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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-23. Synthesis and Projection of Climate Change Impacts on Biological Nitrogen Fixation, and Feedbacks to Forest Productivity and Carbon Storage

Biological nitrogen (N) fixation is the largest natural source of N supplied to terrestrial ecosystems worldwide.  Because N frequently limits plant growth and soil carbon turnover, improved knowledge of biological N2 fixation is critical for predicting how ecosystems will respond to climate change and how they may mitigate increased atmospheric CO2 through biological carbon (C) sequestration (Hungate et al. 2003, Gerber et al. 2010).

Biological N2-fixation is carried out by free-living asymbiotic N2 fixers in soils and detritus (litter, wood), and symbiotic N2 fixers associated with bryophytes, woody plants, and cyanolichens. This diversity of ecological niches for biological N2 fixation is well represented in forests of the Pacific Northwest, owing to low atmospheric N deposition, high detritus accumulations to support asymbiotic N2 fixers, large numbers of N2-fixing bryophytes, structurally complex forest canopies that support abundant biomass of arboreal cyanolichens, and disturbance regimes that facilitate woody N2 fixers such as Alnus and Ceanothus species. Biologically fixed N is at least partially responsible for areas of high soil fertility and exceptionally high C accumulation in many Pacific Northwest forests, which store more C per unit area than any other terrestrial ecosystem, anywhere on Earth.

We seek a postdoctoral candidate to examine the climate-dependency of biological N2-fixation in Pacific Northwest forests.  Existing studies are underway to characterize patterns and mechanisms of climatic controls on coupled C-N dynamics in plants and soils of the Pacific Northwest, yet the climatic factors shaping biological N2-fixation and its importance in driving C accumulation remain poorly resolved. Both a re-analysis of existing literature and new studies along climate gradients are needed to assess biological N2 fixation in response to climate, how this response varies among ecological niches in forests, and whether key soil nutrients (P, Mo) modulate this response.  This information can improve ecosystem simulation models that project future changes in coupled C-N dynamics. We therefore seek postdoctoral candidates to address topics in one or more the following research areas:

Analysis and synthesis of climate-dependency of biological N2 fixation among ecological niches.
This study element will analyze and synthesize existing information on rates of biological N2 fixation in Pacific Northwest forests (and possibly globally), focused on an exploration of climate dependency in biological N2 fixation rates. Older data on N2-fixation rates (Cleveland et al. 1999) will be updated with new data.  A new opportunity to analyze red alder N2-fixation across 30 research installations that span a 3-fold range in mean annual precipitation (125 to 35. cm) and a 10 °C span in mean annual temperature can also be explored.

Characterize field rates of biological N2 fixation across key selected climate gradients.
This element will use a series of well-characterized climate gradients across the Pacific Northwest to generate targeted information on whether and how the climate-dependency of biological N2-fixation varies among ecological niches in forests.  To complement extensive studies of N2-fixation arrayed across climate gradients (e.g., Yelenik et al. 2013), the Fellow may also select intensive study sites to characterize effects of seasonal and interannual climate variation and associated phenology on N2-fixation, or effects of key nutrients (P, Mo) on climate-dependency of N2-fixation.  An “N2-fixation niche” approach to field sampling that considers both structural and functional ecosystem heterogeneity as drivers of N2-fixation inputs can be designed to meld with climate-ecosystem models based on forest structural development, for example, as related to habitat-wildlife studies.

Evaluate climate change impacts on biological N2 fixation, ecosystem N and C balance, and forest productivity by simulation modeling.
Simulation models often parameterize biological N2-fixation as either a fixed static value or adjusted to meet N demand under a simulated CO2 fertilization effect, yet rarely consider climate effects directly.  This study element involves modifying and parameterizing a regional ecosystem model with updated climate functions for niche-based biological N2 fixation, and evaluating the consequences of future climate change scenarios on rates of biological N2-fixation and associated C storage in Pacific Northwest forests. This analysis can take advantage of a range of models that have been used within the Corvallis science community (e.g., Biome BGC, MC1 DGVM, GEM, MEL, VELMA). The Fellow can also interact with Earth System modelers who seek a better representation of the N cycle in their global framework, and with USGS science staff conducting the National Biological Carbon Assessment. The successful candidate will be free to evaluate other ecosystem and environmental factors, such as scenarios of fire disturbance and subsequent forest structural development, land-use change, nutrient availability, range distribution shifts for woody N2 fixers, etc. in simulations.


Cleveland, C.R., A.R. Townsend, D.S. Schimel, H. Fisher, R.W. Howarth, L.O. Hedin, S.S. Perakis, E.F. Latty, J.C. von Fischer, A.R. Elseroad and M.F. Wasson. 1999. Global patterns of terrestrial biological N2 fixation in natural ecosystems. Global Biogeochemical Cycles 13:623-645.

Gerber, S., L.O. Hedin, M. Oppenheimer, S.W. Pacala, E. Shevliakova. 2010. Nitrogen cycling and feedbacks in a global dynamic land model. Global Biogeochemical Cycles, 24, GB1001, doi:10.1029/2008GB00336.

Hungate B.A., J.S. Dukes, M.R. Shaw, Y. Luo, C.B. Field. 2003. Nitrogen and climate change. Science 302:1512-1513.

Yelenik, S.G., S.S. Perakis, D. E. Hibbs. 2013.  Regional constraints to biological nitrogen fixation in post-fire forest communities. Ecology 94:739-750.

Proposed Duty Station: Corvallis, OR

Areas of Ph.D.: Biogeochemistry, geochemistry, ecology, ecosystem modeling, geography, botany (Candidates holding a Ph.D. in other disciplines but with knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications – Research Soil Scientist, Research Chemist, Research Botanist, Research Ecologist, Research Hydrologist, Research Biologist, Research Geographer, Research Geologist.

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of theposition will be made by the Human Resources specialist).

Research Advisors: Steven Perakis (541) 758-8786,; David Myrold (Oregon State U) (541) 737-5737,; Dominique Bachelet (Conservation Biology Institute), (360) 870-5782,

Human Resources Office Contact: Lisa James, (916) 278-9405,

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U.S. Department of the Interior, U.S. Geological Survey
URL: Perakis.htm
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Last modified: 17:58:04 Tue 23 Jul 2013
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