Microbial Responses to Biochar Soil Amendment and Influential Factors: A Three-Level Meta-Analysis

Abstract Biochar is a multifunctional soil conditioner capable of enhancing soil health and crop production while reducing greenhouse gas emissions. Understanding how soil microbes respond to biochar amendment is a vital step towards precision biochar application. Here, we synthesized 3899 observations of 24 microbial responses from 61 primary studies, applied a three-level mixed-effects model to estimate biochar effects, and evaluated the importance of biochar characteristics (feedstock, pyrolysis temperature), soil properties (pH, C:N, cation exchange capacity, bulk or rhizosphere), and treatment protocols (application rate, fertilization, duration, field or laboratory). Biochar significantly boosts microbial abundance (microbial biomass carbon > CFU), nitrite reductase gene ( nirS ), the activity of C- and N-cycling enzymes (dehydrogenase > cellulase > urease > invertase), and potential nitrification rate. Biochar characteristics, soil properties, and treatment protocols strongly determine the direction and extent of microbial response changes. Feedstock, pyrolysis temperature, application rate, and soil pH are important predictors most frequently included in the final models. Our study highlights the promise of purpose-driven biochar production and application such that biochar production parameters can be tuned to elicit the desired microbial responses and application protocols could be optimized to invoke multiple benefits. It also underlines current knowledge gaps and future research needs. Synopsis Meta-analysis reveals overall effect sizes of soil microbial responses to biochar amendment and the most influential factors, highlighting the potential of purpose-driven precision biochar towards sustainable agriculture.