Nitrogen deposition, plant carbon allocation, and soil microbes: Changing interactions due to enrichment

• Premise of the study: Nitrogen (N) inputs to the terrestrial environment have doubled worldwide during the past century. N negatively impacts plant diversity, but it is unknown why some species are more susceptible than others. While it is often assumed that competition drives species decline, N enrichment also strongly affects soil microbial communities. Can these changes affect plant–microbe interactions in ways that differentially influence success of plant species? Furthermore, can altered plant–microbe interactions lead to carbon (C) limitation in plants? • Methods: We focused on a species that increases ( Deschampsia cespitosa ) and one that decreases ( Geum rossii ) in abundance in N‐fertilized plots in alpine tundra. We measured soil microbes using phospholipid fatty acids, and C limitation and transfer using a 13 C tracer experiment, C:N ratios, nonstructural carbohydrates, and leaf preformation. • Key results: While N profoundly influenced microbial communities, this change occurred similarly in association with both plant species. N addition did not alter total C allocation to microbes in either species, but it changed patterns of microbial C acquisition more in Geum , specifically in gram‐negative bacteria. Geum showed evidence of C limitation: it allocated less C to storage organs, had lower C:N and carbohydrate stores, and fewer preformed leaves in N plots. • Conclusions: Carbon limitation may explain why some species decline with N enrichment, and the decline may be due to physiological responses of plants to N rather than to altered plant–microbe interactions. Global change will alter many processes important in structuring plant communities; noncompetitive mechanisms of species decline may be more widespread than previously thought.