This story highlights a current research project at the Hubbard Brook Experimental Forest. To read more about research projects at HBEF, visit Current Research page. Check back regularly to learn about new research projects.
Landscape and Regional Scale Studies of Nitrogen Gas Fluxes

 

  Contact Info:
  Peter M. Groffman
Cary Institute of Ecosystem Studies
2801 Sharon Turnpike
Millbrook, NY 12545 USA
phone: (845) 677-7600, ext. 128
fax: (845) 677-5976
email: groffmanp@caryinstitute.org

UNDERSTANDING THE nitrogen (N) cycle at landscape, regional and global scales is a great current challenge in environmental science. Excess "reactive" N has caused degradation of air and water quality and coastal ecosystems in many areas. The development of solutions to N pollution problems has been hindered by large amounts of "missing N" that dominate N balances at all scales. The uncertainty about N balances is particularly acute in the northeastern U.S., where there are active efforts to address the effects of N pollution on tropospheric ozone levels, coastal eutrophication and drinking water quality, and to determine "critical loads" for atmospheric N deposition. Uncertainty about N balances has led to increased interest in N gas production as a fate of N. The gases nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2) are produced by microbial processes in soils and sediments and can account for a significant percentage of the missing N in balances. However, these fluxes are difficult to quantify because of problematic measurement techniques (especially for N2), high spatial and temporal variability, and a lack of methods for scaling point measurements to larger areas. A particular challenge is that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of N gas flux activity. N gas fluxes have long been assumed to be low in the northeastern U.S., however, recent data that suggest that these fluxes, especially N2, are more important in this region than previously thought. There have also been recent improvements in the development of tools for scaling gas flux measurements, i.e., remote sensing of foliar N derived with imaging spectroscopy, and new isotope methods have been shown to be able to depict patterns in ecosystem N dynamics at landscape scales. The ability of simulation models to depict the hydrologic and biogeochemical controls on N cycle processes has also improved. Thus, the time is ripe for a critical re-assessment of the importance of N gas fluxes in the northeastern U.S.

 
  View of south-facing watersheds at the Hubbard Brook Experimental Forest.

In this project, we are testing the hypothesis that N gas fluxes are more important to N balances in the northeastern U.S. than previously thought, accounting for approximately 50% of the "missing N" in this region. There are three objectives:

  1. Deploy newly developed methods for measuring N2 fluxes at the plot (~ 5 m) scale in multiple watersheds in the White Mountains, NH.
  2. Develop new methods for spatial and temporal scaling of N gas fluxes from the plot to the landscape and regional scale based on new ideas about hotspots and hot moments.
  3. Improve the ability of existing ecosystem biogeochemistry models to depict hotspot and hot moment phenomena.
 
  Figure 1. Taking N2 and N2O samples from a gas flux chamber at theHubbard Brook Experimental Forest for a study of denitrification rates.  

For objective 1, a new soil core gas flow method is being used along with in situ soil oxygen sensors and detailed characterization of denitrifier community and microbial C and N processing to characterize N gas fluxes during rainfall driven hot moments in multiple watersheds. Work under objective 2 focuses on rainfall events as drivers of low soil oxygen levels and hot moments of N gas flux and imaging-spectrometer (AVIRIS)-derived estimates of foliar N and lignin:N ratio to identify landscape scale hotspots of flux. Stable isotopes (ecosystem 15N budgets, and both δ15N- and δ18O of nitrate) are providing integrated estimates of denitrification over multiple spatial and temporal scales. Work under objective 3 involves combination of algorithms and parameter estimates describing physiology, biogeochemistry and N cycling from the PNET-CN and Forest-DNDC models to improve the ability of these models to depict N gas production in soils, with a focus on hotspots and hot moments. The research is addressing a critical basic science uncertainty and producing information relevant to a pressing and globally important environmental problem (N pollution).

Date Prepared: November 2012