Biochar Amendment Effects on Microstructure Resistance of a Sandy Loam Soil Under Oscillatory Stress

Current research on the impacts of biochar (BC), an organic compound used as soil amendment, focuses on soil physical and chemical properties, such as nutrient or water retention, while little attention is paid to the effects on soil mechanical behavior, especially on the microscale. Our objective was to investigate how BC amendment impacts the microscale mechanical behavior of a weathered soil under oscillatory stresses. Substrates were prepared by mixing soil with two BC types obtained from rice and soybean straw (application rate: 10 t ha−1) and incubating the mixture. Disturbed soil samples were collected 15, 30, 45, and 60 days after mixing to conduct rheological tests to characterize the soil microstructural stability and shear behavior. Along the incubation (except for at 45 days), the BC amendment decreased the soil shear strength in the order of treatments: rice BC-amended soil = soybean BC-amended soil > unamended soil (control). A possible cause, the lubricating impact of increased water retention and low aggregation potential of BC, is discussed. The soil viscoelasticity was not affected by biochar application. Already shear stresses of moderate intensity caused structural breakdown of the BC-amended soil, making it more susceptible to deformation and erosion processes.