Comprehensive evaluation of biochar influence on soil microbial diversity, nutrient cycling and contaminant remediation using systematic review and meta-analysis

The increasing degradation of soil health due to intensive agriculture, climate stress, and chemical overuse necessitates sustainable interventions. Biochar, a carbon-rich product from pyrolysed biomass, has emerged as a multifunctional soil amendment with the potential to enhance microbial diversity, nutrient cycling, and contaminant remediation. This systematic review and meta-analysis assessed 43 studies to synthesise the impacts of biochar on soil microbial diversity across various agroecosystems. Biochar consistently increased microbial evenness and richness, particularly for bacterial taxa such as Proteobacteria, Actinobacteria, and Sphingomonas, while fungal responses varied depending on the soil pH and biochar type. The meta-analysis of 18 studies revealed a significant positive effect on the Shannon index (SMD = 3.41, p = 0.031), but no clear impact on Chao1 richness (SMD = 0.54, p = 0.911), indicating metric-specific responses. Biochar amendments also improved soil organic carbon, available phosphorus, and potassium, while reducing heavy metal bioavailability, thereby contributing to long-term carbon sequestration and environmental remediation. Additionally, co-application with nitrogen fertilisers or plant-growth-promoting rhizobacteria (PGPR) enhanced microbial function and crop productivity. However, variation in biochar feedstock, pyrolysis temperature, and environmental conditions contributed to heterogeneity. These findings support biochar's role as a scalable tool for soil restoration, yet highlight the need for long-term field validation, standardised protocols, and advanced molecular tools to decode microbial functions. Therefore, biochar’s integration into climate-resilient and circular agricultural systems represents a critical pathway for sustainable land management. Biochar significantly enhances soil microbial diversity, especially bacterial evenness, across diverse agroecosystems. Meta-analysis revealed a positive effect of biochar on Shannon diversity (SMD = 3.41; p = 0.031), but not on Chao1 richness. Biochar amendments improved soil nutrient retention and reduced heavy metal bioavailability. Enrichment of beneficial microbial taxa such as Proteobacteria and Sphingomonas supports nutrient cycling and stress mitigation. High heterogeneity in outcomes highlights the need for context-specific biochar application and long-term field validation.