Current Research Projects 

The following is a brief overview of current research in the Hobbie lab.

Incorporating mycorrhizal fungi and labile organic nitrogen into ecosystem models of carbon and nitrogen dynamics (Funding: NSF Ecosystems)

Symbiotic fungi form mycorrhizae with tree roots and play a crucial but poorly quantified role in forest carbon and nitrogen cycling. In this study, we are using established and novel techniques to measure carbon flows belowground and to mycorrhizal fungi along a nitrogen availability gradient at the Bartlett Experimental Forest. Whether mycorrhizal fungi take up labile organic nitrogen will also be assessed. The two main types of mycorrhizal fungi, arbuscular mycorrhizal fungi and ectomycorrhizal fungi, differ significantly in their effects on carbon and nitrogen cycling. Changes in the abundance and functions of these two fungal types will be assessed along the nitrogen gradient in collaboration with Tom Horton at SUNY-ESF. Labile organic nitrogen and mycorrhizal fungi will be explicitly incorporated into ecosystem-scale modeling, thereby fully integrating mycorrhizal fungi into our conceptual and quantitative picture of how forests function.

Sample publication: Hobbie, E.A., and T.R. Horton. 2007. Evidence that saprotrophic fungi mobilize carbon and ectomycorrhizal fungi mobilize nitrogen during litter decomposition. New Phytologist 173: 447-449.

Most Arctic plants obtain nitrogen by symbiosis with fungi (Funding: NSF Polar Programs)

Arctic plants are strongly limited by low levels of available nitrogen yet organic nitrogen in the soil is abundanct. To aid in obtaining nitrogen and other nutrients, woody plants such as Salix, Betula, Vaccinium, and Ledum form symbioses with mycorrhizal fungi. In this study, natural abundance ¹⁵N patterns are being measured in many ecosystem pools at the Toolik Lake Long-Term Ecological Research site in Arctic Alaska and the Abisko Scientific Research Station in northern Sweden to estimate carbon and nitrogen dynamics in the plant-mycorrhizal symbiosis. This study is in collaboration with John Hobbie (Marine Biological Laboratory) and Howard Drossman (Colorado College). Masters student Julee Shamhart is using isotopic techniques to examine fungivory in arctic mammals.

Sample publication: Hobbie, E.A., and J.E. Hobbie. Natural abundance of 15N in nitrogen-limited forests and tundra can estimate nitrogen cycling through mycorrhizal fungi: a review. 2008. Ecosystems 11: 815-830.

Effects of precipitation on belowground carbon allocation and storage by fine roots and mycorrhizal fungi in pine savannas (Funding: DOE)

In this study, we will use multiple approaches to examine the key role of fine roots and mycorrhizal fungi in belowground carbon cycling and in ecosystem responses to precipitation shifts. This study will focus on the longleaf pine-wiregrass savanna in southern Georgia in collaboration with Robert Mitchell of the Jones Research Center and Chelcy Ford of the Coweeta Hydrological Laboratory. The responses of ectomycorrhizal fungi, arbuscular mycorrhizal fungi, fine roots, and soil carbon storage to experimental manipulations of precipitation will be examined using stable isotope ratios of carbon, nitrogen and other measurements. (Start date January 2009).

Biotic, chemical and physical controls over organic nitrogen cycling in temperate forest soils (Funding: NSF)

In this study, we are working with Adrien Finzi at Boston University to investigate amino acid cycling in temperate forest ecosystems. Traditional conceptual and computer models assume that plants only take up inorganic forms of nitrogen (ammonium and nitrate). Recent research in arctic, alpine and boreal ecosystems suggest that certain organic forms of N, such as amino acids, can also contribute to plant nitrogen nutrition. Comparable data from temperate forests is lacking; this research is addressing that gap.

Fungal life history strategies and evolution: insights into mycorrhizal and saprotrophic persistence from isotopic measurements (Funding: NSF)

Fungi play important ecological roles and affect society through their activities as decayers, mutualists, and pathogens. The two dominant types, mycorrhizal fungi and saprotrophic fungi, obtain their nutrition differently, with mycorrhizal fungi obtaining simple sugars from plants in exchange for helping their host plants, and saprotrophic fungi obtaining their energy and nutrients from decomposition of dead organic matter. Whether fungi switch between nutritional strategies is a key question in the evolutionary ecology of fungi. In collaboration with David Hibbett at Clark University (http://www.clarku.edu/faculty/dhibbett/) and Jim Trappe, we will study this question using stable isotopes and genetic techniques on archived specimens and in field studies at Harvard Forest LTER.

Experimental constraints on contributions of mycorrhizal symbioses to bedrock weathering of calcium and magnesium (Funding: NSF).

Through the production of organic acids, mycorrhizal fungi may provide their host plants with access to nutrients locked in rocks and soil minerals. In collaboration with Julie Bryce, we are using trace element techniques to examine this important but poorly understood process.

Previous Research Projects 

Controls on biomass dynamics and nitrogen stable isotopes in plant-mycorrhizal symbioses (Funding: NSF Ecosystems)

Nitrogen stable isotopes in the plant-soil system have lacked the rich theoretical framework that has proved so fertile for carbon isotope investigations. One problem is that much of plant nitrogen uptake is mediated by mycorrhizal (symbiotic) fungi, particularly under nitrogen-limited conditions, and the interactions among plants, mycorrhizal fungi, and available nitrogen are difficult to quantify. Biomass allocation, nitrogen allocation, and nitrogen isotope patterns can be theoretically linked in the plant-mycorrhizal symbiosis. In collaboration with Jan Colpaert at Hasselt University in Belgium and Steve Macko at the University of Virginia, we are quantifying biomass and nitrogen dynamics and nitrogen isotopes in laboratory studies of pines cultured with different mycorrhizal symbionts, nitrogen supply rates, and nitrogen forms.

Sample publication: Hobbie, E.A., J.V. Colpaert, M.W. White, A.P. Ouimette and S.A. Macko. 2008. Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus. Plant and Soil 310: 121-136.