Continuous cropping of cut chrysanthemum reduces rhizospheric soil bacterial community diversity and co-occurrence network complexity

Long-term continuous cropping causes a significant decline in the quality of cut chrysanthemum (Chrysanthemum morifolium Ramat.), but the microbial mechanism behind it remains unclear. Here, we examined the impacts of different numbers of years of continuous cropping on the bacterial communities and enzymatic activities in the rhizospheric soil of cut chrysanthemum. Our results showed that continuous cut chrysanthemum cropping for 6 years (Y6) or 12 years (Y12) significantly decreased (P < 0.05) the soil respiration rate and enzymatic activities of alkaline phosphatase and β-glucosidase compared to cropping for only 1-year (Y1). 12 years of continuous cropping resulted in significant decreases in soil bacterial numbers and α-diversity (e.g., Shannon index). Continuous cropping treatments shifted the bacterial community composition, lowering the relative abundances of the phyla Proteobacteria and Acidobacteria and increasing the relative abundances of Actinobacteria and Chloroflexi. Moreover, continuous cropping reduced the bacterial network complexity relative to noncontinuous cropping. Structural equation modeling (SEM) showed that long-term continuous cropping significantly decreased the soil bacterial count and community diversity mainly because of the limitation of soil organic carbon and total nitrogen, which in turn led to a reduction in soil respiration. Overall, our results demonstrated that continuous cut chrysanthemum cropping for 12 years had negative impacts on soil bacterial diversity, community complexity and soil biological activity.