of anthraquic conditions defined in Keys to Soil Taxonomy. The soils within 75 cm in Hsinwu series did not satisfy the requirements of anthraquic conditions, therefore, Hsinwu series were proposed as oxyaquic condition. The duration of saturation and reduction in the soils within 75 cm in both Houhu and Lungchung series exceeded 50% of the year. Those soils displayed positive reactions to α,α''-dipyridyl dye test through the year. Rice production strongly influenced the saturation and reduction conditions of the plowed layer from the soil surface to 50 cm depth. In contrast, the saturation and reduction conditions in the underlying Btv horizons with plinthite mainly varied with fluctuating groundwater table. In the rice-growing red soils of Chungli Terrace, the duration of saturation in the soils of Btv horizon increased with a lower elevation. The saturated time in the soils of 50 cm and 100 cm depths of Lungchung series, having an elevation of 40 m, were 30% and 10% annually. The saturated time in the soils of 50 cm and 100 cm depths of Hsinwu series, having an elevation of 30 m, were 50% and 30% annually. The saturated time in the soils of 50 cm and 100 cm depths of Houhu series, having an elevation of 20 m, were 70% and 50% annually, respectively. The part of above the Btv horizon can be saturated by the local irrigation. In the soil hydrosequence, iron and manganese segregation were very clear because of high air temperature and heavy rainfall. Consequently, various redoximorphic features occurred in the all Btv horizons. Those features were redox depletions with chroma 1 and redox concentrations including Fe-Mn concretions, Fe masses, and pore linings with high chroma. Redox concentrations were found in the interiors of soils. For instance, secondary Fe oxides were originally coated on the surfaces of sand. However, Fe-Mn concretions were progressively formed and impregnated in the matrix by alternative oxidation and reduction cycles. Redox depletion were intensively distributed on the soil surface or along the voids where primary pathways were available for conducting water in the soils. Optimum alternative wetting and drying cycles attributed to frequent seasonal fluctuations of water table facilitated with changing the soil solution composition and increasing in the sizes of Fe-Mn concretion. According to the monitoring data on soil solution composition, pH, and Eh at different depths of three series, and the chemical equilibrium constant, goethite and hematite are the major iron oxide minerals formed in the redoximorphic features. Keys to Soil Taxonomy is a better soil classification system than the others to recommend the differences of the hydrological conditions for the seasonally saturated soils with anthraquic conditions. Houhu, Hsinwu, and Lungchung series were classified as Typic Plinthaquults, Typic Plinthudults, and Plinthic Paleaquults based on Keys to Soil Taxonomy, respectively.