Gas diffusivity and air permeability in a volcanic ash soil profile: Effects of organic matter and water retention

The soil-gas diffusion coefficient (D-p) and air permeability (k(a)) 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 D-p and k(a) in three depths of a volcanic ash soil (Amdisol) 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 psi = - 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 loworganic layer (55-60 cm) and similar for the other 2 organic layers. Soilgas diffusivity varied the most in the high-organic top layer (0-5 cm) and was lower for samples with total porosity exceeding 0.8 m(3) m(-3) likely because of additional inactive air-filled pore space created by interconnected water films. The threshold air-filled porosity (epsilon(th)) where Dp approached zero was on the average 0.05 m(3) m(-3) higher in the highorganic top layer (epsilon(th) around 0.2 m(3) m(-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(C) model compared well with data between pF 1 and pF 3 but typically underestimated ka at pF 4.1 because of a sudden increase in pore connectivity. A recent linear D-p(epsilon) model for Andisols is further developed, with Eth and model slope C predicted 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 D-p(epsilon) models, especially at low soil-water contents.