Aerobic-anaerobic interface plant roots

Once the reductants present in the oxidized root zone are depleted, subsequent oxidation of these compounds will depend on the diffusive resupply across the aerobic-anaerobic interface. The ability of wetland plants to transport oxygen has attracted biologists and engineers to include this process into designing constructed wetlands for wastewater treatment. 

In wetlands N2 fixation can occur in the water colnmn, in the aerobic water-soil interface, in the anaerobic soil bulk, in the rhizosphere, and on the leaves and stems of plants. Phototrophic bacteria in the water and at the water-soil interface are generally more important than non-photosynthetic, heterotrophic bacteria in the soil and on plant roots (Buresh et al, 1980 Roger 1996). The phototrophs comprise bacteria that are epiphytic on plants and cyanobacteria that are both free-living and epiphytic. A particularly favourable site for cyanobacteria is below the leaf surface of the water fern Azolla, which forms a very efficient symbiosis with the cyanobacterinm Anabaena azollae. This symbiosis and those in various leguminous plants have been exploited in traditional rice prodnction systems to sustain yields of 2 to 4 t ha of grain withont fertilizer for hnndreds of years. 

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. 

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. 

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