Global roots of soil fungi: evidence that ectomycorrhizal fungi influence the composition and distribution of saprotrophic fungal communities

Background/Question/Methods

Ectomycorrhizal (EM) fungi and saprotrophic fungi are globally distributed and coexist in many forest ecosystems. There is evidence that these groups of fungi can compete for nutrients, differentiate spatially, and interact antagonistically, which can have implications for forest nutrient cycling. To better understand the distribution of these fungi within and across ecosystems we analyzed fungal community composition across soil horizons from seven EM and non-EM forests spanning from boreal to tropical latitudes. We separately collected O_i-e horizons and mineral soil (0-20 cm) and used 454 pyrosequencing to determine fungal community composition. In two of the EM-dominated ecosystems (a boreal and tropical forest), we used clone library sequencing to assign fungal identity and functional group status (EM vs. non-EM), when possible.

Results/Conclusions

Across forest ecosystems, the presence of dominant ectomycorrhizal trees had a significant influence on the structure of the general fungal community. Pyrosequencing revealed that compared to non-EM forests, fungal community composition was more similar among forests containing EM tree species, even when these forests were in dramatically different biomes (ANOSIM;  r = 0.56, P < 0.05). Fungal communities displayed strong biogeographical patterns, but in the EM-dominated forests, clone library analysis showed that spatial distributions of fungi were similar: saprotrophic fungi dominated the recently-shed leaf litter and EM fungi dominated the underlying soil horizons (ANOVA; P < 0.05). This vertical pattern of fungal segregation has also been found in temperate and European boreal forests, suggesting that these results apply broadly to EM-dominated systems. These results further indicate that measures of fungal community composition in forests may not be complete without separate analyses of organic and mineral soil horizons. Future research should prioritize the functional differences and interactions between EM and saprotrophic fungi to better understand biogeochemical cycling and nutrient sequestration in forests containing EM species.