Surfaces wetland rice soils

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. 

Because reductants are present in most water-saturated soils or sediments and oxygen is relatively unavailable, the Eh declines as the water moves into the subsurface. The decline may be from oxic to anoxic sulfidic or nonsulfidic levels (Table 11.5). The rate and extent of Eh decline with distance from the surface depends on the availability and reactivity of sediment organic matter and other reductants. In the sediments of flooded rice paddies, wetlands, estuaries, and shallow lakes, which may be especially rich in fresh organic matter, the redox front or intetface (also termed a redox barrier or boundary by some), which is the zone of abruptly changing Eh values, may be only a few millimeters or centimeters thick. 

Some plants can induce a release of O2 at the surface of roots and thus an increase in pO2 in the rhizosphere. This process is known to occur in an adaptation of plants to submerged soil conditions, as in wetland plants, and is well documented for lowland rice Oryza sativa). To cope with anoxic or hypoxic conditions occurring in the soil or sediments, such plants have evolved a specialized structure, the aerenchytna, which conducts O2 to root tissues from the atmosphere and the shoots. The portion of O2 that is not consumed in the roots for respiration leaks through the root apoplasm (cell walls) and ultimately into the rhizosphere. Two pieces of evidence support this phenomenon. First, an increase in redox potential... 

Wetlands and rice paddies are major sources of methane and emit approximately up to 50% of annual methane to the atmosphere. Microbial pathways involved in production of methane from wetlands and aquatic systems are discussed in detail in Chapter 5. Until all electron acceptors (oxygen, nitrate, iron and manganese oxides, and sulfate) with higher reduction potentials are exhausted, no methane will be produced. Potentially all these electron acceptors can be present in the same soil profile with electron acceptors with higher reduction potentials utilized in surface layers and the electron acceptors with lower reduction potentials utilized in lower depths (Figure 10.33). 

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