Are interactions between ectomycorrhizal fungi and saprotrophic fungi driven by soil pH constraints on enzyme function?

Christopher Fernandez, Plant & Microbial Biology, University of Minnesota, St. Paul, MN and Craig R. See, Ecology, Evolution, and Behavior, University of Minnesota, St Paul, MN

Background/Question/Methods
Interactions between ectomycorrhizal (EM) fungi and free-living saprotrophic fungi are central to the cycling of soil organic matter (SOM) in forest ecosystems, but the nature of these interactions and subsequent effects on SOM cycling is inconsistent across studies. EM fungi have been found to decelerate SOM decomposition by out-competing saprotrophs for nutrients via ‘nitrogen mining’ mechanisms. In contrast, EM fungi have also been shown to increase SOM decomposition via the production of non-targeted oxidative enzymes, or by providing a labile carbon source to saprotrophs (i.e. “priming”). We contend that the context dependency of these EM-saprotroph interactions can be explained in part by the functional diversity in EM enzymatic capabilities, which in turn depend soil chemistry. Specifically, we hypothesized that nutrient competition between guilds would be more prevalent in systems where SOM substrates favor nutrient limitation (e.g. high C:N), and at higher soil pH (>5), where enzyme hydrolysis is a more effective foraging strategy. We tested this hypothesis using publicly available datasets to examine global patterns of EM and saprotrophic fungal interactions and their influence on soil carbon and nutrient cycling.

Results/Conclusions
Globally, EM dominance of soil communities (measured as the ratio of EM to saprotrophic fungi) was positively related to soil pH (P<0.0001) and was consistent across forest biomes (Biome x Soil pH: P=0.47). EM fungal dominance did not correspond to proxies of carbon and nutrient cycling in a consistent manner across tree host genera and biomes, supporting our hypothesis and emerging evidence for context dependency. Specifically, nutrient competition between EM fungi and saprotrophs appears to be strongly associated with EM fungal communities of Pinus spp., which have relatively low litter quality (e.g. lignin:N ratio) and likely favors N mining by EM fungi. Conversely, EM fungal dominance in angiosperm systems show strong relationships with accelerated N cycling particularly in low pH soils. Proteins represent large pools of organic nitrogen in forest soils and the enzymatic hydrolysis of this otherwise labile pool is largely controlled by hydrogen bonding with polyphenolics, such as tannins, in forest soils with low pH (ca. <5). Our analysis of guild abundance as it relates to pH that tannin-protein complexes may be another key determinant of how EM fungi either prime or suppress saprotrophs, with implications for C and N cycling.