Biochar–Microbe Interactions for Co-Remediation of Heavy Metals and Pesticides in Soils: Mechanisms, Synergies, and a Translational Framework

The co-occurrence of heavy metals (HM) and pesticides (PS) in agricultural soils poses a complex remediation challenge that resists single-amendment strategies. Although biochar (BC) has been widely reported as an efficient adsorbent for specific contaminants, increasing evidence suggests that its role in soil remediation extends beyond adsorption. This review advances a conceptual shift by viewing BC as a keystone microhabitat that structures soil microbial communities and mediates plant–microbe interactions within the rhizosphere. We synthesized evidence from recent literature demonstrating that the porous architecture and functionalized surfaces of BC provide protective habitat that reduces the combined toxicity of HM and PS to microbial consortia involved in PS degradation and HM immobilization. Particular emphasis is placed on how BC feedstock selection, pyrolysis conditions, and modification strategies influence rhizosphere microbiome assembly and activity. Building on this evidence, we present an integrated model of BC–microbe–plant synergy in which contaminant bioavailability is reduced, microbial enzymatic activity is enhanced, and root exudation patterns are modified, resulting in a potentially reinforcing remediation process. Beyond descriptive analysis, this review highlights key mechanistic uncertainties at BC–microbe–plant interfaces, including feedback between BC surface redox chemistry and microbial electron transfer pathways. Finally, we outline a translational research framework that supports the rational design of BC-compatible microbial consortia and the application of multi-omics tools to improve predictability, scalability, and long-term efficacy of BC-based remediation systems for co-contaminated soils.