Quantifying Soil Hydromorphology of a Rice‐Growing Ultisol Toposequence in Taiwan

Seasonally irrigated flooding and fluctuation of groundwater level control the redoximorphic features, saturation, and reduction of rice‐growing Ultisols in different landscape positions of Taiwan. The objective of this study was to quantify soil hydromorphology of an Ultisol toposequence with different saturation and reduction conditions. Three study soils, along a toposequence at the red earth terrace, were selected for monitoring of the water table, matric potential, and redox potential (Eh) at various soil depths in 1996 and 1997. The three soils are Houhu (Typic Plinthaquult) in the toeslope, Hsinwu (Typic Plinthaquult) in the footslope, and Lungchung (Plinthaquic Paleudult) in the lower backslope. The Houhu and Lungchung soils were seasonally flooded for rice (Oryza sativa L.) production and produced perched water tables from March to October. The Hsinwu soil, where rice has not been planted since 1991, still revealed seasonally high water levels and perched water tables during the study period. Redox concentrations originally occurred as soft masses and concrete nodules associated with seasonally high water levels, but irrigation and drainage processes also influenced the development of redoximorphic features. The abundance of Fe–Mn concretions and Fe depletions increased as cycling of oxidation and reduction conditions increased in rice production. The durations of saturation and reduction in the Btv horizons of the Houhu soil in the toeslope position were more than 80% of the year, and the soil had ≈10% of Fe–Mn concretions. The Btv horizons of the Hsinwu soil in the footslope position were saturated for 50% of the year and reduced for 25% of the year, and the soil had ≈20% Fe–Mn concretions. The Btv horizons of the Lungchung soil in the lower backslope position were saturated for 40% of the year and reduced for only ≈10% of the year, and the soil had 15% Fe–Mn concretions. The Houhu and Hsinwu soils had anthraquic conditions and the Lungchung soil, with less reduction, was proposed as having oxyaquic conditions as defined in U.S. soil taxonomy.