Untangling the effect of roots and mutualistic ectomycorrhizal fungi on soil metabolite profiles under ambient and elevated carbon dioxide

Metabolites in soil play an important role in regulating plant-microbe interactions and, thereby, plant performance. Biotic factors such as root exudation and microbial activity or abiotic factors such as the concentration of atmospheric carbon dioxide (CO2) can drive both quantitative and qualitative changes in soil metabolite profiles. While the impact of these factors, either in isolation or in combination, are underexplored in soil systems due to technical challenges, recent technological advances have enabled these hurdles to be overcome. Given the key role that mutualistic ectomycorrhizal (ECM) fungi play in forest soils through their symbiotic interaction with trees, and the foreseen changes in forest dynamics with climate change, we investigated the effect of the Eucalyptus grandis-Pisolithus albus (plant host-fungus) association on soil metabolite profiles under ambient and elevated CO2 conditions (aCO2 and eCO2). We found that significant metabolite enrichment predominately occurred in the rhizosphere where a strong effect by ECM fungus was also observed. Specific ECM fungus-induced metabolites were enriched concurrently with an increased host plant root:shoot ratio, suggesting that the influence of ECM fungus on rhizosphere metabolite profiles may impact plant growth. Strikingly, however, we found no observable differences in soil metabolite profiles between the aCO2 and eCO2 conditions, which may be due to nutrient limitation given the low level of nutrients found in typical eucalyptus forest soils. Overall, our findings increase our understanding of soil metabolic processes at the symbiotic plant-microbe interface under current and future atmospheric CO2 scenarios.