Biochar addition enhances silt loam soil resistance to rill flow: A study based on three years of field monitoring data on China’s Loess Plateau

Biochar addition can change the physiochemical properties of soil, thus likely influencing soil’s resistance to rill flow (reflected by rill erodibility (K_r, s m⁻¹) and critical shear stress (τ_c, Pa). However, the persistent time effects of biochar on K_r and τ_c have remained unexplored. This study aimed to assess the impact of biochar composed of apple branches on K_r and τ_c, and to investigate the relationships between K_r, τ_c and soil properties. The undisturbed soil core samples to a depth of 5 cm were collected from field plots that had received biochar at rates of 0, 1, 2.5, 4, 5.5, and 7 % (w/w) after 1, 2, and 3 years, respectively. The K_r and τ_c of these samples were evaluated through a flume experiment, with scouring soil samples under three flow discharges (e.g., 0.00025, 0.00045, and 0.00065 m⁻³ s⁻¹) and five slope gradients (e.g., 5.24, 8.75, 17.63, 26.79, and 40.40 %). The results revealed that the ranges of K_r and τ_c for no biochar treatments varied from 0.1947 to 0.2107 s m⁻¹ and 1.6971–1.7314 Pa, with the averaged values of 0.2007 s m⁻¹ and 1.7100 Pa, respectively. Compared with no biochar addition, the addition of 1–4 % biochar after 1–2 years generally resulted in a reduction in K_r ranging from 20 % to 59 %, while increasing τ_c by 2–4 %. Conversely, 5.5 and 7 % biochar addition increased K_r by 31 and 5 %, and reduced τ_c by 12 and 6 %. All biochar treatments after 3 years resulted in a 51 % reduction in K_r and a 5 % increase in τ_c relative to bare soil, showing an increasing trend with an increasing biochar addition rate. The fluctuations in K_r and τ_c could be elucidated by changes in cohesion (COH) and mean weight diameter of soil aggregates (MWD), with COH (total effect of −0.32 and 0.17, P<0.01) and MWD (total effect of −0.13 and 0.37, P<0.01) serving as reliable estimators of K_r and τ_c during the 1–2 years following biochar addition. After biochar addition for 3 years, total organic carbon (TOC) (total effect of −0.45 and 0.10, P<0.01) emerged as a significant factor influencing K_r and τ_c, making TOC a potential p_redictor of K_r and τ_c. The results demonstrate that biochar may be an effective measure for enhancing soil resistance to erosion on the Loess Plateau, especially when applied over the long term.