Rapeseed Biofumigation Combined with Biochar Mitigates Continuous-Cropping Obstacles in Chrysanthemum via Soil Improvement, Rhizosphere Microbial-Community Modification, and Wilt Disease Reduction

Long-term monoculture degrades soil ecological conditions and is associated with severe wilt disease in Chrysanthemum morifolium. The effects of combining rapeseed biofumigation and biochar to alleviate these continuous cropping obstacles remain poorly understood. A field experiment compared a control (CK), rapeseed biofumigation alone (R), and biofumigation combined with corn-straw biochar (RB) in soil under eight years of continuous chrysanthemum cultivation. Soil properties, enzyme activities, wilt disease severity, chrysanthemum yield, and rhizosphere microbial communities were assessed. Compared with CK, RB significantly increased available phosphorus (+40.7%), available potassium (+16.4%), total nitrogen (+4.9%), cation exchange capacity (+8.6%), selected enzyme activities, plant height, and flower yield (p < 0.05). The disease index decreased from 8.67 in CK to 2.67 under R and 2.33 under RB, with both reductions significant but no significant difference between R and RB. Both treatments altered microbial community composition, reduced bacterial richness, and enriched MND1; RB additionally reduced bacterial diversity, enriched Acidobacteriota, and decreased the abundances of Firmicutes and Bacteroidota relative to CK. Exploratory Mantel tests indicated that available phosphorus and cation exchange capacity were associated with variation in rhizosphere microbial community structure. Overall, RB improved selected soil fertility parameters and flower yield, accompanied by microbial community changes and lower wilt severity. Broader application requires multi-site and multi-season validation.