Use of oxygen flux density to estimate critical air-filled porosity of a Vertisol

Waterlogging studies of Vertisols require an accurate estimate of critical aeration levels. Gaseous O2 exchange was characterized in the top 0.3 m of a furrow-irrigated Typic Pellustert by measuring quasi-steady state diffusion through intact soil cores sampled in the field over a range of soil-water content. Cores were taken at 0.1-m depth increments from below the cropped ridges at major crack sites, a quarter of the distance between major cracks, and midway between major cracks in order to sample the nonrandom heterogeneity caused by vertical cracks running across the ridges. The relationship between oxygen flux density (F) and air-filled porosity (e) above the 0.2-m depth at the three positions was described by a two-stage linear model and fitted by optimization. Values of F and e for soil below 0.2 m were restricted largely to the first (horizontal) stage of the model. For all other combinations of depths and positions, the model fitted the data with high precision (r2 greater than 0.84). The F of major crack sites was not increased relative to soil at other sites having the same e. In wet soil, crack sites were sealed by the soil swelling and hence diffusion was inhibited. In dry soil, small cracks, fissures, and pores between the major cracks allowed similar rates of diffusion to that in the major cracks at the same e. The minimum e at which gaseous diffusion first occurred (eo) was 0.145 + or - 0.006 m3 m-3. This exceeds the previously accepted limit of e = 0.1 m3 m-3, and may reflect a high proportion of occluded pores in this fine-textured soil. The study evaluated an improved method of quantifying the aeration status of this soil and shifted the emphasis from oxygen content to oxygen flux as a more pertinent parameter of soil aeration in cropping situations. The technique is simple, rapid, and appropriate for routine sampling.