Global evaluation of commercial arbuscular mycorrhizal inoculants under greenhouse and field conditions

Arbuscular mycorrhizal fungi (AMF) are plant symbionts that promote plant growth and provide important plant and ecosystem functions. These abilities have great economical potential which has resulted in an increasing number of commercially available AMF inoculants. Here, we present the results of a global study in which we evaluate the effectiveness of 28 commercial AMF inoculants to colonize host plants under greenhouse and field conditions. This evaluation includes three independent studies across three continents (Australia, Europe, and North America). The Australian and European studies tested 25 different commercial AMF inoculants in non-sterilized and sterilized soils under greenhouse conditions and compared them against laboratory cultures of the AMF Rhizophagus irregularis. This is supplemented by the North American study which evaluated the effects of three commercial inoculants under field conditions. In the greenhouse trials using non-sterilized soil, we observed that the addition of commercial inoculants did not lead to enhanced mycorrhizal colonization and inoculation increased plant biomass in only one out of 25 treatments. In sterilized soil, 84% of the mycorrhizal inoculants did not lead to mycorrhizal root colonization, demonstrating that these products did not contain viable propagules. In contrast, the laboratory cultures of the AM fungus Rhizophagus irregularis resulted in substantial root colonization (48% and 79%) in the Australian and European bioassay. Moreover, only five out of 25 treatments enhanced plant biomass when added to sterilized soil. Metagenomic analysis of field roots in the North American field trial revealed changes in the mycorrhizal community after inoculation. For one inoculant, this was accompanied by increased biomass production. This global evaluation of commercial inoculants raises concerns over unreliable products which do not contain viable propagules and do not result in mycorrhizal root colonization. Under field conditions, effects on plant growth are dependent on changes within the mycorrhizal community. The results of this study highlight the need for standardized quality control of AMF inoculants and further research on their establishment and effects under field conditions.