Bacillus subtilis inoculated in organic compost could improve the root architecture and physiology of soybean under water deficit.

Bacillus subtilis is known to promote root growth and improve plant physiology, while organic compost enhances soil water retention. This study explored the combined effect of inoculating B. subtilis in organic compost on soybean growth under water deficit. The treatments included chemical fertilization, non-inoculated organic compost, and organic compost inoculated with B. subtilis which were assessed under well-watered and water-deficit conditions. The organic compost inoculated with B. subtilis increased root biomass, length, volume, and the number of root tips under well-watered conditions, although it reduced root diameter. Under water deficit, the organic compost inoculated with B. subtilis increased root tip number (∼150%), biomass (∼95%) and number (∼85%) of nodules. Water deficit negatively affected soybean physiology, reduced photosynthesis, transpiration, and stomatal conductance, while increased internal CO₂ concentration. However, the organic compost inoculated with B. subtilis mitigated these effects, enhancing photosynthesis (∼20%) and water use efficiency (∼25%). Under water deficit, this treatment also increased shoot biomass by 15% and the drought tolerance index by 51% compared to the control. The combination of B. subtilis and organic compost improved root architecture, nodulation, and drought tolerance. These results suggest that B. subtilis inoculated in the organic compost is a promising strategy for enhancing soybean productivity and resilience under water stress, offering a novel approach to mitigating drought effects in agriculture.