Grass cover profoundly modifies rhizosphere carbon cycling, yet the mechanistic links between root exudate chemistry and soil carbon stabilization remain elusive. A field experiment in citrus orchards was conducted using four ground-cover species, including spontaneously established native vegetation (Gnaphalium affine) and three sown cover crop species (Lolium multiflorum, Trifolium repens, and Vicia villosa), to investigate how root exudates modulate microbial network architecture and soil carbon fractions. Leguminous Vicia villosa markedly increased soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) by 6.92, 4.37, and 2.55 g kg(-1), respectively, compared with clean tillage. Metabolomic analysis revealed that grass-specific exudates were dominated by lipids (27.7%) and organic acids (19.2%), whereas Vicia villosa released nitrogen-rich metabolites such as lysine-leucine and adenine. These metabolites enriched keystone taxa (e.g., Bacilli, Acidimicrobiia) and enhanced bacterial network connectivity, promoting metabolic cross-feeding and organo-mineral complex formation. Partial least squares structural equation modelling confirmed that root exudates strongly regulated bacterial networks (path coefficient = 0.78, p < 0.001), indicating a strong regulatory influence, which in turn directly increased POC (0.46) and MAOC (0.55). In contrast, fungal networks exhibited non-significant effects on soil carbon fractions (p > 0.05). These findings elucidate a root exudate-bacterium co-regulation mechanism underlying soil carbon stabilization and provide a mechanistic basis for designing carbon-smart orchard management through legume-based grass cover.