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Abstract Global change alters ecosystems and their functioning, and biotic interactions can either buffer or amplify such changes. We utilized a long‐term nitrogen (N) addition and species removal experiment in the Front Range of Colorado, USA to determine whether a codominant forb and a codominant grass, with different effects on nutrient cycling and plant community structure, would buffer or amplify the effects of simulated N deposition on soil bacterial and fungal communities. While the plant community was strongly shaped by both the presence of dominant species and N addition, we did not find a mediating effect of the plant community on soil microbial response to N. In contrast to our hypothesis, we found a decoupling of the plant and microbial communities such that the soil microbial community shifted under N independently of directional shifts in the plant community. These findings suggest there are not strong cascading effects of N deposition across the plant–soil interface in our system. 

Abstract As organisms shift their geographic distributions in response to climate change, biotic interactions have emerged as an important factor driving the rate and success of range expansions. Plant–microbe interactions are an understudied but potentially important factor governing plant range shifts. We studied the distribution and function of microbes present in high‐elevation unvegetated soils, areas that plants are colonizing as climate warms, snow melts earlier, and the summer growing season lengthens. Using a manipulative snowpack and microbial inoculation transplant experiment, we tested the hypothesis that growing‐season length and microbial community composition interact to control plant elevational range shifts. We predicted that a lengthening growing season combined with dispersal to patches of soils with more mutualistic microbes and fewer pathogenic microbes would facilitate plant survival and growth in previously unvegetated areas. We identified negative effects on survival of the common alpine bunchgrassDeschampsia cespitosain both short and long growing seasons, suggesting an optimal growing‐season length for plant survival in this system that balances time for growth with soil moisture levels. Importantly, growing‐season length and microbes interacted to affect plant survival and growth, such that microbial community composition increased in importance in suboptimal growing‐season lengths. Further, plants grown with microbes from unvegetated soils grew as well or better than plants grown with microbes from vegetated soils. These results suggest that the rate and spatial extent of plant colonization of unvegetated soils in mountainous areas experiencing climate change could depend on both growing‐season length and soil microbial community composition, with microbes potentially playing more important roles as growing seasons lengthen.