Effect of static and cyclic loading including spatial variation caused by vertical holes on changes in soil aeration

Gas transport properties are important factors influencing soil quality and crop production, as they directly affect respiration processes of plant roots and microorganisms. They depend not only on soil characteristics such as soil texture, matric potential and soil structure, but also on the type of applied stress (i.e. static loading and cyclic loading). Although there are some studies investigating the effects of compaction on soil gas transport properties, experimental data related to spatial variation caused by vertical holes during static loading are scarce. Therefore, the effect of static, following cyclic loading, and the effect of vertical pores during static loading on total porosity, air-filled porosity (εa), air permeability (Ka) and relative gas diffusivity (Ds/Do) at matric potentials of −60 hPa and −300 hPa were investigated by using repacked soil samples with three different textures (sand, silt loam, clay loam). Total porosity and air-filled porosity (εa) after static loading and subsequent cyclic loading were lower than before loading. Compaction caused a reduction in total porosity mainly due to the decrease of macropores, which can be derived from the reduced air-filled porosity after compaction. The same was also true for air permeability (Ka) and relative gas diffusivity (Ds/Do), except for the investigated clay soil at the matric potential of −300 hPa as it was affected by an enhanced soil shrinkage and crack formation. The higher water content and the lower number of macropores of fine-textured soil might be the reason that the silt loam soil at the matric potential of −60 hPa revealed the lowest values of all observed parameters. The effect of artificially drilled continuous and non-tortuous vertical holes in the samples resulted in a slight but insignificant increase in total porosity and air-filled porosity, as they only account for 0.2% of the total bulk soil volume. Because these samples with vertical holes were equilibrated with the major principle stress, they remained rigid during consecutive static loading. Consequently, Ka significantly increased up to more than one order of magnitude, irrespective of the texture and matric potential. Drilling holes caused Ds/Do to increase by 0.13-14.89 times especially in the fine-textured soil (silt loam) at less negative matric potential (−60 hPa).