Efficiency of four types of biochar to improve soil properties and decrease soil detachment in vulnerable hillslopes to rill erosion

Biochar, as a viable substrate and soil amendment, has the potential to improve the physical and chemical properties of soils, consequently affecting soil erosion. However, few studies have explored the impacts of different types of biochar on soil detachment rates in the hillslope rill erosion process due to overland flow in deforested areas. To fill this knowledge gap, this study evaluated the soil detachment capacity (D_c) and rill erodibility (K_r) of soil samples amended with four different biochars (wood, rice, olive, and almond shells) collected from deforested hillslopes in northern Iran. D_c was measured via a hydraulic flume at three-bed slopes (8.5%, 16.9%, and 25.4%) and five flow discharges (0.21, 0.32, 0.43, 0.55, and 0.63 L/(m·s)). Moreover, key properties of the amended soils and the control soil, including organic matter (OM), aggregate stability (MWD), bulk density (BD), and cation exchange capacity (CEC), were measured. Compared with the control treatment, the application of the four types of biochar significantly (p < 0.01) decreased the D_c (with at least a 41% reduction). The application of almond shell and rice biochars significantly increased the OM and MWD, thus effectively decreasing D_c (−76% compared with that of wood biochar) and (−47% compared with that of olive biochar). The correlation analysis revealed significant associations between OM, MWD, and BD on the one hand and D_c on the other hand. Overall, the soils treated with almond shell and rice biochars could be distinguished from the other soils into distinct groups via principal component analysis. The linear relationship between D_c and shear stress was used to reflect the relationship between the dependent and independent variables (coefficient of determination, R² > 0.71). The multiple regression equation developed to estimate D_c from the OM, MWD, and BD data was also accurate (R² > 0.83). This study demonstrated that almond shells and rice biochars can be effective factors in controlling and reducing D_c and K_r on deforested and steep hillslopes. The findings of this study can help land managers select the most effective organic substrate for soil conservation purposes as well as hydrologists to support the estimation of rill erosion on steep hillslopes.