Nitrogen-fixing bacterial inoculation: a meta-analysis of its effects on crop productivity and methodological considerations

Nitrogen-fixing bacteria (NFB) inoculation can enhance crop productivity and reduce reliance on synthetic fertilizers. This meta-analysis of 80 studies examined factors influencing NFB efficacy: crop-bacterial specificity, inoculation methods, and field-microcosm comparability. Using a random-effects model, we found a significant overall positive effect of NFB inoculation on crop yield (Hedges’ g = 1.263, 95% CI: 1.159–1.367); however, substantial heterogeneity ([Formula: see text]) was observed across studies. Effectiveness varied significantly by crop-bacterial combination; for instance, Azospirillum boosted wheat (Hedges’ g = 1.312) and maize, Rhizobium improved wheat yields (Hedges’ g = 0.883), and Azotobacter significantly benefited chickpea (Hedges’ g = 0.835), underscoring the need for targeted strategies. Soil inoculation demonstrated significantly greater positive effects than did seed inoculation. The advantages of soil inoculation were much more pronounced in field studies than in microcosms. In contrast, for seed inoculation, microcosm studies showed a significantly larger positive effect than did field studies. These findings indicate that while NFB inoculation is a promising approach for sustainable agriculture, optimizing its effectiveness requires careful consideration of crop-bacterial compatibility and inoculation methods. The larger effects observed for seed inoculation in microcosm settings compared to field trials suggest that the direct translation of these specific microcosm results to field performance requires caution.