Biochar as a rhizosphere interface engineer: integrated regulation of microenvironments, biogeochemical cycling, and plant resilience

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Abstract Biochar acts as a rhizosphere interface engineer, reshaping physical, chemical, and biological gradients across the root–soil–microorganism continuum. Physically, it enhances aggregation by 13.9–18.9% and increases porosity by 8.2–41.6%, improving root penetration and creating microbial refuges. Chemically, its functional groups buffer rhizosphere pH, adsorb root-derived organic acids, and mediate redox reactions that govern nutrient speciation and contaminant immobilization. Biologically, it provides protected microhabitats, enriches functional taxa, and stimulates enzyme activities. On average, urease activity rises by 23.1%, alkaline phosphatase by 25.4%, and nitrogen-cycling genes amoA and nosZ increase by 25.3% and 17.0%, respectively. These functional pathways are mechanistically coupled and shift with soil conditions. Enhanced pore connectivity promotes oxygen diffusion toward roots, driving iron redox cycling and iron plaque formation, which modulates phosphorus solubility and selects for microbial consortia coupling nitrogen transformation to iron reduction. The relative importance of these mechanisms varies by context: physical water retention dominates in coarse-textured soils under drought, pH buffering and sorption prevail in acidic soils, and microbial habitat effects are most significant in nutrient-limited systems driven by rhizodeposits. Collectively, these alterations decrease the mineralization of indigenous soil organic carbon by an average of over 5.5%, elevate the retention of root-derived carbon in subsoil by 20% on average, and lower nitrogen leaching by 10.9%. Meanwhile, plant stress resistance is improved via optimized rhizosphere microbial communities and enhanced rhizosphere signal transduction. Translating these benefits into practice requires matching biochar properties to specific soil conditions. For alkaline sandy loam soils, wood- or crop residue-derived biochar produced above 500 °C, applied at 20–40 t ha −1 with 0.5–2 mm particles, is optimal. Acidic soils, however, require lower application rates of 5–25 t ha −1 . Particles smaller than 0.5 mm, as well as sludge- or manure-derived biochar produced below 500 °C, should be avoided without prior ecotoxicological screening mainly due to the risk of its transport or cotransport with contaminants. Long-term efficacy depends on repeated moderate-rate applications to manage aging-related decline and nutrient imbalances. Future research should resolve context-dependent contradictions through systematic dose–response experiments, define safe application windows by tracking aging trajectories, and reconcile conflicting microbial responses using field-based metatranscriptomics paired with enzyme assays. Graphical Abstract Highlights Biochar acts as a rhizosphere interface engineer, regulating root–soil–microbe systems through combined multi-pathways. These changes cut soil carbon and nitrogen losses, boost nutrient retention, and enhance plant stress resilience. Matching biochar to local soil conditions maximizes agronomic benefits and mitigates environmental risks.

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