Regulatory Effects of Different Concentrations of NaCl on Cabbage Seedling Growth and Rhizosphere Microbiome

To investigate the effects of substrate salinity on cabbage seedling growth and the rhizosphere microbial community, this study used cabbage cultivar “Xusheng” as the test material. Four NaCl concentration treatments including 0 (CK), 0.5 (L), 1.0 (M) and 2.0 g/L (H) were established in a seedling substrate with the same basic formula. Seedling agronomic traits were measured, and 16S rRNA high-throughput sequencing was employed to analyze structural and functional changes in the rhizosphere microbial community. The results showed that under the L treatment, leaf area, aboveground dry weight, and belowground dry weight of cabbage seedlings increased by 12.61%, 19.35% and 25.00%, respectively, compared with CK, and the robust seedling index increased by 18.30%, demonstrating the best seedling cultivation performance. The M treatment was broadly comparable to CK, whereas the H treatment significantly inhibited cabbage seedling growth and resulted in the lowest robust seedling index. The microbial community structure of the L treatment did not differ significantly from that of CK, and the L treatment potentially enhanced stress-resistant functions of the microbial community, such as nitrogen respiration, nitrate reduction, and aromatic compound degradation. In contrast, the H treatment markedly weakened both growth-promoting and stress-resistant functions, with the sulfate-reducing bacterium Thermodesulfobacteriota becoming the dominant taxon and posing a potential toxic risk to seedlings. In conclusion, in substrate-based seedling cultivation, low-concentration NaCl may modulate the stress-resistant functions of the rhizosphere microbial community, thereby stimulating the growth potential of cabbage seedlings to a certain extent, whereas high-concentration NaCl leads to rhizosphere micro-ecological imbalance and the proliferation of harmful bacteria, which is detrimental to cabbage seedling growth.