Waterlogged Mineral Soils

Waterlogged mineral soils are those in which part or the whole soil profile is saturated for a sufficient period of time to create distinctive gley horizons in the profile. These soils can be sandy, loamy, or clay soils. The other most commonly referred term is flooded soils. We confine our discussion in this group primarily to mineral soils. The distinctive characteristics of these soils are 

FIGURE 3.13 Concentration of methane and sulfate with depth in a flooded system. (Redrawn from D Angelo and Reddy, 1994b.) 

FIGURE 3.14 Micrograph of a cross-sectional view of aerenchyma tissue of plants. (Courtesy Brix, H., University of Aarhus, Denmark.) 

As the water table is lowered as a result of drainage, the reduced cations and other base cations present in soil pore water are transported downward. The center of the soil aggregate in the drained portion of the soil may remain anaerobic, whereas the surface of the soil aggregate becomes aerobic, as a result of oxygen diffusion. The dissolved Fe and Mn + are transported through the soil matrix until they encounter another aerobic zone, where they are oxidized and precipitated. This process is generally heterogeneous, and leads to soils with a matrix of one color and mottles of another (Schwertmann, 1993). 

FIGURE 3.15 (See color insert following page 392.) Soil profiles showing (a) oxidized and (b) reduced forms of soils. 

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What are some distinct characteristics of waterlogged mineral soils ... 

Mineral soils that are waterlogged may have secondary minerals or iron and manganese concretions. 

Laanbroek, H. J. (1990). Bacterial cycling of minerals that affect plant growth in waterlogged soils a review. Aquat. Bot. 38, 109-125. 

Ferrous iron (Fe " ) appears later than Mn in soils that have been subjected to prolonged waterlogging because, as Table 7.1 shows, the reduction potential of Fe in oxides (and probably in many other soil minerals as well) is lower than that of Mn( + 3,d-4) in Mn oxides. Since Fe +, like Mn +, is rather soluble, it can reach appreciable concentrations in poorly aerated soil solutions. The introduction of dissolved oxygen causes rapid oxidation of Fe " and precipitation of ferric hydroxide if the solution pH is much higher than 6. The rate law of oxidation of dissolved Fe is known to be... 

Weathering occurs because rocks and minerals become exposed to physical and chemical conditions different from those under which they formed. Weathering in the pedosphere takes place close to the surface where the overburden pressure and temperature are low, and H2O is plentiful. Because most soils harbor large numbers of microorganisms, their metabolic activity results in the consumption of O2 and production of CO2. The partial pressure of CO2 in soil may be 10-100 times greater than that of the atmosphere (Holland, 1978). Under waterlogged conditions, where gas diffusion between the soil and the atmosphere is impeded, the oxygen partial pressure may approach zero and the soil solution may contain CO2, CH4, and H2S. Under these conditions, Fe and Mn may become soluble. 

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