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Current Research Projects 

Modeling and isotopes link mycorrhizal fungi to soil carbon and organic nitrogen use (Funding: NSF Polar Programs, Starting June 2011)

Birch, willow, and other shrubs in the Arctic are increasing in response to warming at the expense of grasses and sedges. This increased shrub cover may enhance winter snow capture, increase winter soil temperatures, deepen summer soil thawing, and increase the release of old carbon and nitrogen. The increased flux of sugars from shrubs to shrub-associated symbiotic fungi (termed mycorrhizal fungi) could stimulate decomposition of thawing organic matter and release organic nitrogen for plant uptake previously locked up for millennia. This mycorrhizally-mediated priming of decomposition may influence carbon balances and nutrient availability for further shrub growth. To test this, we will use long-running experiments in Arctic Alaska that are examining plant responses to warming in different tundra types. We will use natural levels of carbon and nitrogen isotopes to track the incorporation of new soil-derived organic nitrogen into shrubs and associated mycorrhizal fungi.

Sample publication: Hobbie, E.A., A.P. Ouimette, E.A. Schuur, D. Kierstead, J.M. Trappe, K. Bendiksen, and E. Ohenoja. 2012. Radiocarbon evidence for the mining of organic nitrogen from soil by mycorrhizal fungi. In press, Biogeochemistry.

Can microbial ecology and mycorrhizal functioning inform climate change models? (Funding: DOE, Starting August 2012)

Feedbacks between forest ecosystems and global climate are regulated in part by the coupled cycling of carbon and nitrogen throughout the atmosphere-plant-soil continuum. However, the current generation of models that link the terrestrial carbon cycle to climate neglect many fundamental plant-microbe interactions that regulate this coupling. One critical link in coupled carbon-nitrogen cycling is the role of organic nitrogen in providing plant nutrition and in contributing to forest carbon storage. This research will investigate the consequences for ecosystem carbon cycling in climate change experiments of microbial decomposition of organic nitrogen and its subsequent uptake by mycorrhizal (plant associated) fungi. Using archived samples and data from the Duke Forest and Oak Ridge National Laboratory Free Air CO2 Enrichment (FACE) experiments, we will use stable isotope methods to estimate the source and quantify the use of organic nitrogen in different mycorrhizal taxa. Using fresh samples from the Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) experiment we will trace carbon from the atmosphere through plants and into the microbial community. Data generated from the three field experiments will be used in conjunction with the MySCaN (Mycorrhizal Status, Carbon and Nutrient cycling) model to explore how organic nitrogen uptake by mycorrhizal fungi affects forest carbon cycling under different climate scenarios. The model explicitly incorporates organic nitrogen movement and key microbial components to predict organic nitrogen changes and soil carbon storage. These measurements and modeling will inform global biogeochemical models by providing new insights into how carbon and nitrogen cycling are linked to plant and microbial dynamics in forest systems. This work will accordingly be useful in efforts to incorporate key processes (such as nitrogen constraints to plant growth and soil carbon storage) into the carbon dynamics of large-scale models used to predict forest-climate feedbacks.

Quantification of organic nitrogen use in pulse-labeling and FACE experiments (Funding: NSF Ecosystems, Starting September 2012)

Much of plant nitrogen supply, particularly for trees, is supplied through symbiotic (mycorrhizal) fungi that obtain sugars from their host trees and in return transfer nitrogen to plants. Conventionally, plant-available nitrogen has been considered to only consist of inorganic nitrogen such as ammonium and nitrate. Although the extensive enzymatic capabilities of mycorrhizal fungi means that organic nitrogen forms are also accessible, difficulties in quantifying organic nitrogen use in the field have limited the acceptance of the key role that organic nitrogen uptake via mycorrhizal fungi may play in meeting plant nitrogen demands. Here, isotopic ratios from two field experiments at Duke University and at Rhinelander, Wisconsin will be used to assess organic nitrogen uptake by different mycorrhizal fungi and plants. We will combine this information with laboratory studies on mycorrhizal plants to provide realistic and quantitative estimates of organic nitrogen use for the first time. In addition to the experiments above, we will measure 14C on archived samples of fungi and plants to expand the diversity of fungal species and geographic extent of our studies. The new methodologies introduced in this proposal will determine without experimental manipulations whether patterns of organic nitrogen use differ across ecosystems and across species of mycorrhizal fungi differing strongly in functional characteristics.

Previous Research Projects 

Biotic, chemical and physical controls over organic nitrogen cycling in temperate forest soils (Funding: NSF, March 2008 – February 2012)

In this study, we worked 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 addressed that gap.

Sample publication: Hobbie, E.A., P. Högberg. Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics. New Phytologist, Tansley Review, in press

Effects of precipitation on belowground carbon allocation and storage by fine roots and mycorrhizal fungi in pine savannas (Funding: DOE, January 2009 - December 2011)

fungi growing on a treeIn this study, we used 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 focused 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 were examined using stable isotope ratios of carbon, nitrogen and other measurements.

Sample publication: Hobbie, E.A., and R. Agerer. 2010. Nitrogen isotopes in ectomycorrhizal mushrooms correspond to belowground exploration types. Plant and Soil 327:71-83.

Fungal life history strategies and evolution: insights into mycorrhizal and saprotrophic persistence from isotopic measurements (Funding: NSF Integrative and Organismal Systems, January 2009 – June 2011)

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 and Jim Trappe, we studied this question using stable isotopes and genetic techniques on archived specimens and in field studies at Harvard Forest Long Term Ecological Research site.

Sample publication: Hobbie, E.A., and R. Agerer. 2010. Nitrogen isotopes in ectomycorrhizal mushrooms correspond to belowground exploration types. Plant and Soil 327:71-83.

Experimental constraints on contributions of mycorrhizal symbioses to bedrock weathering of calcium and magnesium (Funding: NSF Earth and Atmospheric Research, August 2008 – July 2012)

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 at UNH, we are using trace element techniques to examine this important but poorly understood process.

Sample publication: Hobbie, E.A., C.J. Hoff, J.G. Bryce, J.V. Colpaert, and R.A. Hallett. 2009. Nutrient supply rate and mycorrhizal colonization control patterns of element distribution in ectomycorrhizal pine. Communications in Soil Science and Plant Analysis 40:3503-3523.

Incorporating mycorrhizal fungi and labile organic nitrogen into ecosystem models of carbon and nitrogen dynamics (Funding: NSF Ecosystems, September 2006 – August 2010)

Student sampling a soil coreSymbiotic fungi form mycorrhizae with tree roots and play a crucial but poorly quantified role in forest carbon and nitrogen cycling. In this study, we used 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 was also 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 was 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, September 2006 – August 2009)

graph of carbon demand vs mycorrhizal Transfer RatioArctic 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 were 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 was in collaboration with John Hobbie (Marine Biological Laboratory) and Howard Drossman (Colorado College). Masters student Julee Shamhart used 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.