Effects of Biochar on the Mechanical Properties of Bermuda-Grass-Vegetated Soil in China

The effects of biochar on Bermuda grass growth and mechanical properties of vegetated soil were investigated in this study. Six groups of soil column tests were conducted, including two degrees of compaction (DOC) (70% and 90%) and two types of biochar content (5% and 10% by soil dry weight), with two groups of bare soil serving as a reference (soil used in the test was classified as silty sand with gravel, i.e., SM). It was found that biochar increased the effective cohesion by up to 70% and slightly enhanced the effective internal friction angle while mitigating the detrimental effects of wetting–drying cycles, with the effective cohesion and friction angle retaining up to 73% and 99% of their initial values, respectively. Root biomass initially increased and then decreased as biochar content increased, particularly at a low degree of compaction of soil (i.e., 70% DOC was two times that of 90% DOC). The effective cohesion of intact biochar–root–soil initially increased up to 23% (at the biochar content of 5%, 90% DOC) and then decreased as biochar content increased, regardless of DOC. At the optimal biochar content (5%), the effective cohesion and internal friction angle of rooted soil were 1.4 and 1.1 times greater at low DOC (70%). For the remolded biochar–root–soil composite, at a high degree of compaction (90% DOC), the effective cohesion increased with the increase in root and biochar content. For a given root content, the shear strength of the remolded biochar–root–soil mixture was higher than that of intact biochar–root–soil (i.e., the shear strength of intact soil at 5% of biochar content was 87% of remolded soil), suggesting that the remolded soil mixture overestimated the biochar–root–soil strength. Generally, the present study demonstrates that a 5% biochar addition is optimal for enhancing plant root growth and soil strength, particularly under low compaction. Biochar significantly improves the mechanical performance of root–soil composites and mitigates the degradation of soil strength under wetting–drying cycles.