Effect of arbuscular mycorrhizal fungi on metabolic characteristics of microbial community in Solanum nigrum rhizosphere soil with lead stress

Pot experiments were conducted to explore the effect of arbuscular mycorrhiza (AM) fungi on the characteristics and differences in rhizosphere soils of Solanum nigrum under different levels of lead stress0 mg·kg^-1 (CK), 200 mg·kg^-1, 400 mg·kg^-1, 800 mg·kg^-1). To this end, S. nigrum was inoculated with Funneliformis mosseae and Claroideoglomus etunicatum, and the resulting rhizosphere soils collected after 10 weeks of growth under 16 h/8 h, 28℃/20℃ with 12 000 Lux light intensity in a greenhouse. The characteristics and differences in carbon metabolic profiles of microbes were evaluated using the Biolog-ECO micro-plate method. The results showed that:1) the average metabolic activity of microorganisms, described by average well color development (AWCD), decreased-increased-decreased with increasing lead stress. Inoculation with AM fungi improved AWCD significantly under lead stresses, except for 400 mg·kg^-1 Pb⁴⁺. 2) The ability of soil microbes to utilize the four types of carbon substrates (carbohydrate and derivatives, fatty acid and lipids, amino acids, and metabolites) was higher under 400 mg·kg^-1 Pb⁴⁺ than under low (200 mg·kg^-1) and high (800 mg·kg^-1) stresses of Pb⁴⁺. Inoculation with AM fungi significantly improved the ability of soil microbes to utilize amino acid substrates under high lead concentration. 3) Lead stress reduced McIntosh evenness index of rhizosphere soil microbial community, but had no significant impact on richness index, Shannon-Wiener diversity index and Simpson index. Inoculation with AM fungi increased microbial diversity index under lead treatment, and had significant effects on Species richness index, Shannon-Wiener diversity index and Simpson dominance index at medium stress. 4) Under the same Pb⁴⁺ stress, inoculation of AM fungi enhanced metabolic capacity of soil microorganisms for the four major carbon sources, but only had significant effect on amino acids. Principal component analysis showed that metabolites were the most abundant in PC1 and PC2, with respective values of 6 and 4. Sugar was the most abundant in PC3. 5) Under combined effects of lead stress and AM fungi, microbial carbon source utilization was mainly regulated by lead concentration, both had significant interaction effects on microbial carbon source utilization. In conclusion, inoculation with AM fungi increased microbial diversity index of rhizosphere soils of S. nigrum under lead stress, and enhanced utilization of carbon source substrate by rhizosphere soil microbes. This study provided theoretical basis for the application of AM fungi which could strengthen the use of phytoremediation technology in soil remediation for heavy metal pollution.