Soil Gas Diffusivity and Air-Filled Porosity and Their Spatial Patterns in Crop and Grass Systems.

Soil gas diffusivity and air-filled porosity are important indicators of soil structure and soil aeration status, and they exhibit large spatial variability. This study quantified soil gas diffusivity and air-filled porosity and characterized their spatial patterns in crop and grass systems. Sixty undisturbed soil cores were taken from four transects.  Soil cores (356.5 cm³) were sampled at 4-10 cm depths to estimate gas diffusivity and air-filled porosity under different matric potentials: -0.1, -0.5, -1.0, -3.3, and -10.0 m.  Oxygen diffusivity was measured by a chamber method with oxygen concentration being quantified using a gas chromatograph. Harmonic means of relative soil-gas diffusivity and air-filled porosity were higher in the grass system than in the crop system at all measured matric potentials lower than -0.1 m. Harmonic means of relative gas diffusivity and air-filled porosity increased with decreasing matric potential in both land-use systems. Relative gas diffusivity and air-filled porosity exhibited high variation with space and their CV in the crop system was higher than in the grass system at all matric potentials. Spatial variation of relative gas diffusivity and air-filled porosity decreased with decreasing matric potential in both land-use systems. The spatial variability of relative gas diffusivity was structured and the spatial correlation length was longer in the grass system than in the crop system at all matric potentials. Spatial variability of air-filled porosity was structured at all matric potentials in both land-use systems except at -0.1 and -1.0 m in the grass systems.  The spatial correlation length of air-filled porosity was longer in the grass system than in the crop system at -3.3 and -10 m and shorter at -0.5 m matric potentials.  Land-use affected soil structure, with higher relative gas diffusivity and air-filled porosity at the same matric potential in the grass system than in the crop system. Relative gas diffusivity exhibited high, but structured, spatial variability in both land-use systems and at all measured matric potentials.