Ecosystem type drives root-associated AM fungi distribution across North American Grasslands: A functional guild approach

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) are ancient plant symbionts and nutrient exchange forms the basis of this plant-microbe relationship. Plants provide carbon (C) to AMF and the fungi provide the plant with nutrients such as nitrogen (N) and phosphorous (P). Fertilization can reduce plant reliance on AMF, resulting in diminished AMF abundance and diversity or altered community composition, and may select for AMF symbionts with lower C demand. Because the impacts of fertilization on AMF are most described in relatively short, highly controlled experimental systems, or in the context of specific plant hosts, the impacts of fertilization on AMF communities across ecosystems is unclear. We hypothesized fertilization incentivizes plants to reduce carbon allocations to AMF, selecting against AMF with extensive extraradical hyphal networks. We predicted the impacts of fertilization on plant-AMF relationships are additive, with the AMF in N+P amended plots responding most strongly. To test this, we used AMF specific sequencing primers on root DNA to assess the AMF community composition and functional guild distribution across the Nutrient Network, a set of long-term experimental plots amended with N, P, or both. We targeted a range of grassland ecosystems including tallgrass prairies, montane grasslands, shortgrass prairies, and desert grasslands.

Results/Conclusions

We found no consistent response to fertilization within AMF functional guilds or in the full community. There was also no impact of fertilization on AMF percent colonization of roots or alpha diversity. Instead, ecosystem type was the main driver of AMF root colonization (R2 = 0.26), diversity (R2 = 0.21), and community composition (R2 = 0.23). Ecosystem type was also the main driver of plant community composition (R2 = 0.39) and diversity (R2 = 0.44), and AMF community composition and diversity trends matched vegetation patterns. The differences in AMF abundance and diversity appeared to be driven by root moisture (R2 = 0.60) and aridity (R2 = 0.59) in each ecosystem, which strongly correlated with AMF community composition. Rhizophilic AMF, which proportionally concentrate their hyphae inside roots (intraradical) had the highest relative abundance in the driest ecosystems. In contrast, edaphophilic AMF with extensive extraradical hyphal networks had the highest relative abundance in the moist ecosystems. These results emphasize the importance of long-term, large scale research projects to provide ecologically relevant context to laboratory models. By examining a broad sampling of plant roots in different ecosystems, we challenge the current conceptual models on the responses of AMF to long-term soil nutrient additions.