AMF components from a commercial microbiome inoculum fail to colonize roots and lack soil persistence in an arable maize field.

Arbuscular mycorrhizal fungi (AMF) are a group of plant root obligate biotrophs that are considered biofertilizers, providing the host with water, mineral nutrients and pathogen protection, in exchange of photosynthetic compounds. A decline in AMF diversity, e.g. due to agricultural input, can lead to a less efficient ecosystem functioning. A sustainable strategy to re-establish AMF diversity is to supply the target soil with AMF inoculants. After inoculation, it is essential to verify whether the inoculants effectively colonize the host plant and persist into the soil, and if the resident AMF community is affected. The AMF components of a microbiome inoculum (including also PGPR, Trichoderma sp. and Beauveria sp.), applied to the maize rhizosphere in an arable field, were molecularly characterized and traced in field by Next-Generation Sequencing (454-pyrosequencing) of the partial rRNA 18S gene. In addition, mycorrhizal colonization levels were assessed in inoculated and non-inoculated maize. The bioinformatic elaboration of data outputted 61,752 good quality AMF reads that were affiliated to 56 taxa, covering all four Glomeromycota orders. The inoculum consisted of 28 taxa, 26 of which (>99% reads) belonging to Glomeraceae. The inoculated AMF taxa generally failed to colonize roots and lacked soil persistence. Nevertheless, the inoculation process reduced AMF species dominance, triggering higher intraradical AMF diversity. Mycorrhizal levels were optimal in non-inoculated roots and no differences were spotted compared to inoculated roots. The observed increase in inoculated plant biomass was potentially due to the other components of the applied microbiome. We encourage small-scale trials to test commercial inoculum suitability previous to large-scale application, since the combination of several factors, including soil native mycorrhizal status, host-plant preference, and environmental conditions, can influence the success of AMF inoculation.