Viscoelasticity and shear resistance at the aggregate-scale of structured and organic carbon-free chernozems

Soil rheology characterises the flow behaviour of soils at the particle-particle to aggregate-scale. Amplitude sweep tests (AST) are often the method of choice to parameterize soil flow properties such as the shear strain values at the end of the linear viscoelastic range (LVR; i.e., the deformation is mainly elastic), and at the yield point (YP; i.e., elastic equals plastic deformation). Soil samples from seven soil profiles and five soil depths collected in Ukraine were analysed to evaluate the effect of soil organic carbon (SOC) on parameters related to the microstructural soil stability as derived from ASTs. The SOC was removed with H 2 O 2 to determine the soil texture-dependent values of the soil rheological properties and compared to values determined for samples with intact water-stable aggregates. The shear resistance-related parameters were increased for aggregated as compared to SOC-free soil samples indicating an increased soil stability due to SOC. Contrastingly, the values of the overall viscoelasticity, and of the shear strain values were reduced for aggregated as compared to SOC-free soil samples, which indicates a decreased soil stability. Pedotransfer functions were applicable to predict the shear strain-dependent loss and storage moduli, and shear stress values as a function of SOC depletion. The coarse particle (630 – <2000 µm) and volumetric water contents improve the models. We concluded that increased SOC contents, through the gluing and cementing effects of SOC and altered aggregate shapes as compared to the organic carbon-free soil materials result in higher microstructural strength. However, the increased water contents with the SOC-containing soil samples superimposed the soil strengthening effect. This was due to the fact, that a more rapidly increased positive water pressure with shear stress weakened the samples, and the spherical aggregates began more easily to rotate, thus loosing energy, as compared to the platy shape of the organic carbon-free soil materials.