Microbial functional strategies improve cereal yield across soil fertility gradients: A global meta‐analysis

Abstract Beneficial microbial inoculants are widely promoted for improving crop productivity and nutrient‐use efficiency, but their field performance remains inconsistent because microbial functions interact strongly with soil fertility conditions. However, quantitative comparisons explaining how different microbial functional strategies respond across soil fertility gradients and whether combined inoculants produce additive agronomic benefits remain limited. This study conducted a systematic review and random‐effects meta‐analysis to compare the effectiveness of nitrogen‐fixing microorganisms, phosphate‐solubilizing microorganisms, and arbuscular mycorrhizal fungi (AMF) across cereal cropping systems and soil fertility gradients. The analysis included 36 independent field studies comprising 266 paired yield comparisons. Effect sizes were calculated as log response ratios, and meta‐regression models were used to evaluate the influence of soil available phosphorus, soil organic matter, and irrigation regime on yield responses. Results showed that microbial inoculation increased cereal yield by 15.37% (95% confidence interval: 12.94–17.84), apparent nitrogen use efficiency by 16.85%, and apparent phosphorus use efficiency by 15.33%. Among functional groups, phosphate‐solubilizing microorganisms (20.54%) and AMF (19.11%) produced the largest yield gains, followed by nitrogen‐fixing inoculants (13.63%), whereas combined nitrogen‐fixing and phosphate‐solubilizing inoculants showed a smaller response (6.52%), indicating that functional stacking did not necessarily generate additive benefits under field conditions. Crop responses were greatest in maize ( Zea mays L.), 18.34%, followed by rice (Oryza sativa L.), 11.92%, and winter wheat ( Triticum aestivum L.), 5.98%. Soil available phosphorus showed an apparent association with phosphate‐solubilizer responses in the initial model, but this relationship was not robust after accounting for study‐level clustering. Irrigation regime did not significantly affect yield outcomes. These findings show that biofertilizer effectiveness is governed by microbial functional identity, soil nutrient context, and possible nonadditive interactions among microbial groups. Targeted deployment of microbial inoculants according to soil fertility status may improve the reliability of biofertilizer use in sustainable cereal production systems.