Effects of Different Crop Rotations on Microbial Diversity and Enzyme Activities in Brassica napus Rhizosphere Soil

Continuous cropping of <i>Brassica napus</i> impairs sustainable production via soil nutrient imbalance and microecological degradation. We evaluated rhizosphere soil properties and microbial communities under rotations crops (<i>Triticum aestivum</i> [TaBn], <i>Beta vulgaris</i> [BvBn], <i>Glycine max</i> [GmBn], <i>Sorghum bicolor</i> [SbBn], <i>Hordeum vulgare</i> [HvBn], and <i>Brassica napus</i> [BnBn]). BvBn had the highest total nitrogen, total potassium, available potassium, and organic matter contents. TaBn exhibited the highest soil enzyme activities, and its bacterial/fungal Chao1/Simpson indices and unique operational taxonomic units (OTUs; bacteria: 333, fungi: 37) exceeded other patterns. Principal coordinate analysis showed distinct microbial community separation in BvBn/TaBn versus BnBn. TaBn enriched dominant bacterial phyla Pseudomonadota and Actinomycetota; all preceding crops increased fungal phylum Ascomycota while reducing Mucoromycota. Comprehensive assessment confirmed all preceding crops, except oilseed rape altered rhizosphere microbial structure, with TaBn as the optimal preceding crops.