Mixed strain validation reveals dual pathways of heterotrophic ammonia-oxidizers in maize post-drought recovery

Context Water scarcity poses a significant threat to global food security. Enhancing the compensatory growth capacity of crops following post-drought rewatering is therefore a key strategy for water-efficient agriculture. Aims This study investigates the dual regulatory pathways by which heterotrophic ammonia-oxidizing bacteria (HAOB) influence maize compensatory growth after drought stress. Methods Three functionally distinct bacterial strains were selected for co-inoculation experiments: Y2, Shan2, and C5-3. Key results Strain Y2 exhibited strong nitrification ability, Shan2 demonstrated high cytokinin synthesis capacity, and C5-3 possessed both functions. The dual-functional strain C5-3 exhibited the highest biomass at 16.98 g·pot−1, markedly exceeding both drought and rewatered controls. Co-inoculation of Y2 and Shan2 resulted in a biomass of 16.31 g·pot−1, which was significantly higher than that under single-strain inoculation and comparable to that of C5-3, suggesting a synergistic effect between nitrification and cytokinin-mediated growth promotion pathways. Conclusions The HAOB strains exert a dual-function synergistic effect by simultaneously modulating soil nitrogen metabolism and the plant’s endogenous cytokinin signaling pathway. Their overall growth-promoting effect surpasses that of strains acting through a single pathway. Implications This study demonstrates that HAOB inoculation can significantly enhance maize compensatory growth after drought, providing a sustainable microbial strategy to improve water use efficiency and resilience in arid and semi-arid agricultural systems.