Soil microbiomes conditioned by long‐term warming affect plant belowground performance

Abstract Global change affects plant performance, both directly through warming and indirectly through changes in their biotic and abiotic surroundings. Soil microbes can critically influence plant performance, but are vulnerable to warming themselves. Disentangling direct effects of warming on plants from those intermediated by changes in microbial populations is complex under field conditions. To distinguish those effects, we monitored the performance of Agrostis capillaris and Anthoxanthum odoratum grown under uniform and controlled glasshouse conditions in soils inoculated with soil microbiomes conditioned by ambient, medium (14 years; MTW) or long‐term (>55 years; LTW) geothermal warming. This was replicated under normal watering or drought conditions to additionally assess stress resistance. Furthermore, we analysed the microbiome of the inocula through metabarcoding to identify root‐associated fungi and compare their relative abundance under different warming conditions. We found a decreased belowground biomass of both plant species when grown with LTW‐conditioned microbiomes, with an exacerbated effect under drought for Ag. capillaris . We did not observe an associated increase in aboveground biomass, resulting in an increased aboveground biomass:belowground biomass ratio. These changes coincided with concurrent increases in the relative abundance of putative plant pathogens and arbuscular mycorrhizal fungi. We therefore conclude that soil microbes can mediate warming effects on plant performance through reduced belowground biomass.