Human activities have increased nitrogen (N) availability throughout the world. Nitrogen enrichment commonly results in increased plant productivity in terrestrial ecosystems. A large amount of work also indicates that anthropogenic nitrogen enrichment can lead to reduced plant species richness. Reductions in plant diversity have most commonly been attributed to plant resource competition, but other mechanisms may contribute to the observed loss of plant species in fertilized plots. The goal of this research is to include the 'unseen majority' - belowground microbial interactions - in understanding how N enrichment alters plant community structure aboveground. Plant interactions with microbes (e.g., soil parasites, symbionts, decomposers) could be an even more important determinant of plant diversity decline in response to N enrichment than simple plant resource competition. Utilizing microbial surveys of long-term fertilization plots, tissue culture experiments, and field manipulations in the moist meadow tundra and other grassland systems, previous hypotheses of diversity loss with N enrichment will be extended to include belowground microbial interactions. In addition, this research will promote the teaching, training and understanding of issues related to biodiversity and environmental change through virtual field trips accessible on the web, hands-on microbial activity sets distributed to high school classrooms, and the training of women, particular Hispanic women, scientists. Ultimately, the inclusion of belowground microbial interactions will improve predictive abilities regarding how environmental change factors such as N deposition may affect terrestrial biodiversity.
Human activities have played a major role in increasing nitrogen (N) availability throughout the world: agricultural and industrial N inputs to the environment currently exceed inputs from natural N fixation. These anthropogenic sources of N can volatilize into the atmosphere and pollute natural ecosystems far from the source through deposition. N enrichment facilitates plant growth, one reason why we fertilize gardens. However, increased N often negatively affects plant diversity in natural ecosystems. It is often assumed that the decrease in diversity is due to changes in plant competition, with N enrichment allowing one plant species to exclude other species. We explored an alternative explanation: plant-microbial interactions could change under increased N inputs, giving an advantage to a few plant species while causing most plant species to decline. At the Niwot Ridge Long Term Ecological Research (LTER) site in Colorado, we studied the effect of N fertilization on microbes associated with two co-dominant plants: Geum rossii and Deschampsia cespitosa. We chose this system because our previous work has shown strong declines in biodiversity and increased susceptibility of one of the co-dominant plants (Geum) due to N fertilization. We took a variety of approaches to test our hypotheses: we sequenced root-associated fungal and bacterial communities for each species in both ambient and high N conditions, we labeled plants with 13C to follow carbon transfer to the microbial communities, we fungicided and then inoccululated field plots with microbial communities from different N environments, and we created mesocosms that varied in plant species (tundra, grassland, desert), N level, microbial community, and competition. Through these approaches, we learned three main things about the role of microbial communities in determining plant community response to N fertilization: 1. Overwhelming evidence indicates that microbial associations with plants influence how plant species will respond to environmental changes such as atmospheric N deposition. 2. We have increased understanding about the species specificity of plant-microbe associations. Different plant species foster different microbial associations and these associations affect how plants respond to environmental change. For example: a. Two abundant plant species that grow together in alpine tundra share less than 10% of root-associated fungi and 17% of root-associated bacteria taxa. b. The root-associated microbial community of the plant species that declines strongly due to nitrogen fertilization, Geum rossii, is the most affected by increased nitrogen. c. Pathogenic fungi could have an important effect on seedling survival and plant establishment: we find a fungus from the order Helotiales (main fungal order found in Geum roots) regulates multiple genes involved in plant growth and pathogenicity pathways. 3. While most work has assumed that the majority of impact of increased N deposition on plant diversity is via changes in plant competition, our work uncovers the importance of the unseen majority: soil microbes. Across tundra, grassland and desert ecosystems, we find that microbial interactions play just as large role as competitive interactions in determining plant response. This project has resulted in six scientific articles, with several more in preparation. We have presented 33 posters and oral presentations in scientific meetings (20 at the national level and 13 in local meetings). PIs (UCB, UNM and WIU) supported and trained more than 45 undergraduate and graduate students including Hispanic, women and African American students. We also supported 4 high school students from rural Illinois; two of them were recently awarded second place at the national level in the FFA convention. PI and students from this project have participated in multiple outreach activities for school children and high school students. WIU students developed a series of educational websites called mycoblogs and we have also provided training to the international community with the development of tools for analysis of fungal molecular data in collaboration with Los Alamos National Laboratory and the Ribosomal Database project. We have also provided training for more than 40 students and scientists from Latin-American countries and the Caribbean in microbial bioinformatics.
