Aerobic 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... 

Conversely, high levels of nitrate in water column in aerobic soil layers causes a downward diffusion of nitrate, where it is rapidly reduced by microbes. 

FIGURE 6.21 Schematic showing oxidation of reduced compounds in the aerobic soil layer. 

Heterotrophic microbial respiration in aerobic soil layers, where oxygen is nsed as an electron acceptor ... 

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. 

Influence of Added Carbon (as Rice Straw) to a Crowley Silt Loam Soil on the Thickness of Aerobic Soil Layer as Estimated by Redox Profile ... 

FIGURE 6.23 Schematic representation of burrowing by benthic invertebrates in sediments. Aerobic soil layer around irrigated burrow environment. 

The aerobic soil layer functions as an effectiv e sink for reductants diffusing from the underlying anaerobic layer. Both chemical and biological processes regulate the consumption of oxygen in the aerobic layer. Oxygen consumption by anaerobic soils is best described by a two-phase hrst-order reaction (Figure 6.24 Reddy et al., 1980) ... 

Exchange of dissolved nitrogen species between the soil and water column support several nitrogen reactions. For example, nitrification in aerobic soil layer is supported by ammonium flux from the anaerobic soil layer. Similarly, denitrification in anaerobic soil layer is supported by nitrate flux from the aerobic soil layer and water column (see Chapter 14 for discussion on transport processes). 

FIGURE 8.54 Schematic showing the flux of ammonium from anaerobic soil layer to aerobic soil layer and overlying water column, and nitrification of ammonium iu aerobic soil layer. 

Nitrate flux from the aerobic portion of the soil is controlled by (1) labile organic carbon supply in anaerobic portion of the soil, (2) thickness of aerobic soil layer, (3) water column depth, (4) mixing and aeration in the water column, (5) nitrate concentration, and (6) temperature. The flux of nitrate from the floodwater to underlying soil increases with an increase in temperature (Figure 8.59). At low temperatures, nitrate can diffuse to deeper layers into anaerobic zones. Under these conditions, it is likely that nitrate may play significant role in ANAMOX reactions as temperature optima for this reaction is between 10 and 15°C, as compared to denitrification that has temperature optima around 30°C (see Figures 8.39 and 8.47). 

Phosphine is not stable under aerobic environments. The aerobic soil layer can function as the sink for PH3 through abiotic oxidation PHj + 2O2 = H3PO4. Trace amounts of PHj have been detected in wetlands, paddy fields, and sewage treatment systems (Devai et al., 1988 Devai and DeLaune, 1995 Morton and Edwards, 2005). 

The reduction and dissolution of iron and its reprecipitation to form ferrous minerals are thought to be the dominant processes controlling phosphorus solubility in anaerobic systems. Soil pore water profiles shown in Figure 9.56 show that the concentrations of soluble phosphorus and dissolved iron are low in the water column and increase with depth. Similar patterns in these profiles suggest that the solubilities of phosphorus and iron are strongly coupled. In the water and in the aerobic soil layer, iron is in the immobile ferric form, which reacts with phosphate and precipitates. Thus, under aerobic conditions, oxidation and precipitation of iron control phosphorus solubility and limit... 

Mobile pools of iron and manganese are present in water-soluble or dissolved forms in soil pore water. Immobile forms include solid phases such as insoluble precipitates and mineral phases (amorphous and crystalline forms) present both in aerobic and anaerobic soil layers. The flux of dissolved iron and manganese is typically from anaerobic soil layers to aerobic soil layers, where it is oxidized to insoluble precipitates. This results in the establishment of concentration gradients across the aerobic-anaerobic soil interface. Mobilization is also regulated by pH and CEC. Manganese is more soluble in moderately acidic conditions (between pH 5 and 6) than iron. 

In snmmary, the mobility of Fe(II) and Mn(ll) is controlled by (1) diffusion and mass flow to aerobic soil layers, where they are oxidized and precipitated (2) diffusion and mass flow toward plant roots, where they are precipitated as Fe(lll) and Mn(lV) oxides and deposited as encrnstations around roots and (3) mass flow with downward-percolating water to the oxygenated snbsoil layer, where they are oxidized and precipitated. 

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