Short-Term Biochar Effects on Microstructure Viscoelasticity, Shear Resistance, and Resilience of a Weathered Soil

Biochar has been found to be an effective soil amendment in agriculture based upon its manifold functional groups as well as porous structure. However, the impacts of this material on soil mechanical properties are still poorly explored, especially under oscillatory shear conditions (as common due to traffic of agricultural machinery). Hence, our study investigates how short-term application of different rates and types of biochar in successive crops affects soil microstructural resistance, viscoelasticity, and resilience under oscillatory shear. In a completely randomized greenhouse pot experiment, wheat and soybean were grown successively in a sandy loam soil under single addition of two types of biochar (derived from either rice or soybean straw) at application rates (0 – control, 10 and 20 t ha−1). After crop harvesting, disturbed soil samples were collected in three layers to conduct amplitude sweep and thixotropy tests and analyze soil chemical properties. Biochar application resulted in extended elastic behavior, whereas soil strength decreased at low shear strain. Conversely, at high shear strain biochar had a destabilizing effect on soil microstructure, as indicated by the advancement of the flow point and lower overall viscoelasticity in biochar amended soils. Despite reduced microstructure stiffness exhibited in thixotropy tests, soil amended with biochar almost recovered completely its stiffness after high shear impact. However, significant effects were only noticed in topsoil layer independent of biochar type applied. Hence, accumulated biochar on soil surface layer had an overall negative impact on soil mechanical stability.