Effects of plant roots on the erodibility of compacted soils can be explained by a biological indicator derived from Wilson’s model

Abstract Aim Compacted slopes demand herbaceous cover for erosion control. This study aims to develop improved understandings on how compacted soil properties––particle size and relative density––interact with grass roots to regulate soil erodibility to resist rill detachment. Method Sandy soils sieved to maximum particle sizes of 5 and 2 mm were compacted to 90%, 80%, 70% relative densities. One side of compartmentalised pots was cultivated with Paspalum Notatum (rooted soils) and mycelia developed colonised the other side (bonded soils). Plant growth, root traits and soil aggregate stability were determined. Flume tests were conducted to measure the detachment rates of the rooted and bonded soils under a wide range of shear stresses. Erodibility parameters, $${\tau}_{c}$$ τ c and $${K}_{d}$$ K d from the excess shear stress model and also $${b}_{0}$$ b 0 and $${b}_{1}$$ b 1 from Wilson’s model were calibrated against test data. Results High-density coarse soil exhibited lower erodibility coefficients ( $${K}_{d}$$ K d and $${b}_{0}$$ b 0 ) and higher shear thresholds ( $${\tau}_{c}$$ τ c and $${b}_{1}$$ b 1 ). Roots or bonding consistently reduced $${K}_{d}$$ K d and $${b}_{0}$$ b 0 , and increased $${\tau}_{c}$$ τ c and $${b}_{1}$$ b 1 , with more pronounced effects in finer, medium-dense soils. Overall, the Wilson’s model fitted the data better. A dimensionless, cohesion-related indicator, $${K}_{bio}$$ K bio , was derived from this model to quantify soil reinforcement by roots and reveal the contributions given by the biochemical bonding and physical binding mechanisms. Conclusion Roots can more effectively reduce the erodibility of finer, medium-dense soils, where root biochemical bonding played a dominant role. Effects of roots on the erodibility of compacted soils can be explained by a physically meaningful biological indicator.