Catch crop amendments and microbial inoculants differently modulate apple rhizosphere microbiomes and plant responses

Plant-soil feedback and soil microbial legacies play crucial roles in replanting success of apple. This study investigated how different soil amendment strategies influence these factors in replant disease-affected soil. Two approaches were tested: (i) the preculture and amendment of catch crops-either a single species, Tagetes patula, or a diverse catch crop mixture (CCM), and (ii) the inoculation of plant-beneficial microbes-bacteria, arbuscular mycorrhizal fungi, or their combination (SynC). Apple rootstock M.26 was grown for seven weeks in a greenhouse, and plant growth, soil nutrients, root phytoalexins, and microbial communities in rhizosphere and root-affected soil were analyzed. Catch crop amendments but not microbial inoculations, significantly altered soil nutrients. Root length increased significantly under CCM, and in tendency in Tagetes and SynC. Phytoalexin contents were lowest in Tagetes and highest in CCM, both differing from the control in specific compounds. Microbiome analysis revealed that catch crops strongly modulated fungal communities in rhizosphere and root-affected soil, favoring potentially beneficial Linnemannia and Mortierella, while microbial inoculations predominantly modulated bacterial/archaeal rhizosphere communities. Our results suggest that catch crops and microbial inoculants induced distinct shifts in soil-plant-microbe interactions under replanting conditions.