From plant functional traits to ecosystem biogeochemistry
Since soils contain the largest reservoir of terrestrial carbon (C), it is crucial to understand the interactions between plants and soils, how they drive soil C dynamics and how these relationships may respond to global change. An understanding how plant-soil relationships are coupled to cycles of nitrogen (N) and phosphorus (P) is central for this goal because plant demand for these elements drives patterns root form and function. Much of what we know about plant traits is restricted to the above ground portions of plants. We also have some knowledge about how different root symbioses (e.g. N2 fixation and mycorrhizal fungi) acquire N and P for plants. The goals of our work are to understand the linkages between these above- and belowground traits, how they influence the storage of soil C and how their expression interacts with cycles of N and P. We are currently exploring these ideas in temperate (Coweeta Hydrologic Lab, North Carolina) and tropical forests (Barro Colorado Nature Monument, Panama).
How will a changing climate shape southern Appalachian forests?
Southern Appalachian forests are experiencing an increased frequency of drought events during the growing season. Drought has the potential to discriminate against tree species with large xylem vessels and may also lead to additional stresssors such as outbreaks of insect herbivory. In collaboration with Chelcy Ford (USDA FS Southern Research Station) we are exploring how a decrease in rainfall may shift patterns in plant competition and ecosystem processes in early successional forests. In a controlled greenhouse experiment we explored the influence of reduced soil moisture on plant growth and water exchange for four targeted early successional species that differ in hydraulic architecture and nutrient acquisition strategies (root symbioses). Of particular importance is the fate of Robinia pseudoacacia, an N2-fixing species that brings in a substantial amount of N in early successional forests.
We are currently establishing a rainfall reduction experiment in the Cowee valley, NC where we will determine how reductions in rainfall during the growing season will influence plant-soil relationships and the trajectory of forest community development. Our long term goal is to forecast how this climatic stressor will influence ecosystem biogeochemistry and water budgets of regenerating forests.
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.