Transformation Mechanisms of Major Nutrients and Metals in Wetlands

The three most important physicochemical properties of the soil that are affected by flooding are pH value, ionic strength, and oxidation-reduction potential (Eh or redox potential) (Patrick et al., 1985).

Wetland soils and overlying waters occur in a wide range of pH values. Organic soils in wetlands are often acidic, particularly in peatlands in which there is little groundwater inflow. On the other hand, mineral soils often have more neutral or alkaline conditions (Mitsch and Gosselink, 2000). The pH of most soils tend to change toward the neutral point after flooding, with acidic soils increasing and alkaline soils decreasing in pH. Increases as great as 3 pH units have been measured in some acid soils. The equilibrium pH for waterlogged soils is usually between pH 6.5 and 7.5 (Patrick et al., 1985). The tendency of soils of low pH to decrease in acidity and for soils of high pH to increase in acidity when submerged indicates that the pH of a submerged soil is buffered around neutrality by substances produced as a result of reduction reactions. Among the more likely compounds involved in buffering the pH of waterlogged soils are Fe and Mn compounds in the form of hydroxides and carbonates, and carbonic acid (Patrick et al., 1985). For some organic soils high in iron content, submergence does not always increase pH (Ponnamperuma, 1972). Peat soils often remain acidic during submergence through the slow oxidation of sulfur compounds near the surface, producing sulfuric acid and the production of humic acids and selective cation exchange by Sphagnum moss (Mitsch and Gosselink. 2000).

Flooding the soil causes an increase in the concentration of ions in the soil solution, although the increase may not persist throughout the growing season. In slightly acid and acid soils, the reduction of insoluble Fe, and possible Mn compounds, to more soluble forms accounts for much of the increase in cations. In neutral to slightly alkaline soils, Ca²⁺, and Mg²⁺ in the soil solution make significant contributions to the ionic strength. Ferrous and manganous ions produced through reduction reactions displace other cations from the exchange complex to the soil solution (Patrick et al., 1985).