Gas diffusivity in peat soils across Denmark is controlled by soil-water characteristics and effective porosity

The soil-gas diffusivity (ratio of gas diffusion coefficients in soil and pure air), Dp/D0, controls the mobility of gases in variably saturated soils, including aeration, emission and uptake of greenhouse gases. Previous soil-gas diffusivity studies have focused on lower-organic soils. Here, Dp/D0 was measured on intact soil cores at six different soil-water matric potentials between -30 and -1000 cm H2O (pF between 1.5 to 3) on 127 Danish peat top soils (12-52% SOC). The SOC level was not found to be main control of Dp/D0 for peat soils. Dp/D0 versus soil-air content (ε) curves varied as much within a narrow SOC interval (e.g., 30-36% SOC) as for the whole range. In contrast to previous observations for lower-organic soils, it was found that at each pF level, Dp/D0 increased linearly with ε, the slope, the Penman pore continuity index P, increased linearly with pF. Previous Dp/D0 models developed for lower-organic soils (typically < 4% SOC) failed to describe Dp/D0 for peat soils. A soil-water characteristic curve-dependent model (Buckingham-Burdine-Campbell, BBC) well predicted Dp/D0 for peat soils. A modified WLR model with inputs of actual and effective air-filled porosity (ε*, taken as ε around pF3) was developed. The BBC and modified WLR models were successfully validated against independent data for high-organic soils representing different climate zones and organic matter quality. This work provides a better understanding of and models for gas diffusion in high-organic soils, setting a platform for including soil-air phase properties when evaluating soil functions, security, and capital.