Microbial inocula enhance effects of biochar amendments on crop productivity, soil health, and microbial communities: A meta-analysis

Biochar as a soil amendment can increase soil carbon sequestration, soil microbial diversity, overall yields, and general soil functioning. To accelerate these effects, biochar is often activated with beneficial soil microbes such as arbuscular mycorrhizal fungi or plant growth promoting bacteria via microbial inocula. However, there has been no comprehensive review of the effects of microbial inoculum additions for biochar amendments. We conducted a meta-analysis to quantify the crop and soil effects of adding biochar alone compared to adding biochar with a microbial inoculum. The meta-analysis included 56 studies and examined whether the effects depended on the source of inoculum, inoculum type, or experiment type. We found that microbial inocula increased soil N and soil organic carbon concentrations and crop productivity compared to adding biochar alone. However, these effects were limited to locally sourced and research-grade inocula, while commercial inoculum products only slightly increased soil P. Fungal inocula had stronger effects than bacterial inocula. Inoculum effects were the strongest in greenhouse studies, increasing N, plant productivity, and fungal abundance, while field studies only increased plant productivity, suggesting that biochar activation with inoculum in on-farm settings may not provide intended positive effects, and thus alternative methods for biochar activation may be needed.