Human activities have increased the amount of available nitrogen (N) globally. Increased N-availability can change the community structure and diversity of both plants and their belowground microbial associates. We investigated how N enrichment can alter plant-associated fungal communities in moist meadow alpine tundra. We focused on two co-dominant plants with differing responses to N-enrichment: Deschampsia cespitosa (Poaceae) increases in abundance in response to N while Geum rossii (Roceaceae) decreases. We sampled an 8-year N fertilization experiment at Niwot Ridge, on the Front Range of Colorado, using microscopy and molecular methods to study changes in root fungal communities of the two species due to N-enrichment. We hypothesized that fungal associations shift to being parasitic for Geum at high N availabilities, while Deschampsia can better reduce colonization at high N. To test our hypothesis, we evaluated roots for presence of vesicles, arbuscules and other mycorrhizal and non-mycorrhizal fungal structures and identified fungal communities using fungal specific primers of the nrITS region in both unfertilized and fertilized treatments.
Results/Conclusions
Geum and Deschampsia were both highly colonized by arbuscular and ericoid mycorrhizal fungi. Dark septate fungi were also observed. Culturable endophytes of Geum were dominated by fungi in the order Helotiales that included several hyaline ericoid and pseudomycorrhizal fungi such as Phialocephala fortinii. Root-associated fungi were closely related to fungi found in roots of alpine ericaceae plants, Antarctic plants such as Colobanthus quitensis and Deschampsia antartica and other plants in alpine tundra. Deschampsia isolates were less abundant and roots were mainly colonized by bacteria. N fertilization increased AMF and ericoid colonization of Geum roots, but did not alter the colonization of Deschampsia roots. N fertilization also decreased dark septate fungi for both plant species. Our findings partially support our hypothesis: fungal communities associated with Geum increased and potentially shifted towards more parasitic fungi. However, Deschampsia colonization did not change due to N fertilization. Although further quantification of the benefits and costs of particular fungal groups is needed, our results suggest that shifts in fungal associations may contribute to the declines in plant species abundance due to N enrichment.
