Although microbial inoculants are promoted as a strategy for improving compost quality, there is no consensus in the published literature about their efficacy. A quantitative meta-analysis was performed to estimate the overall effect size of microbial inoculants on nutrient content, humification and lignocellulosic degradation. A meta-regression and moderator analyses were conducted to elucidate abiotic and biotic factors controlling the efficacy of microbial inoculants. These analyses demonstrated the beneficial effects of microbial inoculants on total nitrogen (+30%), total phosphorus (+46%), compost maturity index (C:N ratio (-31%), humification (+60%) and the germination index (+28%). The mean effect size was -46%, -65% and -40% for cellulose, hemicellulose, and lignin respectively. However, the effect size was marginal for bioavailable nutrient concentrations of phosphate, nitrate, and ammonium. The effectiveness of microbial inoculants depends on inoculant form, inoculation time, composting method, and experimental duration. The microbial inoculant effect size was consistent under different feedstock types and experimental scales. These findings imply that microbial inoculants are important for accelerating lignocellulose degradation. Higher mean effect sizes have tended to be published in journals with higher impact factors, thus researchers should be encouraged to publish non-significant findings in order to provide a more reliable estimation of effect size and clarify doubts about the benefits of microbial inoculants for composting.

CGIAR Research Program on Climate Change Agriculture and Food Security P O Box 30709 00100 Nairobi Kenya

Czech University of Life Sciences Prague Department of Agro environmental Chemistry and Plant Nutrition Kamýcká 129 16500 Prague Czech Republic

Jimma University College of Agriculture 307 Jimma Ethiopia

Soka University Department of Science and Engineering for Sustainable Innovation 1 236 Tangi machi Tokyo Japan

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