Microbial biomass and enzymatic activity in the rhizosphere of prickly-pear cactus genotypes inoculated with Bacillus subtilis and Paenibacillus Sp.

Some bacterial taxa, such as Bacillus and Paenibacillus, are known to colonize the rhizosphere and promote plant growth. However, little is known about their effect on microbial biomass and enzymatic activity in the rhizosphere of plants under semi-arid conditions. This field study assessed the effects of B. subtilis and Paenibacillus sp. in the rhizosphere of two prickly-pear cactus genotypes on microbial biomass of C, N, and P, and on enzymatic activity during early and late growth stages. The analysis of variance showed that microbial biomass and enzymatic activity were significantly influenced by the interaction between PGPB taxa (B. subtilis and Paenibacillus sp.), prickly-pear cactus genotypes ('Baiana' and 'Doce'), and plant growth stage (90 and 270 days). Specifically, PGPB inoculation increased microbial biomass P, β-glucosidase, and acid phosphatase, while microbial biomass of C and N were primarily driven by differences between cactus genotypes 'Baiana' and 'Doce'. At the early growth stage (90 days), the highest values of microbial biomass C, P, and acid phosphatase were observed, whereas N biomass was higher at the later stage (270 days). B. subtilis increased microbial biomass P in the 'Doce' genotype and acid phosphatase in 'Baiana,' while Paenibacillus sp. increased β-glucosidase in 'Baiana.' The combination of the 'Doce' genotype with B. subtilis enhanced phosphorus availability, suggesting that specific plant-microbe interactions may benefit nutrient acquisition in arid, nutrient-poor soils; however, further research is needed to confirm whether this effect extends to other genotypes.