Floodwater soil zones

The penetration of the O2 into the soil depends on its rate of consumption in aerobic processes and its rate of transport by mass flow and diffusion, and in the floodwater-soil interface, mixing by burrowing invertebrates. Various aerobic processes take place in these oxygenated zones. 

Eive zones can be distinguished the floodwater standing on the soil per se, the floodwater-soil interface, the anaerobic bulk soil, the rhizosphere, and the subsoil. These are to some extent continuous with each other, and they are certainly linked so that the function of the system as a whole is greater than the sum of its parts. But they provide convenient boundaries for discussion. 

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. 

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. 

In paddy soils, oxygen is introduced through the floodwater and consumed at the soil-water interface, and to some extent oxygen is also introduced through the plants into the root zone. Manganous manganese and ferrous iron formed in the anaerobic zone of surface layer diffuses in two directions (1) to the surface layer, where it is oxidized and (2) to the subsurface layer, where it is oxidized. 

Distinct Eh gradients are present (i) at the soil-floodwater interface, (ii) at the root zone of wetland plants, and (iii) aronnd the soil aggregates in drained portions of wetlands dnring low water table depths. 

The thickness of the aerobic layer varies from <1 mm to 3 cm. In relation to anaerobic soil volume, the aerobic soil volume at the soil-floodwater interface is small. However, this thin aerobic interface in the proximity of anaerobic soil is key to many unique biogeochemical processes functioning in wetlands. The differentiation of a wetland soil or sediment into two distinct zones as a result of limited oxygen penetration into the soil was first described by Pearsall and Mortimer (1939) and Mortimer (1941). 

The dominant and most reported component of root plaques is various oxidized compounds of iron. Microscopic observations of root plaques show a highly heterogenous morphology composed mostly of an amorphous material dispersed throughout nodules (50-300 nm in diameter), needles (50-100 nm in length), and filaments with variable lengths. This iron plaque formation on roots results from diffusion of Fe + toward the root zone in response to concentration gradients at the interface (similar to those observed at the soil-floodwater interface). The oxidized rhizosphere functions as a sink for Fe + and other reduced substances. 

Pathway 6 shows dinitrogen fixation, which is the reduction of atmospheric, inert nitrogen to ammonia that plants and microbes can then use. It occurs in the water column, soil-floodwater interface, root zone, and anaerobic soil in wetlands. It is carried out by free-living bacteria (Clostridium), bacteria living in symbiotic association with plants (Rhizobium), cyanobacteria (Anabaena), and periphyton. 

Because of limited oxygen availability in wetland environments, nitrification is restricted to the (1) aerobic water column, (2) aerobic soil-floodwater interface, and (3) aerobic root zone (Figure 8.34). In all these zones, nitrification is supported predominantly by chemoautotrophic bacteria, which use oxygen as their electron acceptor and ammonium as their energy source. Nitrification in these zones is often limited by the availability of ammonium. 

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. 

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). 

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. 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

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