Functional diversity of arbuscular mycorrhizal fungi drives divergent plant resource allocation strategies under nitrogen limitation

Arbuscular mycorrhizal (AM) fungi shape plant nutrient acquisition, yet their functional roles under nitrogen (N) limitation remain unclear. Using Plantago lanceolata inoculated with five AM fungi strains under N-limited conditions, we demonstrate that fungal identity critically regulates symbiotic outcomes. Despite interspecific variation in extraradical mycelium production and hyphal traits (carbon/nitrogen/phosphorus concentrations [C/N/P], diameter), AM fungi universally failed to enhance plant biomass. We identified three plant-AM fungal phenotypic strategies under N-limitation: (1) high mycelial investment with carbon drain (Rhizophagus irregularis WFVAM23, Funneliformis mosseae WFVAM45), where sustained fungal growth reduced shoot biomass; (2) early mycelial growth with root trait modulation (R. irregularis DAOM10, Scutellospora calospora WFVAM35), showing rapid initial mycelial growth that plateaus, exerting neutral effects on plant biomass. Meanwhile, S. calospora WFVAM35 reduced root diameter while R. irregularis DAOM10 increased RTD; and (3) low mycelial production with minimal host impact (Gigaspora margarita WFVAM21), with minimal mycelial biomass across all growth stages. AM fungi functional diversity reshapes plant resource allocation across a mutualism-parasitism spectrum under N limitation, mediated by fungal mycelium traits and root architectural reconfiguration. N availability and fungal identity emerge as pivotal determinants of mycorrhizal phenotypic plasticity, emphasizing the critical role of trait-based frameworks to predict plant-fungal adaptation dynamics in nutrient-limited ecosystems.