Reduced anaerobic soil layer

The thickness of this oxidized soil layer and associated processes are discnssed in Chapter 5. Ecologically, the thickness of this oxidized layer has a significant effect on regnlating nitrogen reactions (as discnssed in Chapter 8) and functions as a sink for reduced compounds diffusing from the underlying anaerobic soil layer. 

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... 

Ammonium flux from anaerobic soil layer is governed by the (1) concentration gradient established as a result of ammonium consumption in the aerobic zone due to nitrification and ammonia volatilization, (2) ammonium regeneration rate in the anaerobic soil layer, (3) adsorption coefficient for ammonium, (4) soil CEC, (5) intensity of soil reduction and accumulation of reduced cations, (6) bioturbation at the soil-floodwater interface, and (7) soil porosity. 

Several factors influence the thickness of an oxidized surface layer. These include the oxygen concentration of the floodwater, the soil organic matter content, the amount of reduced compounds in the reduced or anaerobic soil zone, the photosynthetic activity of periphyton and aquatic macrophytes, and the bioturbation by macroorganisms. 

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. 

In flooded or waterlogged soils, as for example, in a continuously flooded rice paddy, anaerobic (reducing) conditions exist in the soil below the surface, and hence nitrification of ammoniacal nitrogen does not occur in this layer. Therefore, when ammonia, ammonium salts, or urea (which hydrolyzes in the soil to... 

Coprecipitation of copper can occur within the larger mass of ferric oxyhydroxide. Most mineral soils contain significant amounts of active iron, up to 1%. The iron can be reduced to its soluble ferrous form when the system becomes anaerobic or it can be oxidized to the insoluble ferric form when the system is oxidized. As the ferrous ion is precipitated into more insoluble ferric form, it forms coatings on clay and silt particles often as a ferric oxyhydroxide layer, which can potentially coprecipitate trace metals. This is an important sink apparently for copper as well. The copper trapped in this material is released only under reducing conditions. 

Reduced iron Poorly drained soils (e.g., subsurface layer with a high bulk density causes waterlogging, or a very fine textured soil where permeability is very low), anaerobic environmental conditions... 

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