GAS TRANSPORT PARAMETERS ALONG FIELD TRANSECTS OF A VOLCANIC ASH SOIL

Abstract

Variations in gas transport parameters at the field scale govern the transport, fate, and emission of greenhouse gases and volatile organic chemicals in soil. In this study, we evaluated predictive models for soil-gas diffusivity (D p /D o ) and air permeability (k a ) based on measurements along a 117-m transect and a parallel 33-m transect of a humic volcanic ash soil (Andisol) in Nishi-Tokyo, Japan. Measurements were done on 100-cm 3 undisturbed soil samples, with 3-m spacing between sampling points, and included water retention, soil-gas diffusion coefficient (D p ), k a at .different soil-water matric potentials, and saturated hydraulic conductivity. Traditionally used predictive gas diffusivity models underestimated D p /D o in wet soil and largely overestimated D p /D o under dry conditions because of soil aggregation effects. A linear model for D p /D o as a function of air-filled porosity (e), taking into account inactive/ remote air-filled pore space, accurately described D P (e)/D o from wet to oven-dry conditions and well captured the spatial variations in D P /D o along the transects. The k a exhibited a nonlinear relation with e, and k a (e) was best predicted from a recently presented power-law model, with measured k a at -100 cm H 2 O of soil-water matric potential (k a,100 ) as a reference point. Trends of decreasing soil-water retention and increasing e along transects were observed. Similar trends in k a and saturated hydraulic conductivity were not observed because the convective fluid transport parameters were mainly governed by soil structure and not by fluid phase contents. Autocorrelograms suggested a spatial correlation range of 10 to 20 m for gas transport parameters (D p /D o and k a ). Measurements of e and k a at conditions close to -100 cm H 2 O of soil-water matric potential are suggested for rapid assessment of the magnitude and spatial variations in gas transport properties at the field scale.