Soil-Gas Phase Transport and Structure Parameters for a Soil Under Different Management Regimes and at Two Moisture Levels

Measurements of diffusive and convective gas transport parameters can be used to describe soil functional architecture and reveal key factors for soil structure development. Undisturbed 100-cm(3) soil samples were sampled at the Long-term Research on Agricultural Systems experiment located at the University of California, Davis. The 18 plots used in this study represented fairly wide ranges in organic carbon (0. 0072-0.0153 kg kg(-1)) and clay (0.30-0.44 kg kg(-1)). Soil-air permeability, k(a), and soil-gas diffusivity, D-P/D-0, were determined at field-moist conditions (fin) and, subsequently, after saturation and drainage to - 100 cm of matric potential (pF2). Gas diffusivity in intact samples at fm conditions exhibited a general, linear relationship with air-filled porosity (epsilon), independent of soil texture and treatment. Comparing intact and repacked samples drained to pF2, repacked soil displayed markedly lower D-P/D-0 values at similar air-filled porosity, illustrating soil structure effects on D-P/D-0. The Currie tortuosity-connectivity parameter, X=Log(D-P/D-0)/Log(epsilon), decreased with increasing bulk density in the intact samples at both moisture conditions, suggesting less tortuous and well-connected pathways for gas diffusion at higher bulk density. Pore organization, PO = k(a) / epsilon, showed a treatment effect with typically higher values for the organic plots, implying that an improved possibility for formation of organomineral soil aggregates resulted in better-connected macropore networks. Fitting a linear model to D-P/D-0 versus epsilon measurements revealed different slopes at the two moisture conditions, suggesting short-term nonsingularity (hysteretic) effects after rewetting and drainage.