Ecosystem recovery from disturbance
N2 fixation is central to recovery from disturbance in longleaf pine savannas. These ecosystems are inherently poor in N due to frequent disturbances from fire and human activity. In collaboration with Lars Hedin (Princeton University) and Erik Hobbie (University of New Hampshire), we are examining the ecology of N2 fixation and the role it plays in recovery.
The goals of our work are to reduce uncertainties about the process of N2 fixation, including the contribution of taxonomic groups of N2-fixers (herbaceous legumes, soil crusts and free-living bacteria) to fixation and how nutrients (soil N, P and Mo) regulate fixation. We also aim to quantify the importance of fixation at the landscape level and to determine if these inputs are sufficient to counteract losses through fire and disturbance.
Our research sites are located at Fort Benning (GA) and Eglin Air Force Base (FL), where we have over 50 ha of research plots that range in stand age (from 2 year old plantations to mature) and fire frequency (from 1-2 year fire return intervals to stands that have not burned in the last 15 years).
Plant-soil feedbacks in ecosystem biogeochemistry
Soils contain the largest reservoir of terrestrial C, making it critical to understand the interactions between plants and soils that drive soil C dynamics. A fundamental way that plants modulate soil C is through their demand for nutrients such as N. Plant demand for N drives the expression root traits, including associations with mycorrhizal fungi and soil microbes, which vary in their capacity to scavenge or mine N, thereby promoting or suppressing soil C loss. This theme of plant-soil feedback motivates several research studies in the lab.
PhD student Carly Phillips is leading project on the plant-soil feedback of shrub expansion on the north slope of Alaska. Carly is studying how the expansion of Betula, Alnus and Salix into formerly grass-dominated tundra, is changing the cycles of C and N in soils. Carly's research combines field and lab-based approaches focused on unraveling the feedback between shrubs, soil microbial communities and soil biogeochemistry.
Global change factors may alter tree species assemblages due to migration or pest and pathogen introduction, leading to questions about the consequences of such changes on soil C. MS student Melanie Taylor is studying how tree species migrations will affect soil C in temperate forest soils. Will soil C be determined by the quality of litter inputs from new species, the pre-existing structure of the soil microbial community, or an interaction of the two? Melanie is answering this question in a laboratory mesocosm experiment where she crossed leaf and root litter from eight tree species with the soils of those eight tree species and is monitoring soil CO2 efflux.
Ecosystem resiliency and climate change
Southeastern US is experiencing an increased frequency of drought events during the growing season. In collaboration with Chelcy Miniat (USDA FS Southern Research Station) we are exploring how drought may shift patterns in plant competition and ecosystem processes in early successional forests of the southern Appalachians. Of particular importance is the fate of Robinia pseudoacacia, an N2-fixing species that contributes to ecosystem resiliency by supplying new N to the ecosystem.
In a greenhouse experiment we reduced soil moisture and monitored plant growth and water exchange for four early successional species. We found that moderate drought enhanced both N2 fixation and the competitive ability of Robinia pseudoacacia. This finding suggests that under scenarios of moderate drought, N2 fixation may alleviate N constraints resulting from low soil moisture and supply more to N to the ecosystem. But, how will Robinia pseudoacacia and N2 fixation respond to more frequent or more severe drought events?
Wurzburger, N. and Miniat, C.F. 2014 Drought enhances symbiotic di-nitrogen fixation and competitive ability of a temperate forest tree. Oecologia 174:1117-1126.
We are answering this question with two research studies led by PhD student Jeff Minucci. First, Jeff is maintaining a rainfall reduction experiment in the Cowee valley, NC where we are studying how reductions in rainfall during the growing season influence N2 fixation and the trajectory of forest community development. Our long term goal is to understand how drought will influence ecosystem biogeochemistry of regenerating forests.
Second, Jeff is conducting a greenhouse experiment with Robinia pseudoacacia to understand the impact of the frequency and duration of drought events on N2 fixation and other physiological processes.
Nutrient limitation on asymbiotic nitrogen fixation in tropical forests
Nitrogen fixation is a major source of nitrogen input to tropical ecosystems but we poorly understand its limits. Phosphorus has been considered the primary element that restricts nitrogen-fixers in nature, and tropical forests in particular. Through a collaboration with Alex Barron, J.P Bellenger, Anne Kraepiel and Lars Hedin (Princeton University) and Joe Wright (Smithsonian Tropical Research Institute), we demonstrated that molybdenum, a trace metal and component of the nitrogenase enzyme, can limit free-living, nitrogen-fixing bacteria in tropical soils. This work (Barron et al. 2009) exposed the significance of an underappreciated trace metal in the tropical nitrogen cycle.
But, why and when do molybdenum and phosphorus limitation arise? To answer this question, we conducted field experiments across lowland tropical forests of Panama and assays of soil chemistry in the lab. We found that free-living nitrogen-fixing bacteria are constrained by the interaction of phosphorus and molybdenum in soils at two scales: within local soil layers and across landscape gradients in soil phosphorus. This work (Wurzburger et al. 2012) provides a mechanistic framework for the nutrient limitation of free-living nitrogen-fixing bacteria. See UGA press release here.
Barron, AR, Wurzburger, N, Bellenger, JP, Kraepiel, AML, Wright, SJ, and Hedin, LO 2009. Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils. Nature Geoscience 2: 42-45.
Wurzburger, N, Bellenger, JP, Kraepiel, AML, and LO Hedin 2012. Molybdenum and phosphorus interact to constrain asymbiotic nitrogen fixation in tropical forests. PLoS ONE 7(3):e33710.
Plant-soil feedbacks in the nitrogen cycle
Mycorrhizal fungi not only aid in nutrient acquisition for plants, but some have co-evolved strategies with their host to regulate nutrient cycles for their own coupled benefit. In collaboration with Ronald Hendrick (Ohio State University) I documented a nitrogen feedback between plant litter chemistry and mycorrhizal roots allowing the host plant, rhododendron (Rhododendron maximum) to maintain control of the nitrogen cycle in a temperate forest of the southern Appalachians. We proposed that ericoid mycorrhizal fungi mediate this feedback with their diversity of oxidative extracellular enzymes that break down rhododendron’s litter and acquire nitrogen within it. Rhododendron has long been the focus of research at the Coweeta Hydrologic Lab, and this research provides a new perspective on the biogeochemical impact of rhododendron in southern Appalachian forests. More broadly, this research was one of the first in-situ demonstrations of a plant-soil-nutrient feedback that can facilitate niche differentiation, plant competition and regulation of the nitrogen cycle in terrestrial ecosystems.
In a related study, we characterized the assemblage of fungi associating with ericoid mycorrhizal roots of Rhododendron maximum. We observed 71 fungal taxa using culture- and cloning-based techniques. Many of these fungi are not considered symbionts in a strict sense, but their association with R. maximum roots raises questions about their functional role in the rhizosphere and their broader contribution to nutrient cycling in these forest ecosystems. From the genomes of root-associating ascomycetes we observed multiple copies of multi-copper oxidases -- a suite of enzymes that can oxidize organic matter. Documenting the diversity and expression of multi-copper oxidase genes may provide a mechanistic approach for understanding how ericoid mycorrhizal root fungi regulate the cycles of nutrients between the host plant and surrounding soil environment.