<p>Biochar has been widely incorporated into landfill cover soils to enhance moisture regulation due to its porous structure and strong water retention capacity. Plant fibers, as abundant and renewable materials, also offer potential as soil amendments. However, little research has addressed the coupled effects of biochar-fiber composites on soil hydraulic and mechanical behavior under unsaturated conditions. This study investigates their influence on CO₂ permeability (<i>k</i>_<i>a</i>), hydraulic conductivity (<i>k</i>_<i>w</i>), and shear strength in sandy loam. Laboratory experiments examined the effects of reed straw biochar (5% and 10% by mass) and fibers (0.1% and 1% by mass) on <i>k</i>_<i>a</i> and shear strength, while monitoring suction and volumetric water content. Results showed that, compared with the bare soil (S), <i>k</i>_<i>a</i> increased by 117.46% with 5% biochar (SRS-5%) due to macropore formation, but only by 2% with 10% biochar (SRS-10%), where a potential reduction in effective gas channels counteracted the continued increase in total porosity. Fiber addition further enhanced <i>k</i>_<i>a</i> by 22.25–64.79%. Saturated hydraulic conductivity (<i>k</i>_<i>s</i>) increased by 186% with SRS-5% and 32.67% with SRS-10%, with fibers contributing additional gains of 78.09% and 15.58%, respectively. Shear strength improved with confining pressure (50–200 kPa), rising 27.7–124.22% with biochar alone, and by a further 25.42–52.31% when fibers were included. The combined treatment of biochar and straw fiber increased the shear strength by 94.49%-198.81%. Improvements are potentially attributed to pore-filling by biochar and three-dimensional bridging of fibers. These findings underscore that biochar and straw fibers modify gas permeability, hydraulic conductivity, and mechanical stability, aiding landfill cover optimization.</p> Graphical abstract <p></p>