AMF genotype and host identity determine AMF inoculant performance.

The use of arbuscular mycorrhizal fungi (AMF) as biofertilizers is limited by variable outcomes. Although differential outcomes among AMF and hosts are well documented, the relative importance of fungal identity, host identity, and soil nutrient availability in determining symbiotic outcomes remains unclear, particularly in soils containing resident AMF communities. We crossed five AMF isolates spanning a gradient of phylogenetic relatedness and ecological strategies with four host plant species under two soil nutrient levels in a greenhouse experiment. We quantified inoculant establishment and target gene abundance using isolate-specific digital droplet PCR assays and measured host responses as mycorrhizal responses for root biomass, shoot biomass, and shoot phosphorus content. Differential effects occurred in both directions: AMF isolates differed in their performance among hosts, and hosts differed in their responses to AMF isolates, including conspecific Rhizophagus isolates. AMF isolate identity was the strongest predictor of inoculant establishment and target abundance in roots and soil, whereas host identity explained much of the variation in plant responses. Rhizophagus irregularis isolates consistently established more successfully than other taxa and reached target gene root abundance several orders of magnitude greater than Acaulospora and Gigaspora. Plants inoculated with Gigaspora experienced the most positive growth response overall. Our results show that the ability of AMF inoculants to establish and proliferate within resident communities is largely independent of their capacity to benefit host plants. Thus, evaluating biofertilizer candidates solely on measures of host or fungal performance is insufficient; effective AMF inoculants must be assessed for both competitive establishment and mutualistic function across diverse host and soil communities.