Aerobic surface soil layer

Decomposition of organic matter is rapid under aerobic conditions, which predominate in the water column and surface soil layers. These decomposition rates typically decrease with soil depth. Carbon dioxide fluxes from soil into the atmosphere increase with lowered water table depth or drained soil conditions. An enhanced rate of soil respiration in response to both drainage and nutrient enrichment has been previously reported for organic soils. For example, the interaction between hydrology and nutrient enrichment is graphically shown for the Everglades soils in Figure 5.46 (DeBusk and Reddy, 2003). 

Oxygen supply in wetlands is restricted to the water column and to a thin layer of surface soil. Oxygen is also transported by wetland macrophytes to their root zone, resulting in the creation of aerobic conditions on root snrfaces (see Chapters 3 and 6 for a detailed discussion on aerobic-anaerobic interfaces in wetlands). 

The supply of oxygen to the soil surface and the consumption of oxygen in the soil are recognized as the major regulators determining the thickness of the aerobic soil layer. 

At many sites, the subsurface environment will be anoxic or even anaerobic due to the activity of aerobic and facultatively anaerobic bacteria in the surface layers of the soil. It is therefore essential to take into consideration the extent to which anaerobic degradation may be expected to be significant. Reactions may take place under sulfidogenic or methanogenic conditions, and the occurrence of sulfate at sites containing building material waste and the metabolic versatility of sulfate-reducing bacteria makes them particularly attractive. 

AU fungi are heterotrophs and are predominantly aerobic, and hence are usually found at or near the surface of any organic matter upon which they may be growing. In the soils they are limited largely to the plow layer, or A-horizon, but this is because an abundant supply of energy sources is available only in this upper region [2]. 

The physical location of Fe(III) and Mn(IV) oxides in the soil profile can also influence their utility as electron acceptors by microorganisms. As discussed before, oxidized forms of iron and manganese are insoluble/immobile and they are usually present in oxidized/aerobic portions of the soil. Thus, the decomposition of organic matter with Fe(III) and Mn(lV) as electron acceptors will be restricted to this zone. If this layer is disrupted as a result of organic matter loading, oxygen depletion may promote the reduction of Fe(III) and Mn(IV) oxide precipitates by facultative bacteria. Microbes growing directly on Fe(III) and Mn(IV) oxide mineral surfaces may solubilize and utilize them as electron acceptors. 

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