Asymmetric priority effects between commercial and native arbuscular mycorrhizal fungi in grapevine

Arbuscular mycorrhizal fungi (AMF) are increasingly recognized as a sustainable alternative to conventional fertilization in viticulture. This study evaluated how inoculation sequence between the pure commercial AMF Rhizophagus irregularis (Błaszk., Wubet, Renker & Buscot) Walker & Schüßler and native AMF communities influences grapevine (Vitis vinifera L. cv. Merlot) performance, and root-associated AMF community assembly. Six treatments were compared: a sterile control, R. irregularis (Rhizophagus), native AMF (Natural), simultaneous co-inoculation (Mix), and two sequential inoculation strategies (RNat and NatR). Plant biomass and photosynthetic activity were assessed, while 18S rRNA amplicon sequencing was used to characterize AMF community composition. Soil microbial metabolic activity was assessed to evaluate the influence of the accompanying bacterial community, revealing no significant differences. AMF inoculation significantly enhanced grapevine biomass and photosynthetic performance compared with the control. Sequential inoculation with the commercial inoculum applied first (RNat) resulted in the greatest biomass accumulation and markedly influenced AMF community composition. Although alpha diversity did not differ significantly among treatments, prior establishment of the native AMF community (NatR) favoured the persistence of several low-abundance native taxa, whereas early establishment of R. irregularis generated communities dominated by the R. irregularis. From an applied perspective, these results suggest that delaying commercial inoculation until native AMF are established may preserve a more diverse mycorrhizal community, albeit at the cost of a reduced growth benefit, a trade-off with direct implications for sustainable inoculation practices in viticulture.