Water Retention, Gas Transport, and Pore Network Complexity during Short-Term Regeneration of Soil Structure

Soil structure maintains prime importance in determining the ability of soils to carry out essential ecosystem functions and services. This study quantified the newly formed structure of 22-mo field-incubated physically disturbed (2-mm sieved) samples of varying clay mineralogy (illite, kaolinite, and smectite) amended with organic material (7.5 Mg ha⁻¹). The newly formed structure was compared with that of sieved, repacked (SR) and natural intact samples described in previous studies. Assessment and comparison of structural complexity and organization was done using water retention (pore size distribution), soil gas diffusivity, air permeability, and derived pore network complexity parameters. Significant decreases in bulk density and increases in pores >100 μm were observed for incubated samples compared with SR samples. For the soils studied, the proportion of pores >100 μm increased in the order: smectite < illite < kaolinite, with no effect of organic amendment. Soil structural complexity, quantified by soil gas diffusivity, air permeability, and derived pore network indices, was greater for incubated than SR samples. For illitic soils, incubated samples had lower water content and higher air-filled porosity and air permeability than natural intact samples at a matric potential of −10 kPa. Despite this, soil pore organization was similar for both natural and incubated soils, but pore network complexity increased in the order: SR < incubated < natural soils. Finally, the air permeability percolation threshold corresponding to the physically based diffusion threshold increased with structural complexity (SR = 0.02 μm²; incubated = 0.20 μm²; natural = 0.70 μm²). Thus, critical reexamination is needed of the often-used 1.0-μm² percolation threshold for convective air transport when analyzing pore network complexity. Lack of a clear effect of organic amendment for incubated samples suggests using higher application rates in future studies.