Ectomycorrhizal fungi: mediators of plant-microbial interactions and terrestrial biogeochemistry

Abstract:

Ectomycorrhizal fungi (EMF) that are symbiotic with tree roots can shape forest biogeochemical cycling by mining soil for nitrogen (N), increasing plant carbon (C) allocation belowground, and interacting with free-living saprotrophic (SAP) microbes that decompose soil C. Hypothetical plant-EMF-SAP interactions can alter our predictions of forest biogeochemistry to the point that it affects projected future climate stability, yet the mechanisms of these interactions – and the conditions under which they occur – remain unclear. We aimed to test the hypothesis that EMF slow decomposition and reduce soil CO2 release by competing with SAPs for access to N in soil organic matter (i.e. the Gadgil effect) and that these processes would be exacerbated under high plant C availability to EMF, but suppressed under high soil N. To test this hypothesis, we performed a series of greenhouse-based synthetic ecosystem experiment using Pinus taeda seedlings with and without their EMF symbiont (Suillus cothurnatus) under high and low levels of soil carbon (C), soil nitrogen (N), and plant C (ambient vs. elevated carbon dioxide (CO₂)). We found that EMF primed decay of soil organic matter under low soil C, increasing expression of genes involved in fermentation and soil C and N depolymerization by both SAPs and EMF. By contrast, EMF slowed decay under high soil C and elevated soil N suppressed the EMF effect on soil C-derived CO₂ losses. Elevated CO₂ reversed these trends: EMF suppressed soil CO2 release under low soil C but had no effect under high soil C. Together, these results suggest that the direction of EMF-SAP interactions is highly dependent on soil C availability to SAPs and might reverse according to plant C allocation belowground to access soil N.