Anisotropy of intensity–capacity parameters on Aquands with contrasting swelling–shrinkage cycles

Abstract

The Ñadi soils (Aquands) are volcanic ash soils with an iron–cemented layer that are subject to different hydraulic stresses caused by high levels of exposure to weathering agents and high water–tables. Two Aquands pedons with high (ALC) and low (CHA) hydraulic stresses were evaluated to consider their capacity and intensity parameters, the effect of shrinkage on carbon stocks and the anisotropy of their physical functions. In undisturbed soil samples, water retention curves and air permeability (Ka) were measured. C stocks were calculated using bulk density corrected for shrinkage. Air conductivity (ka), gas diffusion anisotropy of (AR) and X–ray computed tomography (μCT) were used to assess soil structural changes and the related modifications of pore anisotropies. Both pedons showed andic properties (Al o+½Feo≥2%) dominated by organo–mineral complexes (Al p/Alo≥0.5). The (ALC) pedon exposed to high hydraulic stresses has 58% more volumetric water content compare to waterlogged (CHA) pedon after shrinkage, however, the air permeability decreases similarly in both soils (ΔKa = 0.40 – 0.36). Shrinkage depends of SOC (R2 = 0.69) and it determine the differences in soil C stocks (6.6 – 7.6 Mg C ha−1). Convective and diffusive horizontal air fluxes were reported on the surface for ALC (ka = −3.0 log cm s−1; AR = 3.1) and at depth (ka = −2.6 log cm s−1; AR = 2.4) for CHA. With the μCT, the highest Euler numbers were used as indicator of intricate pore matrix in ALC (−1.92 × 105) and CHA (−2.35 × 105). The two Aquands pedons responded differentially to shrinkage and was considered in the C stocks calculation. And horizontal anisotropy is well defined in A and BCr horizons and are analogous with the pore connectivity evaluated with μCT.