Denitrification: Workshop on Advanced Approaches to Quantify Denitrification

Overview

During the last few decades, the introduction of reactive nitrogen (N) into the biosphere by food and energy production has been greater than rates of N fixation in native terrestrial ecosystems, and this anthropogenic input has been steadily increasing. By far the largest uncertainty about the human domination of the N cycle on all scales is the amount of reactive N that is converted back to N₂ during the last step of denitrification.

Unfortunately, we have little knowledge about how much N is denitrified, when, and in what location. We have a good understanding of the environmental conditions under which denitrification occurs, but reliable quantification of N₂ production in the field is still relatively rare. There are three primary reasons for lack of knowledge about N₂ production:

• It is difficult to measure due to the high atmospheric background concentrations.
• N₂ production rates are very heterogeneous in space and time.
• There is a lack of synergy between the scientific communities that determine N₂ production rates.

To address this deficiency, the INI held a 3-day workshop in the spring of 2004, comprising approximately 50 participants, to focus on quantification of N₂ production via biological denitrification. The objectives of this workshop were:

• Evaluate the state of our knowledge of denitrification rates in a wide range of terrestrial and aquatic ecosystems
• Compare methodologies that have been adopted by different scientific disciplines, with the expectation that collaborations across disciplines could yield methodological advances
• Chart out the current weaknesses and the actions needed to address those weaknesses for an improved global assessment of where, when, and how much reactive N is converted to N₂ in the biosphere

Two important considerations were superimposed over a matrix of ecosystem types and methodological approaches:

• What are the appropriate scales over which N₂ production can and should be measured?
• How can models be integrated with the measurements to improve mechanistic understanding of the processes and permit estimation of fluxes at larger spatial scales, longer temporal scales, and across a range of forecasted scenarios?

The results of the workshop will be published as one or more synthesis papers in a peer reviewed journal.

Scientific Steering Committee

Co-chairs, Eric Davidson (USA) and Sybil Seitizinger (USA). Members, Elizabeth Boyer (USA), Mary Firestone (USA), Anne Giblin (USA), J. Wendell Gilliam, (USA) Peter Groffman USA), Luiz Martinelli (Brazil), Lars Peter Nielsen (Denmark), and Mary Scholes (Republic of South Africa).

Sponsors

National Science Foundation (Division of Environmental Biology and Biogeosciences), Environmental Protection Agency, National Aeronautical and Space Administration, and International Biosphere-Geosphere Programme (IGAC).

Products

Workshop Final Report

The workshop identified recent technological developments that hold promise for future denitrification research, such as the application of Membrane Introduction Mass Spectrometry (MIMS) to measure N₂ / Ar ratios in soils. 

However, there is no single methodological approach  -- or 'silver bullet' -- that will solve the problem of balancing nitrogen budgets for ecosystems.  Instead, a novel combination of several promising methodological techniques reviewed during this workshop should be applied in an integrated manner in one or more 'flagship' studies along the continuum from upland soils to rivers and estuaries. 

New advancements in quantification of denitrification will require inter-disciplinary collaboration to simultaneously combine multiple approaches across landscapes and waterscapes. Improved understanding and quantification of denitrification will promote our ability to manage reactive nitrogen in the biosphere and to avoid harmful effects of excess reactive nitrogen on water quality, air quality, and human health.

The more detailed results of the workshop will be shared with the broader scientific community through publication of synthesis papers in a special "Invited Feature" section of Ecological Applications.  At this time, a summary of the findings can be found in the Workshop Final Report.

Additional Information

E. Davidson, edavidson(at)whrc.org

http://whrc.org/nitrogen/index.htm