Emerging evidence from field experiments and global scale analyses suggest ectomycorrhizal fungi can alter plant growth and soil carbon (C) cycling via competition with free-living microbial decomposers for nitrogen (N). However, substantial variation exists in the sign and magnitude of measured ectomycorrhizal-decomposer interactions. This lack of mechanistic understanding of ectomycorrhizal-decomposer interactions inhibits the incorporation of mycorrhizal ecology into predictive models of ecosystem and Earth C cycling. We hypothesized ectomycorrhizal fungi affect both plant growth and soil C cycling, and that these effects are modulated by environmental N availability. To test these hypotheses we used a laboratory model ecosystem with Pinus taeda and ectomycorrhizal fungi and tracked mycorrhizal effects on ecosystem C and N cycling using natural abundance 13C stable isotopes. We crossed the presence or absence of the model ectomycorrhizal fungus Suillus cothurnatus with four levels of N fertilization to understand how ectomycorrhizal fungi affect plant growth, free-living soil microbial activity and soil biogeochemistry, and the degree to which these interactions are mediated by environmental N availability.
Results/Conclusions
We found strong interactive effects of ectomycorrhizal fungi and N fertilization on above and belowground processes. Both Suillus inoculation and N fertilization increased plant biomass relative to control, but these effects interacted such that Suillus increased plant biomass at low levels of N fertilization, but reduced plant biomass at high levels of fertilization. This appeared to be linked to soil N availability. Both inoculation with Suillus and N fertilization increased inorganic N pools, yet there was an interaction such that Suillus reduced inorganic N pools at the highest levels of N fertilization. This was likely linked to the effect of Suillus on decomposition overall. Suillus inoculation and N fertilization significantly increased soil organic matter decomposition and CO2 losses, yet these effects interacted such that the effect of Suillus on decomposition was undetectable at the highest level of N fertilization. These results demonstrate the importance of ectomycorrhizal symbionts for ecosystem processes, and interactions with N fertilization suggest that environmental N availability alters plant-mycorrhizal-decomposer interactions with direct consequences for ecosystem scale C cycling. Ongoing work is probing changes in gene expression and the metabolome of these soils to understand the mechanisms behind these emergent changes in ecosystem C and N cycling.