Field inoculation with a local arbuscular mycorrhizal (AM) fungal consortium promotes sunflower agronomic traits without changing the composition of AM fungi coexisting inside the crop roots

Improving reliability and effectiveness of microbial inoculants in crops is a pressing necessity due to recent in- creases in price of synthetic fertilizers and environmental concerns related to their application. Usually, field in- oculation of beneficial microbes, such as arbuscular mycorrhizal fungi (AMF), manipulates abundance and species composition, making it difficult to disentangle their independent effects. In this study, we investigated for the first time the mechanisms behind the agronomic performance of sunflower after field inoculation with a local AM fungal consortium under high and low soil fertility. The abundance of AMF in roots was promoted by inocu- lation more in low than high soil fertility. In both soil conditions, up to 68 % of the AM fungal taxa retrieved in roots were shared between the inoculated and control plants, confirming minor changes in AM fungal commu- nity composition. On the contrary, the structure of AM fungal community was modified by inoculation. Inocula- tion improved grain yield by 16 % in low soil fertility, oil yield up to 36 %, and enhanced grain content of nutri- ents under both soil conditions. The best predictor of agronomic performance of sunflower was percentage of AM fungal root colonization in high soil fertility and percentage of vesicles in low fertility. The structure of AM fun- gal community was not correlated with crop functional parameters under high soil fertility, while under low fer- tility the occurrence of Rhizophagus sp. VTX00105 in roots was the best predictor. Overall, our results demon- strated that local AM fungal inoculants do not affect root AM fungal composition, but increases abundance and modifies the structure of AM fungal community in roots. These modifications are associated with improvements in sunflower grain and oil yield, and in seed nutritional value, especially in low soil fertility. However, the mech- anisms behind the functioning of field inoculum on crop performance were revealed to be context-dependent.