ORIGINAL ARTICLE Is a mixture of arbuscular mycorrhizal fungi better for plant growth than single-species inoculants? Thomas Crossay 1 & Clarisse Majorel 1 & Dirk Redecker 2 & Simon Gensous 1 & Valérie Medevielle 1 & Gilles Durrieu 1 & Yvon Cavaloc 1 & Hamid Amir 1 Received: 7 January 2019 /Accepted: 17 May 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Inoculation of arbuscular mycorrhizal fungi (AMF) as plant growth promoters has mostly been conducted using single-species inoculum. In this study, we investigated whether co-inoculation of different native AMF species induced an improvement of plant growth in an ultramafic soil. We analyzed the effects of six species of AMF from a New Caledonian ultramafic soil on plant growth and nutrition, using mono-inoculations and mixtures comprising different numbers of AMF species, in a greenhouse experiment. The endemic Metrosideros laurifolia was used as a host plant. Our results suggest that, when the plant faced multiple abiotic stress factors (nutrient deficiencies and high concentrations of different heavy metals), co-inoculation of AMF belonging to different families was more efficient than mono-inoculation in improving biomass, mineral nutrition, Ca/Mg ratio, and tolerance to heavy metals of plants in ultramafic soil. This performance suggested functional complementarity between distantly related AMF. Our findings will have important implications for restoration ecology and mycorrhizal biotechnology applied to ultramafic soils. Keywords Arbuscular mycorrhizal fungi . Mixed inocula . Abiotic stress . Heavy metals . Restoration ecology . Ultramafic soil Introduction Arbuscular mycorrhizal fungi (AMF) are obligate symbionts having a huge potential to produce biostimulants for plants (Berruti et al. 2016). AMF improve plant growth by different mechanisms, including increased mineral nutrition (Hawkins et al. 2000; Smith and Read 2008; Feddermann et al. 2010), enhanced defense against pathogens and insects (Pozo and Azcón-Aguilar 2007; St-Arnaud and Vujanovic 2007; Shrivastava et al. 2015), and increased abiotic stress tolerance (Ruiz-Lozano and Aroca 2010; Bárzana et al. 2012; Augé et al. 2015; Ferrol et al. 2016). However, the effects of AMF Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00572-019-00898-y) contains supplementary material, which is available to authorized users. * Thomas Crossay thomasjc.crossay@gmail.com * Hamid Amir hamid.amir@univ-nc.nc Clarisse Majorel clarisse.majorel@ird.fr Dirk Redecker dirk.redecker@inra.fr Simon Gensous s.gensous@gmail.com Valérie Medevielle valerie.medevielle@unc.nc Gilles Durrieu gilles.durrieu@univ-nc.nc Yvon Cavaloc yvon.cavaloc@univ-nc.nc 1 Present address: Institut des Sciences Exactes et Appliquées (ISEA), Université de la Nouvelle-Calédonie, BP R4, 98851 Nouméa Cedex, New Caledonia 2 Agroécologie, AgroSup Dijon, CNRS, INRA, Universite Bourgogne Franche-Comté, F-21000 Dijon, France Mycorrhiza https://doi.org/10.1007/s00572-019-00898-y