Global responses of soil bacteria and fungi to inoculation with arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi exist naturally in most ecosystems and have significant effects on other soil microbes, but how these effects vary across ecosystem types and experimental conditions remains unclear. We compiled a global dataset to identify the responses of soil bacteria and fungi to AM fungal inoculation, and whether these responses vary or are consistent across ecosystem types (cropland, grassland, and forest) and experimental conditions (sterilized vs. non-sterilized soils; mixed vs. single AM fungal inoculation; and field studies vs. greenhouse experiments). Our analysis of the global data showed that AM fungal inoculation increased almost all soil bacterial and fungal parameters examined (1.4-45.0 %), except for fungal abundance which decreased significantly (-13.6 %). Plant, shoot, and root biomass were also increased by 23.3 %, 15.7 %, and 30.1 % after AM fungal inoculation, respectively. Additionally, the effects of AM fungi on most soil bacterial and fungal metrics did not differ significantly among ecosystem types and experimental conditions applied. Changes in pH and soil organic carbon were key factors contributing to the responses of bacteria and fungi. These findings highlight the consistent positive effects of AM fungal inoculation on soil bacteria and fungi across various ecosystem types, which may simplify the prediction of AM fungal functionality and related important ecological processes at large spatial scales. Moreover, the roles of bacteria and fungi in supporting ecosystem functionality may be overestimated without considering the effects of AM fungi. Therefore, the effects of AM fungi on soil bacteria and fungi should be explicitly disentangled when evaluating the roles that bacteria and fungi play in terrestrial ecosystems.