GAS DIFFUSIVITY AND AIR PERMEABILITY IN A VOLCANIC ASH SOIL PROFILE

The soil-gas diffusion coefficient (Dp) and air permeability (ka) govern the transport and emission of greenhouse gases and volatile organic chemicals in the unsaturated zone. The effects of soil organic matter and water retention on the two gas transport parameters are not well known. In this study, we measured Dpand kain three depths of a volcanic ash soil (Andisol) profile, with organic matter contents of 17% (0-5 cm depth), 4.7% (15-20 cm), and 0.2% (55-60 cm), respectively. Measurements were made on undisturbed samples at soil-water matric potentials from = −10 cm H2O (pF 1) to −12,600 cm H2O (pF 4.1) and, for Dp, also on air- and oven-dried samples. Soil-water retention was larger in the low-organic layer (55-60 cm) and similar for the other 2 organic layers. Soil-gas diffusivity varied the most in the high-organic top layer (0-5 cm) and was lower for samples with total porosity exceeding 0.8 m3m−3likely because of additional inactive air-filled pore space created by interconnected water films. The threshold air-filled porosity (th) where Dpapproached zero was on the average 0.05 m3m−3higher in the high-organic top layer (tharound 0.2 m3m−3) compared with the lower layers. For air permeability, the low-organic layer (55-60 cm) behaved differently because of a different soil structure. A recent power law ka() model compared well with data between pF 1 and pF 3 but typically underestimated kaat pF 4.1 because of a sudden increase in pore connectivity. A recent linear Dp() model for Andisols is further developed, with thand model slope Cpredicted from soil total porosity and volumetric content of intra-aggregate pores (soil-water content at pF 3). The linear model performed better than frequently used nonlinear Dp() models, especially at low soil-water contents.