Soil-Gas Diffusivity-Based Characterization of Variably Saturated Agricultural Topsoils

Abstract

Soil-gas diffusivity and its variation with soil moisture plays a fundamental role in diffusion-controlled migration of climate-impact gases from different terrestrial agroecosystems including cultivated soils and managed pasture systems. The wide contrast in soil texture and structure (e.g., density, soil aggregation) in agriculture topsoils (0–10 cm) makes it challenging for soil-gas diffusivity predictive models to make accurate predictions across different moisture conditions. This study characterized gas diffusivity and gas-phase tortuosity in soils sampled from managed pasture and cultivated sites in Sri Lanka at 0–10 cm depth, together with selected soil-gas diffusivity data from the literature. Soil-gas diffusivity was measured using a one-chamber diffusion apparatus using N2 and O2 as experimental gases. The measured diffusivity, together with literature data representing both intact and repacked soils, were tested against five existing widely known gas diffusivity predictive models. The tested models tended to mischaracterize the two-region behavior in some of the aggregated soils, suggesting the need of soil-specific diffusivity models to better describe gas diffusivity in agricultural soils. We suggested a new parametric two-region model, developed in line with literature-based models, to represent both unimodal and bimodal/two-region behavior of selected soils. The new model statistically outperformed the existing predictive models for both intact and repacked soils and, hence, demonstrated its applicability to better characterize site-specific greenhouse gas emissions under different soil water regimes.

Soil Bulk Density and Matric Potential Regulate Soil CO2 Emissions by Altering Pore Characteristics and Water Content

“…A majority of studies have calculated D p /D o through measuring soil porosity or water content or through model assessments due to the challenges in directly measuring soil gas diffusivity [4,5,14,17,18]. The relationship between D p /D o and soil porosity can vary with changes in total porosity or the continuity of soil pores due to differences in soil bulk density or water content [5,17].…”

A new exponential model for predicting soil gas diffusivity with varying degree of saturation

“…Normalizing D p by the diffusion coefficient of the same gas in free air ( D o , m 2 s −1 ) at the same temperature and pressure yields the dimensionless soil–gas diffusivity (the soil gas diffusion coefficient [ D p ]–free‐air diffusion coefficient [ D o ] ratio), D p / D o , which becomes a function only of the soil air‐filled porosity, pore connectivity, and the tortuosity of the functional gaseous phase. Essentially, D p / D o is highly soil moisture dependent because of the two‐pronged effects of soil moisture on D p / D o —decreased air‐filled porosity and increased gaseous phase tortuosity in the presence of soil moisture (Abeysinghe et al., 2022; Thorbjørn et al., 2008). High intrinsic variability because of random connection and disconnection of moisture bridges constraining gas diffusion, together with the measurement uncertainty in the presence of water, make the estimation of D p / D o practically challenging at high‐moisture regimes.…”