Soil Oxygen Content

FIGURE 6.13 Air reservoirs in soil using copper tubing and rubber stoppers, and an oxygen analyzer system showing specially machined analyzer cell. Bottom view shows a specially machined cell on the left and an oxygen electrode on the right (Faulkner et al., 1989). 

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FIGURE 6.9 Redox potential and soil oxygen content after short-term flooding (Meek and Grass, 1975). 

The diffusion of oxygen in air can be described by Pick s first law. Redox potential, which measures electron activity in soils, can be nsed as an indicator of soil aeration status. Aerated soils have Eh greater than +300 mV. Redox potential or Eh below +300 mV indicates little or no presence of oxygen. In addition to redox potential measurement, ODR and soil oxygen content can be used to determine the aeration status of wetland soils. 

Patrick, W. H., Jr., R. D. DeLaune, and R. Bugler. 1973. Soil oxygen content and root development of cotton in Mississippi River alluvial soils. Louisiana State University, Agricultural Experiment Station Bulletin No. 673. pp. 28. 

As mentioned before, soil is a physically, chemically, and biologically complex system. Factors that affect corrosion in soil, in addition to specific ions, are resistivity of soil, oxygen content, and acidity. Field measurements of soil resistivity are covered in ASTM G 57, Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method, which is the most widely used test, and using the proper meter produces accurate and reproducible results. Conducting field measurements of soil pH is covered in ASTM G 51, Test Method for pH of SoU for Use in Corrosion Testing. The corrosion resistance of lead and its alloys depends mainly upon the presence of silicate, carbonate, and to a lesser extent sulfates, in contributing to the passive film formation. 

Factors affecting the corrosion of pipe include soil resistivity, moisture content of the soil, pH, permeability of soil to moisture and air, soluble ion content of the soil, oxygen content, and the presence of corrosion-activating bacteria. 

Results from other studies support the rapid degradation of methyl parathion in soils with a high water (i.e., low oxygen) content (Adhya et al. 1981, 1987 Brahmaprakash et al. 1987). Experiments in flooded and nonflooded soils showed that the redox potential affected both the rate of degradation and the transformation products of methyl parathion (Adhya et al. 1981, 1987). Transformation to volatile products was suggested by Brahmaprakash et al. (1987) as the reason that significant amounts of " C from labeled methyl parathion could not be accounted for, especially in flooded soils. 

The effects of physical factors such as temperature, soil moisture content, pH and oxygen availability on microbial survival and activity in soil are well documented (for a review, see Ref. 90). It is also widely acknowledged that these same factors may also influence plant growth and can therefore be presumed to influence both root exudation and rhizosphere microbial populations. 

Two bioassays are employed to evaluate the effect of samples on terrestrial life forms. For gas samples, the plant stress ethylene test is presently recommended. This test is based on the well-known plant response to environmental stress release of elevated levels of ethylene (under normal conditions plants produce low levels of ethylene). The test is designed to expose plants to various levels of gaseous effluents under controlled conditions. The ethylene released during a set time period is then measured by gas chromatography to determine toxicity of the effluent. For liquid and solid samples, a soil microcosm test is employed. The sample is introduced on the surface of a 5 cm diameter by 5 cm deep plug of soil obtained from a representative ecosystem. Evolution of carbon dioxide, transport of calcium, and dissolved oxygen content of the leachate are the primary quantifying parameters. 

This process consumes oxygen and produces C02. As a result, the oxygen content of air in soil may be as low as 15%, and the carbon dioxide content may be several percent. Thus, the decay of organic matter in soil increases the equilibrium level of dissolved C02 in groundwater. This lowers the pH and contributes to weathering of carbonate minerals, particularly calcium carbonate. 

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