Effects of biochar amendment and root density on water and gas permeability of unsaturated soils under wetting and drying cycles

Low water and gas permeability (WAGP) is essential for landfill covers to limit greenhouse gas emissions and water infiltration. Biochar and plant roots can modify soil WAGP, yet their combined effects in unsaturated soils are poorly understood. This study examined soils amended with 0%, 5%, and 10% biochar and vetiver grass roots at three densities (0, 3, and 6 clumps), compacted to 90% relative density. Matric suction, volumetric water content (VWC), and gas permeability ( k a ) were monitored during drying, and three wetting-drying cycles were assessed for stability. Results showed that biochar and roots significantly reduced WAGP. Fine biochar (< 0.5 mm) decreased k a by 43.27%–82% and hydraulic conductivity ( k w ) by 96.61%–99.7%, with the 5% biochar amendment resulting in greater reductions in both k a and k w than the 10% amendment. Roots reduced k a and k w by 26.81%–65.7% and 1.22%–46.74%, respectively. The synergistic interaction between roots and biochar further reduced k a and k w by 60.06%–88.67% and 96.65%–99.78%, respectively. Additionally, wetting-drying cycles may increase the WAGP of bare soil by promoting the formation of macropores, whereas in root-permeated soil, continuous root growth occupies macropores and reduces WAGP. Scanning electron microscope (SEM) observations confirmed that biochar filled silt pores and promoted aggregation, while roots enhanced pore tortuosity, collectively restructuring soil microstructure. These findings highlight the potential of combined biochar–root strategies for lowering WAGP in landfill cover systems.