Oxidized soil-floodwater interface

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

The dominant and most reported component of root plaques is various oxidized compounds of iron. Microscopic observations of root plaques show a highly heterogenous morphology composed mostly of an amorphous material dispersed throughout nodules (50-300 nm in diameter), needles (50-100 nm in length), and filaments with variable lengths. This iron plaque formation on roots results from diffusion of Fe + toward the root zone in response to concentration gradients at the interface (similar to those observed at the soil-floodwater interface). The oxidized rhizosphere functions as a sink for Fe + and other reduced substances. 

Wetland soils and aquatic sediments are uniquely characterized by aerobic and anaerobic interfaces at the soil-floodwater interface or in the root zone of wetland plants (see Chapter 4 for details). Aerobic oxidation of Fe(II) and Mn(ll) is restricted to the thin aerobic layer at the soil-floodwater interface or in the root zone. Thus, the extent of aerobic oxidation of Fe(ll) and Mn(ll) is dependent on the flux of dissolved species from anaerobic soil layers to aerobic zones. At circumneutral pH, concentrations of dissolved Fe(ll) and Mn(II) are very low, thus restricting flux into aerobic portions of the soil. At this pH level, the majority of Fe(II) and Mn(ll) compounds are present as immobile solid phases such as FeCOj, MnCOj, FeS2, Fe(OH)2, and Mn(OH)2. These compounds can be oxidized only when the water table is lowered, thus exposing top portion of the soil profile to aerobic conditions. 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

The second reason is that as soil gases are transported to the surface they must pass through the thin aerobic soil-floodwater interface where methane oxidation occurs. 

In paddy soils, oxygen is introduced through the floodwater and consumed at the soil-water interface, and to some extent oxygen is also introduced through the plants into the root zone. Manganous manganese and ferrous iron formed in the anaerobic zone of surface layer diffuses in two directions (1) to the surface layer, where it is oxidized and (2) to the subsurface layer, where it is oxidized. 

The accumulation of reduced compounds in the anaerobic or reduced soil layer results in the establishment of concentration gradients across the aerobic-anaerobic interface. The concentration of reduced compounds is usually higher in the anaerobic layer, which results in upward diffusion into the aerobic soil or floodwater where they are oxidized. Similarly, some of the dissolved oxidized compounds diffuse downward, that is, from the floodwater or aerobic soil layer into the underlying anaerobic soil layer, where they will be reduced. For example, the steep gradients in ammonium concentrations in the soil profile are due to diffusion into aerobic soil layer or floodwater, and subsequent oxidation to nitrate (Figure 3.11). The nitrate formed diffuses downward into the anaerobic soil layer and is consumed as electron acceptor by microorganisms (Figure 3.12). Similar oxidation... 

---