Wetlands soil conditions

Fungi, which are active in upland environments, cease to exist in wetland soils. This is primarily due to the absence of oxygen and alteration in soil pH (acid to neutral) under anaerobic conditions. Overall, microbial biomass decreases under saturated soil conditions. The metabolic activities of anaerobic bacteria depend on alternate electron acceptors, such as oxidized forms of nitrogen, iron, manganese, and sulfur. Under wetland soil conditions, rates of many microbially mediated reactions decline, and some reactions may be eliminated and replaced by new ones. New microbial reactions are involved in the reduction of oxidized compounds during respiratory processes, resulting in the production of reduced compounds. 

The saturated soils that occur during wetland, or lowland, rice cultivation give rise to a set of physical, chemical, and biological properties that are quite different from upland soils. Rice is the only major row crop produced under flooded-soil conditions and the absence of air-filled pores along with reduced soil-atmosphere interactions result in an almost entirely different set of processes than those occurring in upland cropping systems. 

The principal distinguishing feature of wetland soils is that they develop under predominantly anoxic conditions. Although anoxia is also sometimes found in other ecosystems, it prevails in wetlands and dominates soil properties. Because of the very large organic matter content of some wetland soils, a rough separation into organic and mineral types based on organic matter content is a useful delineation. 

Of wetland plants, rice has been studied the most extensively, and nitrogen has been the most extensively studied element. In this section the rates at which rice roots can absorb nitrogen are discussed and whether this is affected by the morphological and physiological adaptations to anoxic soil conditions. 

Terrestrial/lotic linkages in the form of fast near-surface flowpaths, such as overland flow, require certain soil conditions together with rainfall patterns. Where these are met, a hillslope terrestrial signal, with some resemblance to a wetland signal, is transmitted to the stream channel. 

Some plants can induce a release of O2 at the surface of roots and thus an increase in pO2 in the rhizosphere. This process is known to occur in an adaptation of plants to submerged soil conditions, as in wetland plants, and is well documented for lowland rice Oryza sativa). To cope with anoxic or hypoxic conditions occurring in the soil or sediments, such plants have evolved a specialized structure, the aerenchytna, which conducts O2 to root tissues from the atmosphere and the shoots. The portion of O2 that is not consumed in the roots for respiration leaks through the root apoplasm (cell walls) and ultimately into the rhizosphere. Two pieces of evidence support this phenomenon. First, an increase in redox potential... 

Certain areas of the environment are extremely sensitive to certain chemicals, or may present enormous cleanup problems. Wetlands, and rivers and lakes that serve as water supplies, are areas of the environment that are highly sensitive to chemical contamination. The problems created by these environmental features may be exacerbated by topography and soil conditions which speeds the spread of chemical contamination. 

High moisture contents in alkaline soils have been shown to increase the availability of Mo (Nayyar, 1972). Poonamperuma (1972) observed an increase in soluble-Mo concentration under wetland rice conditions. Flooding of soils increases the availability of Mo. Soils in the tropics that remain flooded for considerable periods have relatively high contents of Mo (Lopes, 1980), because a high moisture content in the soil reduces ferric iron to the ferrous form, which favors fixation of Mo (Jones, 1956). 

Hydrophytic vegetation (wetland plants adapted to saturated soil conditions)... 

FIGURE 3.5 Wetland soils under different hydrologic conditions. 

Upland soils can be transformed into wetland soils as a result of excessive rainfall, poor drainage, and high oxygen demand in the soil. Under these conditions, oxidized forms are reduced as a result of the respiratory requirements of anaerobic bacteria. Similarly, when wetland soils are drained, they function as upland soils, and under these conditions many of the reduced compounds are oxidized by either chemical or biochemical reactions. 

In upland/drained soils, oxidized forms of chemical species dominate the system, whereas in wetland soils reduced forms dominate the system (Figure 3.6). During flooding or in the absence of molecular oxygen, the oxidized forms are converted into reduced forms, through several microbially mediated catabolic processes. Under drained conditions, many of the reduced forms are converted into oxidized forms through chemical and biological processes. Presence of reduced forms indicates soil wetness or anaerobic soil conditions, which are used as indicators of hydric soil identification. 

When oxygen is limited, as is the case in wetland soils, unique conditions are set in motion that differentiate wetlands from uplands in snch a way as to increase organic matter in the soil, which may even resnlt in the formation of thick layers of peat, and a change in the distribntion of microorganisms (with anaerobic bacteria being more active) and chemical properties of wetland soil. 

Plants have adapted to the harsh anaerobic conditions of wetland soils. Development of aerenchyma tissnes permit oxygen pumping to the roots, to support root respiration and aerobic bacteria in the root zone. 

Anaerobic conditions drive the dominant processes in wetland soils ... 

It is evident that oxygen-, nitrate-, sulfate-reducing and methanogenic conditions have a profound effect on various biogeochemical properties regulating organic matter decomposition in wetland soils (Table 5.14). A review on the comparison of microbial dynamics in marine and freshwater system as influenced by the availability of electron acceptors is presented by Capone and Kiene... 

Selected Biogeochemical Properties Related to Soil Organic Matter Decomposition in Wetland Soils Incubated under Various Redox Conditions... 

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