Plant species and pH interactions shape rhizosphere bacterial diversity in peat-based systems

Soil pH shapes rhizosphere microbial structure and diversity, influencing nutrient cycling, plant growth, and ecosystem health, but how pH affects the microbiome in greenhouse-grown ornamental plants in soilless substrates is less understood. This study examined the interactions between substrate pH and plant species in shaping bacterial communities within a peat-based substrate. Using a randomized complete block design, four plant species (petunia, marigold, tomato, and geranium) and an unplanted control were grown at four pH levels (4.5, 5.5, 6.2, and 7.0), and all pots were fertilized at each irrigation. After eight weeks, 16S rRNA amplicon sequencing revealed significant shifts in rhizosphere microbial composition and diversity. PERMANOVA analysis revealed that pH was the primary driver of community composition (F = 41.54, R 2 = 0.565, p = 0.001), with plant species exerting secondary but significant effects (F = 3.86, R 2 = 0.140, p = 0.001). Redundancy analysis identified pH and plant species as the most significant predictors, with iron emerging as the key micronutrient driver of community structure. Acidobacteriota dominated at low pH, while Bacteroidota and Proteobacteria increased with higher pH. Several bacterial genera showed strong correlations with iron in both leachate and plant tissue, highlighting micronutrient feedback in shaping rhizosphere communities. Petunia exerted the least selective effect, with microbial composition resembling unplanted controls. This study demonstrates that substrate pH and plant species interact to modulate bacterial communities in greenhouse systems, with broader implications for optimizing pH management to enhance nutrient cycling and improve sustainability in soilless horticultural systems.