TRANSIENT-STATE OXYGEN DIFFUSION IN SOIL

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Abstract
Experimental work showed that the rate of O2 consumption after wetting air-dry Brookings silty clay loam could be adequately described by the function a + bekt, where a, b, and k are constants and t is time. It was shown that the consumption rate was not greatly influenced by variations in moisture content in the range of low soil -moisture tension. A mathematical description of the diffusion process, based on classical diffusion theory which was modified to include the activity factor, was developed for a soil layer having an impermeable boundary, while the other face is exposed to an atmosphere of constant O2 concentration C0 The initial O2 concentration in the soil is also C0. Soil oxygen concentrations were monitored at depths of 15, 75, and 135 cm in Brookings soil with the platinum electrode for approximately 1 week after wetting air-dry soil. The concentration decreased rapidly and reached a minimum at all 3 soil depths during the early period after wetting. For the remainder of the experiment, the concentration increased slowly at the 15-cm soil depth; a time lag occurred, however, before a slow increase was noted at the 2 deeper soil depths. Theoretical O2 concentrations were computed for the 15-cm soil depth, using a diffusion coefficient calculated from the cube root of the soil porosity, to show the relative effect of changes in porosity on soil O2 relationships. The work here demonstrates that low O2 concentrations could exist temporarily under certain conditions in an otherwise seemingly well-aerated soil.