Progress 09/01/03 to 08/31/07
Outputs
OUTPUTS: Our research addresses a current debate in ecology centering on whether ecosystem function depends on plant and microbial biodiversity. This debate has been fueled by the documented global loss of species and unresolved concerns regarding our ability to experimentally detect the influence of biodiversity on ecosystem function. The results of our USDA-sponsored research provide new evidence that heterotrophic microbial communities in soil are one mechanism linking plant diversity to ecosystem function. This link occurs because organic compounds contained in plant detritus limit and structure soil microbial communities, which, in turn, control key pathways of ecosystem carbon and nitrogen cycling. The insights we have revealed have important implications for the management of terrestrial ecosystems. Plant diversity is a key factor regulating soil microbial communities as well as the cycling of carbon and nitrogen in soil. This link demonstrates that greater plant diversity
increases microbially mediated processes in soil that sustain nutrient supply to plants. Thus, plant diversity appears to be an important factor in sustaining the long-term productivity of these grassland ecosystems. An important output of our USDA-sponsored research was the study of fungal community composition and function in a field experiment in which the diversity of grassland plants has been experimentally manipulated for a decade. This effort provided an opportunity for a postdoctoral scholar (Dr. Mark Waldrop) to study with D. Zak and to learn molecular techniques necessary to understand how plant diversity influenced fungal community composition and function. We completed our proposed plan of study, and we have actively worked to communicate our results to the broader scientific community. To date, we have given presentations describing our USDA-sponsored research at national scientific meetings, we have given 5 invited talks at other universities, and we have prepared and
published peer-reviewed manuscripts (see Publications below). Dissemination - Scientific Presentations Resulting from Support Waldrop, M.P. and D.R. Zak. 2004. Is microbial diversity dependent upon plant diversity? Results from a common garden experiment. International Society for Microbial Ecology (ISME), Cancun, Mexico. Waldrop, M.P. and D.R. Zak. 2007. Microbial resource competition: A new link between plant diversity and ecosystem processes Ecological Society of America Meeting, San Jose, CA. Zak., D.R. 2007. Microbial Responses to a Changing Climate: Implications for the Future Functioning of Terrestrial Ecosystems. Soil Science Society of America Meeting, New Orleans, LA.
PARTICIPANTS: This grant provided support for Dr. Donald R. Zak, who served as project director. He was involved in all aspects of this research, from the field collection of soil samples to the preparation of manuscripts for publication in scientific journals. He supervised all project personnel during the duration of this award. This grant also provided Dr. Mark Waldrop with training as a postdoctoral scholar in molecular microbial ecology. It exposed him to new techniques in molecular biology and provided him the opportunity to communicate his new knowledge in the peer-reviewed publications and presentations at national and international scientific meetings. An undergraduate student, Kalub Fedack, was provided with a research opportunity using support from this grant. He learned to conduct the biochemical analysis of root litter, which was a key result for our research project.
TARGET AUDIENCES: Our target audience is the scientific community involved with understanding whether plant diversity contributes to a wide range of ecosystem processes. Our results clearly demonstrate that plant diversity is a determinant of ecosystem processes, like soil nitrogen availability, which contribute to ecosystem services derived by society. Developing this understanding forms the foundation for developing informed policies regarding the conservation of biological diversity.
PROJECT MODIFICATIONS: None
Impacts
With support from this USDA grant, we have obtained experimental evidence that plant diversity influences ecosystem processes in soil by altering the composition, diversity, and function of heterotrophic microbial communities. Our research was conducted at a field study in which the diversity (1, 2, 4, 8, and 16 species) of grassland plants has been experimentally manipulated and maintained for a decade. Our work has revealed two important patterns: i) the relative abundance of soil fungi increases with plant species richness, and ii) fungal diversity is a unimodal function of resource availability (i.e., plant detritus), which increases with plant species richness. We examined three mechanisms by which plant diversity may increase fungal abundance in soil: changes in i) plant biochemistry, ii) mycorrhizal abundance, and iii) competitive interactions among bacteria and fungi for limiting resources. We observed no change in any biochemical constituent of root litter,
and conclude that plant diversity did not alter the lignin or cellulose concentration of plant litter. We therefore reject the hypothesis that diversity-mediated changes in detritus biochemistry were responsible for the greater abundance of soil fungi. It could be argued that the increase in fungal relative abundance with greater plant diversity may be due to increasing mycorrhizal infection of plant roots, particularly AM fungi which are more prevalent in grasslands. There are two pieces of evidence that suggest mycorrhizal fungi are not responsible for the increase in fungal abundance. First, fungal clone libraries constructed for these communities indicate that AM fungi are a small proportion (~4%) of the total fungal community in soil. Moreover, as plant diversity increased, we observed an increase in the abundance of functional genes and degradative enzymes in soils which are indicative of saprotrophic fungi and not AM fungi. As such, we found no support for the idea that that AM
fungi increase in abundance, suggesting that saprophytic fungi increased with greater plant diversity. The consequences of greater saprophytic fungal abundance with increasing plant diversity have clear implications for links between plant diversity and microbially mediated ecosystem processes. Soil fungi are the primary organisms metabolizing lignin and humus, compounds that are among the most abundant and long-lived natural polymers on Earth. Greater fungal abundance, caused by increasing plant diversity in our experiment, should therefore result in greater lignolytic activity. Consistent with this expectation, we observed a positive relationship between phenol oxidase activity and plant diversity in our experiment. Moreover, higher rates of phenol oxidase activity occurred in concert with the greater abundance of the basidiomycete laccase gene, the transcription of which produces a phenol oxidase enzyme. The greater fungal abundance we observed, in combination with hyphal-growth
form and phenol oxidase activity, suggest that soil fungi out-compete bacteria as detritus production becomes greater in species-rich plant communities.
Publications
- Waldrop, M.P., D.R. Zak, C. Blackwood, C.D. Curtis, and D. Tilman. 2006. Resource availability controls fungal diversity across a plant diversity gradient. Ecology Letters 9: 1127-1135.
- Waldrop, M.P., D.R. Zak. C.B. Blackwood, and C. D. Curtis. 2007. Microbial resource competition: a new link between plant diversity and ecosystem processes. Ecology in review.
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