Working in Tandem: PGPR and AMF Co-Inoculation Sustains Maize Productivity Under a 33% Reduction in Phosphorus Fertilization

Reducing phosphorus (P) fertilization is essential for sustainable agriculture but may compromise crop establishment and productivity. Microbial inoculants, particularly plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF), may help reduce fertilizer dependence, yet field evidence for their combined effects under reduced P inputs remains limited. In a single-season field trial under semi-arid Mediterranean conditions, we tested whether single or combined PGPR and AMF inoculations could compensate for a 33% reduction in P fertilization in maize (Zea mays L.). Without inoculation, reduced P decreased hay yield by 17% and plant density by 11%, while grain yield remained unaffected. Single inoculations provided little benefit and did not prevent these declines. In contrast, PGPR-AMF co-inoculation under reduced P fully restored hay yield (8.0 t ha−1) and plant density to levels comparable to those of the 100% P treatment, representing a 21% increase in hay yield relative to uninoculated reduced-P plots. Grain yield per hectare and nutrient concentrations in hay and grain were unaffected by either P reduction or inoculation. Correlation analyses indicated negative associations between stand density and per-plant grain yield and between biomass production and leaf water index, suggesting shifts in resource allocation and plant performance. These findings provide field-scale evidence that PGPR-AMF consortia can buffer the effects of reduced P fertilization on maize establishment and vegetative productivity, supporting their integration into sustainable nutrient management strategies. Multi-season studies are needed to assess the persistence and scalability of these benefits across environments.