Interactions between soil microbes can drastically alter ecosystem processes both above and belowground, but the mechanisms by which these microbes interact, and their impacts on soil biogeochemistry remain elusive. We aimed to characterize interactions between coniferous plants, their major root fungal symbionts (ectomycorrhizal fungi, EMF), and free-living saprotrophic decomposers (SAPs) in soil along gradients in resource availability. We performed a greenhouse-based synthetic ecosystem experiment with Pinus taeda seedlings growing 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 expected that under low soil C, EMF prime decomposer activity and increase the release of soil C as CO2, while under high soil C, EMF slow decomposition and reduce soil CO2 release, with EMF competing with SAPs for access to soil organic matter (i.e. the Gadgil effect). These processes would be exacerbated under high plant C availability to EMF, but suppressed under high soil N.
Results/Conclusions
We found that EMF prime decay of soil organic matter under low soil C, but slow decay under high soil C. Elevated soil N suppressed the EMF effect on soil C-derived CO2 losses. Under elevated CO2, soil CO2 release was always higher with more soil C. There was a tendency for greater soil CO2 release when N was added to soils with high soil C in the presence of EMF, although we didn’t find strong evidence of this pattern. Together, our results suggest that the direction of EMF-SAP interactions is highly dependent on soil C and N availability to SAPs and might change according to plant C level.