Combined microbial inoculation improves plant growth, nutrient acquisition, and soil biological functioning in Opuntia ficus-indica under water deficit

The productivity of Opuntia ficus-indica (L.) Mill. in arid and semi-arid environments is strongly constrained by water scarcity and limited soil biological activity. This study evaluated the effects of plant growth-promoting rhizobacteria (PGPR) consortia (Cs1 and Cs2), arbuscular mycorrhizal fungi (AMF), and their combined inoculations (Cs1&AMF and Cs2&AMF) on plant growth, nutrient acquisition, soil physicochemical properties, enzymatic activities, and microbial functional diversity under non-stressed and water-stressed conditions (95% and 20% of field capacity, respectively). Over a six-month greenhouse experiment, microbial inoculation significantly enhanced plant growth compared with the non-inoculated control, with the strongest responses observed under combined PGPR&AMF treatments. Microbial inoculation also improved plant mineral nutrition in the cladodes, particularly nitrogen, phosphorus, and potassium contents (up to +32.88%, +50.00%, and +12.74%, respectively, under non-stressed conditions), while reducing residual mineral accumulation in soil, indicating enhanced nutrient uptake. Soil organic matter content increased markedly in inoculated treatments, especially under combined inoculations, accompanied by significant stimulation of phosphatase (+ 282.27% for Cs2&AMF treatment), β-galactosidase (up to +789.25% for Cs2&AMF), and urease (up to +40.68% for Cs1&AMF) activities under both water regimes. Biolog EcoPlate analyses revealed higher average well color development (AWCD) under both non-stressed (+ 352.61% for Cs1) and water-stressed conditions (+ 129.04% for Cs2&AMF), broader carbon substrate utilization, and increased microbial metabolic functioning, reflected by significantly higher functional diversity indices (Shannon diversity and substrate richness) in inoculated soils. Combined inoculations exhibited the greatest metabolic versatility and resilience under water stress. Multifactor analysis of variance (ANOVA) identified microbial inoculation as the dominant factor driving variability in soil biological and functional traits, while water regime primarily influenced plant growth and nutrient parameters. Principal component analysis highlighted a clear functional shift from physico-chemically driven control soils toward biologically active soil–plant systems promoted by PGPR&AMF synergy. Overall, these results demonstrate that combined PGPR and AMF inoculation represents an effective and sustainable strategy to enhance soil biological functioning, nutrient cycling, and drought tolerance of O. ficus-indica in water limited environments.