Preferential movement of oxygen in soils?

Plant roots and soil biota require [O.sub.2] to function normally. When the soil [O.sub.2] is depleted, it must be replaced by [O.sub.2] from the atmosphere. The transport occurs primarily by gaseous diffusion. Although [O.sub.2] diffusion has been researched in homogeneous soils, and in tight clay soils with cracks, little is known about the effects of macropores on [O.sub.2] transport in typical agricultural soils. In this study we examined the impact of large pores on [O.sub.2] concentration in a sandy loam soil. Experiments were carried out with homogeneous and simulated-macropore columns with a steady-state rainfall of 3 cm [d.sup.-1] in which the [O.sub.2] concentration in the soil matrix and in the macropore was measured at 15-cm vertical intervals. The diameter of each column was 20 cm and the height was 105 cm. Groundwater depth was systematically varied between 30 and 105 cm. Soil water content did not vary with distance above the water table for the 30-, 60-, and 90-cm groundwater depths. While both the soil water for the entire column and the [O.sub.2] concentration from the surface to the 30-cm depth was the same for the homogeneous and macropore columns, the [O.sub.2] penetrated deeper in the macropore column and was 4% greater below the 30-cm depth in the macropore column than for the homogeneous column.