Density-Driven Root Exudate Remodeling Promotes Pathogen Enrichment and Exacerbates Negative Plant–Soil Feedback in Panax notoginseng Monoculture Systems

Negative plant–soil feedback (NPSF) drives yield decline in monocropping systems, yet how intraspecific competition modulates NPSF across planting densities remains unclear. We conducted a two-stage plant–soil feedback experiment using five crops (Triticum aestivum L., Zea mays L., Solanum lycopersicum L., Cucumis sativus L., and Panax notoginseng (Burkill) F.H. Chen) with contrasting NPSF intensities under four planting densities (30 × 30 to 8 × 8 cm). Crops with stronger NPSF (P. notoginseng) showed pronounced density-dependent biomass reductions, whereas those with moderate (S. lycopersicum, C. sativus) or low (Z. mays, T. aestivum) NPSF were largely density-insensitive. Given its sensitivity, P. notoginseng was used to explore mechanisms. High-density planting (8 × 8 cm) intensified NPSF, reducing seedling survival by 88.54% and biomass by 56.08% compared with low-density controls (30 × 30 cm). Microbiome profiling showed enrichment of pathogenic Fusarium spp. and depletion of beneficial Humicola spp. under high density. Metabolomic analysis identified linoleic acid and oleamide as key root exudates upregulated under high-density stress, which selectively stimulated Fusarium growth as preferred carbon sources. Collectively, these results reveal a density-dependent feedback in which intensified competition reshapes root exudation, promotes pathogen proliferation, and suppresses beneficial taxa, thereby amplifying NPSF. This provides mechanistic insights into microbially mediated NPSF under density stress and highlights the importance of optimizing planting density to sustain crop productivity.