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 C-13 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.