Meta-analysis reveals globally sourced commercial mycorrhizal inoculants fall short

Introduction

Several researchers have highlighted the potential of microbial inoculants to advance sustainable agriculture (Elnahal et al., 2022; O'Callaghan et al., 2022). Among microbial inoculants, arbuscular mycorrhizal (AM) fungi have garnered attention for their ability to enhance soil health and plant fitness. AM fungi can increase plant growth through enhanced access to limiting soil resources, improve plant defense against herbivores and pathogens, increase tolerance to drought and salinity stress, and increase carbon sequestration (Reynolds et al., 2006; Bennett et al., 2009; Ji & Bever, 2016). With this promise, the commercial market for AM inoculants is rapidly growing, approaching 995 million USD globally (Mordor Intelligence, 2024). AM inoculants, often referred to as ‘endomycorrhizal’ inoculants on commercial product labels, are easily and widely available in many regions of the world.

Despite the optimism surrounding microbial inoculants, global studies have revealed inconsistencies with commercial products, including instances of crop mortality, unlabeled fertilizers, and nonviability (Corkidi et al., 2004; Tarbell & Koske, 2007; Faye et al., 2013; Duell et al., 2022; M. Salomon et al., 2022; Koziol et al., 2024). The benefits of commercial products can be limited by their narrow inclusion of the same four to five species, with many containing a single AM fungus in the Rhizophagus genus (Basiru et al., 2020), despite evidence that a more diverse AM fungal consortium may increase crop growth (Magnoli & Bever, 2023), nutrient uptake (Reynolds et al., 2006), and other benefits. Concerns regarding product mislabeling and contamination by fungal pathogens further highlight the potential risks associated with these products (Tarbell & Koske, 2007; Vahter et al., 2023). The lack of accountability for product viability is compounded by scientific assessments that often do not report product identities, although some do (Wiseman et al., 2009; Faye et al., 2020), making it a challenge for both the inoculant industry and for users to be informed on product quality concerns. Regulatory frameworks for mycorrhizal inoculants remain limited in many regions (Carrazco et al., 2024; M. J. Salomon et al., 2022), exacerbating challenges related to product viability and identity of mycorrhizal fungi in products. The United States fully lacks regulations on the import/export of mycorrhizal fungal products or quality control, despite the United States having a 25% share in the mycorrhizal inoculant industry, representing 249 million USD annually (Mordor Intelligence, 2024). Moreover, the global transport of inoculants may present a risk for future invasion of non-native microbes (Schwartz et al., 2006; Hart et al., 2017). While there are multiple causes for concern, to date there is no synthesis of the viability or efficacy of commercial inocula.

To comprehensively assess the global quality of mycorrhizal inoculants, we collected published comparisons and analyzed them using meta-analyses, mixed models, and categorical comparisons. Inoculants were assessed for their impact on crop growth and mycorrhizal viability. To avoid positive publication bias, only studies assessing five or more commercial products were included. This approach accounts for the high likelihood of selecting nonviable inoculants in certain regions (M. Salomon et al., 2022). Studies evaluating fewer inoculants may have an increased likelihood of publication bias for studies that happened to select a viable inoculant, skewing the results. In total, the analysis encompassed 302 inoculant trials, including 7 field soil trials, 28 laboratory-grown inoculant trials, 17 no inoculant added or sterilized inoculant controls, and 250 commercial AM product trials on the growth and mycorrhizal symbiotic associations among seven crop species. Only half of the observations reported the names of the products tested. Of the studies that reported product names, the same inoculants were not tested across multiple publications, but some were used across multiple experiments within a publication that tested inoculants in another context (e.g. different crops assessed). In total, a pool of 94 unique, globally sourced inoculants were evaluated. In contrast to commercial products, which do not report on propagation and quality control practices for their products, research laboratory-grown fungi (henceforth laboratory-grown) are meticulously evaluated through peer-reviewed scientific experiments. This process involves comprehensive documentation of inoculant propagation, nutrient additions, storage, handling, application methods, viability, and plant growth responses. Therefore, data are presented for studies that provided positive controls of field soil or laboratory-grown inoculants to compare to commercial products, which often lack transparent quality control frameworks.