Macrogenomic Technology Reveals that Hartz Mycorrhizal Fungicide and Intercropped Garlic Ameliorate Succession Disorders in Continuous Nonheading Chinese Cabbage (Brassica rapa L. subsp. chinensis)

Using metagenomic sequencing, we assessed the effects of Trichoderma harzianum and garlic intercropping on the growth, soil microbial community, and sustainability of continuous nonheading Chinese cabbage (NHCC; Brassica rapa L. subsp. chinensis ). Four treatments were evaluated: NHCC monoculture control treatment; NHCC monoculture inoculated with T. harzianum (T1); NHCC–garlic intercropping with water (T2); and NHCC–garlic intercropping inoculated with T. harzianum (T3). Growth parameters, physiological indices, soil quality, and rhizosphere microbial composition were analyzed. The results showed that T1, T2, and T3 significantly altered fungal and bacterial community composition in NHCC rhizosphere soil, and that T1 increased the relative abundance of Lysobacter antibioticus and decreased the relative abundance of Fusarium oxysporum . The T2 treatment increased the relative abundances of Dactylellina haptotyla , Bacillus subtilis , Nitrospira japonica , and Lysobacter antibioticus as well as suppressed Macrophomina phaseolina . The T3 treatment increased the relative abundances of Pochonia chlamydosporia , Bacillus subtilis , and Nitrospira japonica as well as decreased the relative abundances of Macrophomina phaseolina and Fusarium oxysporum . The Kyoto Encyclopedia of Genes and Genomes analysis revealed heightened enrichment of metabolism-related microorganisms in T2 and T3. The T1 treatment upregulated phoD (alkaline phosphatase), nirK, and nirS (nitrate reduction). The T2 treatment increased phoD, narG (nitrate reduction), nirS, and amoA (ammonia oxidation). The T3 treatment exhibited the broadest activation, elevating phoD, narG, nirK, nirS, and amoA. The T1, T2, and T3 treatments significantly increased soil available nitrogen and soil available potassium content. The T2 and T3 treatments increased exchangeable calcium (Ca 2+ ). The T3 treatment uniquely elevated available phosphorus. Organic matter, total phosphorus, and total potassium remained unaffected. All treatments improved NHCC leaf length, width, and fresh weight; T2 and T3 increased plant height; and T3 enhanced root length. The T1, T2, and T3 treatments reduced leaf soluble protein, superoxide dismutase, and malondialdehyde. The T2 and T3 treatments significantly increased the content of soluble sugar in leaves and significantly decreased the content of peroxidase. The T3 treatment significantly increased the content of chlorophyll in leaves. These results demonstrate that T. harzianum , NHCC–garlic intercropping, and NHCC–garlic intercropping inoculated with T. harzianum enhance NHCC growth, soil quality, and beneficial microbiota while suppressing pathogens. Furthermore, NHCC–garlic intercropping inoculated with T. harzianum yielded a superior effect, while the effect of NHCC monoculture inoculated with T. harzianum had the weakest effect.