Enzyme production as a key mycorrhizal trait

Background/Question/Methods: Mycorrhizal fungi exist at the interface of the producer-decomposer continuum within an ecosystem. Nearly all plants on Earth associate with some form of mycorrhizal fungi and there is tremendous diversity among the species of fungi that participate in this symbiosis. The trait most central to the potential effects of mycorrhizal fungi on ecosystem element cycling is the production of extracellular enzymes that degrade soil organic matter. The ability to degrade organic matter has been argued to be the mechanistic foundation for biome-scale differences in the distributions of mycorrhizal functional types and the consequences of these distributions for biogeochemical cycles. We contend that production of nitrogen-degrading enzymes by ectomycorrhizal fungi drives inhibition of free-living decomposition via competition for soil nutrients, while the lack of nitrogen-degrading enzyme production by arbuscular mycorrhizal fungi selects for facilitation of free-living decomposers. We test these hypotheses by conducting experimental exclusions of arbuscular and ectomycorrhizal fungi and reciprocally transplanting soils between sites of contrasting mycorrhizal dominance.

Results/Conclusions: Exclusion of mycorrhizas in ectomycorrhizal-dominated systems increased CO₂ respiration, nitrogen mineralization rates, and production of hydrolytic enzymes per unit microbial biomass. This is indicative of competition between ectomycorrhizal fungi and free-living decomposers for soil nutrients. The magnitude of the exclusion effect also scaled with ectomycorrhizal abundance: sites with higher levels of ectomycorrhizal fungi exhibited greater increases in carbon and inorganic nitrogen cycling upon mycorrhizal exclusion. Excluding arbuscular mycorrhizal fungi in the arbuscular mycorrhizal system had no detectable effect on carbon or nitrogen cycling. Reciprocal transplants showed that net nitrogen mineralization rates significantly declined when arbuscular mycorrhizal soils were moved to the ectomycorrhizal stand. In contrast, more nitrogen was mineralized when ectomycorrhizal soils were moved to the arbuscular mycorrhizal stand. While these results are limited to a single set of experimental conditions in a particular temperate forest, evidence is mounting from many systems that this distinction between mycorrhizal classes is useful for predicting mycorrhizal effects on ecosystem level carbon and nitrogen cycling rates. Further distinctions in fungal enzyme production, such as different types of oxidative and cellulolytic enzymes, may be useful for understanding soil and ecosystem level processes.