Short-Term Effects on Rill Erosion and Flow Resistance in a Wood-Biochar Amended Soil

Abstract

Despite growing interest in biochar as a soil amendment, its effects on erosion control remain uncertain, particularly for rill erosion. In this investigation, the short-term (i.e., 1 month after biochar addition) effects of wood-biochar on rill erosion and flow resistance were evaluated. At first, plot experiments were performed discharging a constant clear inflow into a rill incised in the original soil (rill₀) and two rills, named rill₃ and rill₅, incised in clay loam soil amended with different initial biochar concentrations BC (3% and 5% by weight). For each rill, the outflow discharge of the entire run was collected by a storage tank and the run was stopped when a 55 L volume was reached. The collected volumes were used to measure the corresponding weights of the eroded mixture (sediments and biochar), biochar and sediments. 3D photo reconstruction was used to obtain the Digital Elevation Models (DEM) before and after the runs and by the corresponding DEM of difference, the total volume of the eroded mixture was calculated. Measurements of water depth, hydraulic radius and flow velocity were used to calculate the Darcy–Weisbach friction factor f values. The results demonstrated that in the short term, the addition of BC = 3% is effective in reducing soil erosion by 9% as compared to the control condition, while adding BC = 5% determined an increase of 140% in eroded volume as, for this latter, the short-term soil particle aggregation effects are too weak and the mixture remains cohesionless. Moreover, the analysis demonstrated that the proposed flow resistance equation gives a reliable f estimate and this equation gives, for fixed hydraulic conditions, flow resistance that decreases with increasing BC. Ultimately, for the short term, BC higher than 3%–4% cannot be considered a good soil conservation practice.