Density-Dependent Changes in Root Exudates, Rhizosphere Microbes, and Soil Metabolites of Solanum rostratum

Intraspecific competition shapes high-density monodominant communities of invasive plants, but how density is associated with rhizosphere microbial reassembly and soil metabolite accumulation via root exudates remains unclear. We established four planting densities (1, 3, 5, and 10 plants per pot) and integrated growth and photosynthetic physiology, root exudate metabolomics, rhizosphere metagenomics, and soil metabolomics using Mantel tests and variable-level correlation analysis. Increasing density reduced all traits except plant height, which showed a slight, non-significant increase followed by a significant decrease. Root exudates shifted from phenolic acid/terpenoid dominance at low density to sugar/peptide dominance at high density. Low-density-enriched bacterial genera were positively correlated with biomass, whereas high-density-enriched genera (e.g., Ilumatobacter, Allosphingosinicella) were negatively correlated. Soil metabolites such as Glutarate Semialdehyde and Sutherlandin D were negatively correlated with biomass, root traits, and photosynthesis. Mantel tests showed strong correlations between plant phenotypes and root exudates (r = 0.80) and between exudates and soil metabolites (r = 0.83). Variable-level analysis linked phenolic acid exudates to putatively growth-associated bacteria (e.g., Flavitalea, Blastococcus) and sugar/peptide exudates to putatively stress-adapted bacteria (e.g., Allosphingosinicella, Ilumatobacter), which were positively correlated with Glutarate Semialdehyde and Sutherlandin D accumulation. These results indicate that density-associated exudate changes synchronized with rhizosphere microbial reassembly and soil metabolite accumulation, forming an “exudate–microbe–metabolite” cascade. This cascade may buffer against high-density intraspecific competition and help explain monodominant community formation in invaded ranges.