Depth-Dependent Responses of Microbial Community Structure and Function to Reductive Soil Disinfestation

Reductive soil disinfestation (RSD) is an effective approach for controlling horticultural plant diseases by improving soil properties. However, its effects on microbial communities and their functional characteristics across soil depths remain poorly researched. In this study, we evaluated the impacts of RSD using solid (rice bran, RB) and liquid (molasses, MO) organic amendments in a Fusarium-infested field. Changes in biotic and abiotic properties were examined at two soil depths (0–15 cm and 15–30 cm) and the potential of different amendments to restore microecological functions in deeper soil was assessed. Both RSD treatments alleviated soil acidification and salinization compared with the control. The absolute abundances of Fusarium oxysporum and Fusarium solani were significantly reduced under both treatments, with MO-RSD showing stronger pathogen suppression in the 15–30 cm layer. MO-RSD exerted a greater influence on microbial community structure across soil depths, resulting in bacterial-fungal co-occurrence networks with higher complexity. Metabolic activity and carbon source utilization increased significantly following both RSD treatments, with the greatest enhancement observed in the 0–15 cm layer under MO-RSD. Furthermore, MO-RSD enriched a higher diversity and abundance of beneficial microorganisms such as Bacillus, Paenibacillus, and Tumebacillus in the 0–15 cm layer, and Azotobacter, Penicillium, and Neurospora in the 15–30 cm layer. These microbes were closely associated with enhanced metabolic activity and pathogen suppression. Overall, MO-RSD established a more integrated and functionally diverse microbiota across the 0–30 soil profile, likely due to the greater permeability and mobility of liquid organic amendments in shaping deeper soil microbial communities.