Data extraction and effect size estimation
From each study, we extracted the mean above-ground biomass with its associated measure of dispersion (i.e. standard deviations or standard errors) for control and inoculated plants, the number of replicates per treatment, the duration of plant growth, the environmental condition of the site, the type of inoculated microorganism (i.e. bacteria or fungi), the composition of the inoculated microorganisms (i.e. individual or in a consortium), and the source of microbial inoculum (i.e. native, foreign or commercial inoculant). Native microbial inoculants were defined as microbes that were isolated from soils within the local study area. Foreign microbes were those obtained from the same country but in a different study area. Commercial microbes were those obtained from commercial companies. Commercial microorganisms were separated from foreign microbes in that the former often has additional plant growth-associated components in addition to the microbial communities. Therefore, in the analysis comparing effects between commercial and native inoculation inoculants, foreign microbial communities were excluded.