Effects of soil pore structure on gas diffusivity under different land uses: Characterization and modelling

Abstract

Soil gas diffusion significantly affects soil respiration and greenhouse gas emissions, and is strongly determined by soil structure (mainly pore structure) and water content. Given the challenge of quantifying soil gas diffusivity and pore structure, the effect of soil structure on gas diffusion under different moisture conditions remains elusive. Here we measured soil gas diffusivity (Ds/D0) of intact and repacked soils under four typical land uses, including irrigated cropland (IC), native grassland (NG), native forest (NF), and wetland (WL), at different soil matric potentials (ψ, –1 ≤ψ≤–1500 kPa) using one-chamber method. The pore structure of intact soils was quantified using micro-computed tomography (µ-CT) and soil water retention curve. The Ds/D0 for repacked soils with different bulk densities (1.2–1.5 g cm−3) and clay contents (14–35%) were further determined. Land use significantly affectedDs/D0with NF > NG > IC > WL, and disturbing soils via repacking generally decreased Ds/D0, especially for NF. Increased bulk density and clay content decreasedDs/D0, particularly at low ψ (–50 ≤ψ≤– 1500 kPa), under which Ds/D0 was controlled by volume of pores with diameter > 6 µm and particularly 6–300 µm. By contrast, Ds/D0 at relatively high ψ (–1≤ψ≤–10 kPa) was driven by volume of pores with diameter > 300 µm. Furthermore, an empirical model of Ds/D0 was developed by incorporating a scale parameter, which quantifies the effects of soil pore structure on gas diffusivity. The developed model predicted the Ds/D0 under different land uses and moisture conditions better than most currently used models. Overall, this study reveals how soil structure quantitatively affected gas diffusivity under different land uses and moisture conditions, and provides a feasible model to predict soil gas diffusivity, which is critical for accurately estimating and predicting greenhouse gas emissions from soils. This study suggests the importance of considering soil structural parameters derived from soil water retention curve and/or µ-CT in predicting gas diffusion across different soil types and land uses.