Aerobic-anaerobic interface zones

Pimenov, N.V., Rusanov, I.I., Yusupov, S.K., Fridrich, j., Lein, A., Wehrli, B. and Ivanov, M.V. (2000) Microbial processes at the aerobic-anaerobic interface in the deep-water zone of the Black Sea. Microbiology 69, 435 48. 

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). 

Once the reductants present in the oxidized root zone are depleted, subsequent oxidation of these compounds will depend on the diffusive resupply across the aerobic-anaerobic interface. The ability of wetland plants to transport oxygen has attracted biologists and engineers to include this process into designing constructed wetlands for wastewater treatment. 

Nitrification-denitrification reactions are the major pathway for N loss from wetland ecosystems to the atmosphere. The process involves a series of sequential microbial processes that include mineralization of organic nitrogen to ammonium, oxidation of ammonium to nitrate, and denitrification of nitrate to nitrous oxide or dinitrogen gas. Nitrification-denitrification in wetlands occurs primarily in two zones the aerobic-anaerobic interface at the surface of the flooded soil or sediment, and the oxidized rhizosphere of wetland plants (Figure 16.6). 

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. 

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. 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

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). 

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. 

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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