Nitrogen deposition alters plant–fungal relationships: linking belowground dynamics to aboveground vegetation change

Nitrogen ( N ) deposition rates are increasing globally due to anthropogenic activities. Plant community responses to N are often attributed to altered competitive interactions between plants, but may also be a result of microbial responses to N , particularly root‐associated fungi ( RAF ), which are known to affect plant fitness. In response to N , D eschampsia cespitosa , a codominant plant in the alpine tundra at N iwot R idge ( CO ), increases in abundance, while G eum rossii, its principal competitor, declines. Importantly, G . rossii declines with N even in the absence of its competitor. We examined whether contrasting host responses to N are associated with altered plant–fungal symbioses, and whether the effects of N are distinct from effects of altered plant competition on RAF , using 454 pyrosequencing. Host RAF communities were distinct (only 9.4% of OTU s overlapped). N increased RAF diversity in G . rossii, but decreased it in D . cespitosa . D . cespitosa RAF communities were more responsive to N than G . rossii RAF communities, perhaps indicating a flexible microbial community aids host adaptation to nutrient enrichment. Effects of removing D . cespitosa were distinct from effects of N on G . rossii RAF , and D . cespitosa presence reversed RAF diversity response to N . The most dominant G. rossii RAF order, H elotiales, was the most affected by N , declining from 83% to 60% of sequences, perhaps indicating a loss of mutualists under N enrichment. These results highlight the potential importance of belowground microbial dynamics in plant responses to N deposition.