OXYGEN CONCENTRATION AND REDOX POTENTIAL GRADIENTS IN UNSATURATED MODEL SOIL AGGREGATES

Anoxic microsites in unsaturated soils are often discussed in the context of denitrification or favorable conditions for plant root growth. To verify the existence of such anoxic microsites in the center of aggregates in unsaturated soils, O2 partial pressure, and redox potential were studied in model soil aggregates of spherical shape with microelectrodes. Soil material was taken from six soil horizons varying in texture and content of organic C. The measurements showed significant changes of the aeration status of these soil aggregates (diameter of 23 mm) as a function of pore water pressure in the range of p(m) = -1 to -6 kPa. At pore water pressures less than -4 kPa, all aggregates exhibited oxic centers. At higher soil moisture contents, the radii of anoxic centers at different pore water pressures could be related to soil respiration as the sink for O2, with parameters such as air-filled porosity and pore continuity controlling the O2 flux to the aggregate center. The values of diffusion constant covered the range from 4 x 10(-16) to 40 x 10(-16) mol Pa-1 m-1 s-1 and were related closely to pore water pressure, but didn't show a close relation to air-filled porosity. The occurrence of anoxic aggregate centers was not always accompanied by a decrease in redox potential. It was assumed that these effects are induced by different microbial activities as well as by a different redox buffering of the six soils investigated.