Rhizobacteria-induced soil biochemical and hydrological modifications improve medicinal plant productivity under water deficit and elevated CO2

The growing popularity of herbal medicines has prompted a need for more efficient cultivation of medicinal plants. Among the commonly used plant growth-promoting approaches, rhizobacteria inoculation has been in the research limelight. However, its effectiveness on medicinal plants in complex environments, especially with soil water deficit and elevated carbon dioxide concentrations (eCO2), remains unclear. This study examines the influences of two rhizobacteria, Bacillus subtilis and Pseudomonas fluorescens, on the productivity of a medicinal plant, Pseudostellaria heterophylla, through greenhouse experiments. Sufficient (31%, w/w) and reduced (22%, w/w) soil water levels were considered at atmospheric CO2 (aCO2) (425 ppm) and eCO2 (1000 ppm). Soil biochemical and hydrological properties, including water retention capacity, organic matter content and enzyme activities, were determined. Plant productivity and soil quality were also evaluated. Results show that rhizobacteria inoculation, particularly Bacillus subtilis, enhances soil water retention capacity by 7 – 19% compared to the control under reduced soil water levels by promoting aggregate formation and organic matter production. To drive the dynamics of organic matter under reduced water conditions, rhizobacteria inoculation can stimulate the activities of β-glucosidase and acid phosphatase at aCO2, as well as urease activity at eCO2. Shoot and root biomass are also improved by up to 59% and 36%, respectively, under reduced water level at aCO2. Once eCO2 is applied, rhizobacteria promotes biomass allocation for root growth to facilitate water and nutrient uptake. Consequently, the inoculation enhances plant productivity by at least 29% relative to the control under water deficit at eCO2. This finding demonstrates that rhizobacteria inoculation would be an effective strategy for improving medicinal plant productivity against water deficit and eCO2. Nevertheless, the observed decline in soil quality at eCO2 suggests that integrating inoculation with other agricultural practices is necessary to sustain enhancements in plant productivity over the long term.