Effects of Sphingomonads on sugar beet growth and rhizosphere microbiota under continuous cropping

Introduction Sugar beet is a crucial sugar crop, and its yield and quality are vulnerable to the adverse effects of continuous cropping. Plant growth-promoting rhizobacteria function as biological control agents and exhibit high potential for crop growth promotion. Methods In this study, soil subjected to continuous sugar beet cropping was selected as the experimental substrate to evaluate the effects of Sphingobium abikonense strain W2, Sphingomonas panni strain W9, Sphingomonas sp. strain W13, and their mixed bacterial suspension on sugar beet seedling growth and soil properties using pot experiments. High-throughput sequencing was used to characterize changes in the rhizosphere soil microbial community structure. Results The results indicated that Sphingomonads inoculation significantly improved the agronomic performance of sugar beet seedlings, as evidenced by increased plant height, stem diameter, aboveground and root fresh weight, and enhanced nitrogen and phosphorus uptake. In addition, inoculation increased soil pH, available potassium content, and sucrase activity. Microbial community analysis revealed that all inoculation treatments markedly altered the diversity and composition of the rhizosphere microbiome. Compared with the continuous cropping control, the inoculated soils exhibited a significantly higher abundance of Pseudomonadota, exceeding that observed under crop rotation. Moreover, beneficial genera (e.g., Pseudomonas , Cupriavidus , Massilia , and Novosphingobium ) were enriched. Functional prediction demonstrated a significant enhancement of key metabolic processes, including ureolysis and xylanolysis. Conclusion Overall, Sphingomonad inoculation effectively regulated the structure and function of the rhizosphere microbial community, improved soil enzyme activity and nutrient availability, and promoted sugar beet seedling growth. This study provides a theoretical foundation and potential biocontrol strategy for mitigating continuous cropping obstacles in sugar beet cultivation.