Interactions between microbial plant growth promoters and their effects on maize growth performance in different mineral and organic fertilization scenarios

We examined interactions between microbial plant growth promoters and their single and combined effects on maize plant growth with and without mineral NPK fertilization and with and without organic fertilization in terms of maize stover. Two similar greenhouse pot experiments were performed one with disinfected soil and another with non-disinfected soil as growth substrate. Three commercial microbial biofertilizers were examined including the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis, the saprotrophic fungus Trichoderma harzianum and the rhizobacterium Azospirillum brasilense. In general microbial inoculations had limited effects on maize growth compared to mineral and organic fertilization. Among the microbial plant growth promoters examined only T. harzianum caused plant growth promotion, and only in combination with mineral fertilization and with disinfected soil as growth substrate. Despite of the limited effects of microbial inoculations on plant growth strong microbial interactions were observed. Population density of diazotrophic bacteria in soil was markedly higher with mineral fertilizers. Combination of A. brasilense with one or both of the two other microbial biofertilizers reduced the population density of diazoptrophic bacteria in disinfected soil. Both mineral and organic fertilization reduced AM fungus root colonization in non-disinfected soil, which on the other hand was counteracted from inoculation with microbial biofertilizers. In conclusion, despite limited effects on plant growth performance inoculation with commercial microbial biofertilizers can have strong effects on native populations of AM fungi and diazotrophic bacteria, which should be considered when integrating microbial biofertilizers in agroecosystems.