Why is increased nitrogen associated with declines in plant diversity? Is it competitive interactions? Changed plant-microbial interactions?
Human activities have played a major role in increasing nitrogen (N) availability throughout the world. In many plant communities, N enrichment facilitates plant productivity but negatively affects species diversity, in that a few species increase in abundance but many others decline. It is often assumed that competition drives this diversity decline; however N enrichment also greatly alters soil microbial communities. Most plants form symbiotic associations with mycorrhizal and endophytic fungi, thus if the microbial players are different or if the nature of the interaction changes (mutualistic to parasitic), this could have profound effects on plant performance under N enrichment. We ask:
1. Are plant-microbe interactions changing under N enrichment (e.g., possible increases in parasitism by soil fungi)? and does this lead to carbon limitation in plants? We are focusing on one species that increases with N enrichment (Deschampsia cespitosa) and one that decreases with N (Geum rossii) in the alpine tundra. We are using a 13C tracer experiment to quantify carbon transfer from plants to the soil microbial community and using a variety of measurements to quantify carbon limitation in plants, including C transfer to storage organs, C:N ratio, preformed leaf formation, and nonstructural carbohydrates. Results suggest that N did not alter rates of carbon loss from either species, but for Geum carbon transfer to soil microbes was altered, specifically a group of gram negative bacteria acquired a lot of carbon from Geum at high N.
2. How do fungal and bacterial root endophyte communities differ under N enrichment? To address this we are using molecular techniques to characterize microbial communities in roots of a species that increases with N enrichment (Deschampsia cespitosa) and one that decreases with N (Geum rossii) in the alpine tundra. We will identify whether parasitic and pathogenic taxa may increase in roots of the species that declines with N enrichment. Preliminary results suggest that there are a few pathogenic fungal species that increase in Geum roots at high N.
3. Do mechanisms of N effect on diversity differ across sites? We propose that competition will drive diversity decline in more productive systems, but is less productive, more stressful systems microbial communities will play a larger role. We are testing this in three grassland sites, alpine tundra, desert grassland, and tallgrass prairie, in greenhouse experiments in which we manipulate both the microbial communities (from collecting soil inoculum from control and long-term N fertilized plots in the field) and competition (by varying planting densities in large greenhouse pots).
Overall, as N deposition continues to increase worldwide, impacting biodiversity and associated ecosystem function, understanding mechanisms of diversity loss is essential to target susceptible species and reverse these effects.