Gas diffusion through soil crumbs: the effects of compaction and wetting

SUMMARY Samples of 1–2 mm crumbs from a clay loam under permanent pasture were equilibrated at ‐5 kPa water potential then compacted to varying degrees. Gas diffusion coefficients D , (hydrogen through air), were measured immediately on compaction, again after re‐equilibration at ‐5 kPa, then at other water contents between saturation and dryness. The relationship between diffusion coefficient and air content, was, as elsewhere, in two parts ( dD / d small for drainage of pores within crumbs; large for pores between crumbs), but the transition from one part to the other occurred at smaller air contents with increased compaction. The air content at which D approached zero as the samples wetted was greatest in the loosest soil. Compaction from a bulk density of 0.86–1.29 g cm −3 decreased the relative diffusion coefficient, D / D 0 ( D 0 is the diffusion coefficient without impedance), from 0.35 to 0.22 (by 38%) at complete dryness, but from 0.19 to 0.035 (by 82%) in the soil initially at ‐5 kPa. On re‐wetting and re‐equilibrating at −5 kPa, D / D 0 decreased further to 0.008 (total 97%) because of extra water held in the now smaller pores of the compacted soil. No single relationship between D / D 0 and fitted the results for even this one soil.