Importance of oxygen diffusion rate in soil
Answers
Gases, including oxygen, move in the soil according to diffusion laws. The main parameter related to gas diffusion in soil is the gas diffusion coefficient of the soil (Dp), which is a property of the medium and the gas under study and depends upon the texture, structure, distribution, size and connectivity of the pores as well as their tortuosity (Schjonning et al., 1999; Moldrup et al., 2001). Compaction and water saturation of soils are the main barriers to soil oxygen transport, water being a more effective barrier (Papendick and Runkles, 1965; Moldrup et al., 2000a; Neale et al., 2000). The diffusion of gases in water is slower than their diffusion in air by a factor of 104 (Call, 1957; Moldrup et al., 2000a; 2004; Thorbjorn et al., 2008).
The oxygen present in the atmosphere of the soil is used in different processes and may be limited by flooding or by soil compaction, affecting plant growth (Hillel, 2003; Lal and Shukla, 2004). There are three conditions that relate soil oxygen concentration to plants. In normoxia (normal conditions) respiration is aerobic, metabolism proceeds normally and most ATP is produced by oxidative phosphorylation. Under hypoxia oxygen concentration begins to be limiting for ATP production by the oxidative phosphorylation pathway; while under anoxia ATP is generated by glycolysis since no oxygen is available (Saglio et al., 1988; Horchani et al., 2011).
The effects of oxygen deficiency in the soil may be either direct or indirect; direct effects deal with the lack of oxygen for plant processes, while indirect effects are related to the physical and chemical properties of the soil that use oxygen (Lal and Shukla, 2004). Direct effects restrict processes such as plant respiration, water and nutrient absorption; they produce a change in root metabolism towards fermentation. A critical and limiting value of soil oxygen diffusion for crops is variable and is called the oxygen diffusion rate or ODR (Glinski and StÄpniewski, 1985), which refers to the availability of oxygen for plants. It has been found that soil oxygen diffusion rates below 20 x 10-8 g O2 cm-2 min-1 do not permit plant emergence (Stolzy and Latey, 1964; Lal and Shukla, 2004). However, plants have the capability to adapt to a deficiency of oxygen in the soil by mechanisms such as developing aerenchyma or by hormonal adjustment (Armstrong et al., 1994).
Indirect effects are produced when anaerobic conditions in soil produce denitrification and reductions in some elements such as Mn, Fe, and sulfate; indirect effects may be divided into chemical and biological. At the chemical level, organic compounds from the decomposition of organic matter, such as ethylene, phenolic acids and acetic acid are produced, which are toxic to plants (Hillel, 2003; Lal and Shukla, 2004); low oxygen concentration also increases the solubility of calcium carbonate, affecting the solubility of Fe and inducing ferric chlorosis in trees. Manganese is also reduced in soils with poor aeration; when the reduced form accumulates it generates toxic conditions for plants (Taylor and Ashcroft, 1972). At the biological level a decrease in oxygen produces anaerobic decomposition of soil organic matter (SOM), facilitating, for example, the generation of ammonia from protein decomposition instead of forming nitrates as would occur under normal conditions (Taylor and Ashcroft, 1972; Lal and Shukla, 2004).
The aim of this chapter is to discuss the theory of gas diffusion, oriented in oxygen movement, and the soil properties that affect the movement of oxygen in soil.
Answer:
For a better understanding of the causes and effects of soil aeration, two quantitative indicators that reflect oxygen availability and redox state in soils are oxygen diffusion rate (ODR) and redox potential (E H).
Explanation:
The rate of oxygen diffusion to a platinum wire buried in the ground, where the oxygen is polarographically decreased, is used to calculate ODR. Amperometric detection of the platinum wires decreases the current intensity and its relationship to the oxygen flu
Since these indicators are very geographically and temporally changeable, reliable in situ monitoring requires continuous measurements and sufficient numbers of repeats. Here, we provide a brand-new, entirely automated recording technique for in situ measurements in which ODR and EH are monitored at the same platinum electrode. By shortening the polarisation duration and adding a recovery period between two successive measurement cycles, the contradiction between electrode polarisation for ODR and the ensuing biassed EH measurements is resolved. Accurate EH measurements are ensured by the reduced polarisation time.
Additionally, it causes ODR measurements to be somewhat overstated, however, this may be fixed before data processing. The recovery interval limits the EH-ODR data pairs' temporal resolution to 8 hours. We utilise measurements from a field experiment in Zürich, Switzerland, to demonstrate how to use the system. Although ODR was significantly more responsive to precipitation, ODR curves at various depths approximately paralleled the corresponding curves of O2 concentration in soil air. Declining EH was a required but insufficient factor for low ODR. Instead of ODR, EH paralleled the O2 content in soil air. With just minimal labour requirements, the fully automated technology enables time series of duplicate measurements in multifactorial field research. It could be especially well suited for research on the impacts of soil tillage, compaction, and irrigation, where soil variables linked to structure, such as porosity, gas permeability, and soil aeration, are crucial.
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