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Using soil isotopes as an indicator of site-specific to national-scale denitrification in wetlands
Citation:
Nahlik, A., M. Kentula, W. Mitsch, M. Hernandez, K. Song, B. Bernal, M. Fennessy, D. Sobota, AND J. Renee Brooks. Using soil isotopes as an indicator of site-specific to national-scale denitrification in wetlands. Presented at 2013 Society of Wetland Scientists Annual Meeting, Duluth, MN, June 02 - 06, 2013.
Impact/Purpose:
Wetlands are recognized for their significant role in providing a range of ecosystem services, including provision of clean water, which is critical for long-term sustainability of communities. Denitrification, one process by which wetlands provide clean water, provides a permanent pathway of nitrogen (N) removal from the biosphere through the conversion of N from land-based sources (such as fertilizer and manure) into N gases. This is a desirable process because it removes excess nitrate-N from waters, including those people depend on for drinking water and healthy fisheries; thus, there are human health and economy impacts directly associated with denitrification. Most direct measurements of denitrification are difficult, time-consuming, and costly. We are currently developing an indicator of denitrification by defining the relationship between soil N isotopes (δ15Nsoils) and direct measurements of denitrification. By establishing this relationship, we may be able to simply measure δ15Nsoils, which is easy, fast, and inexpensive, in lieu of more difficult, direct measurements of denitrification. Pilot studies conducted in Ohio wetlands demonstrate that δ15Nsoils are positively correlated with denitrification. As such, we are currently evaluating δ15Nsoil collected from approximately 1000 wetland sites from across the conterminous United States that were sampled for the National Wetland Condition Assessment. By mapping δ15Nsoils across the US landscape, we will identify areas important for reducing N loading to the nation’s water resources.
Description:
Denitrification is an anaerobic, microbial process that converts nitrate to inert dinitrogen (N2) gas and nitrous oxide (N2O), a potent greenhouse and ozone depleting gas. High rates of denitrification can be found in wetlands, resulting in the removal of large quantities of nitrate from land-based sources, thereby potentially reducing nutrient pollution of sensitive freshwater and coastal marine ecosystems. Currently, most direct measurements of denitrification are expensive, time-intensive, and limited by several important methodological artifacts. Abundance of the 15N isotope in wetland soil organic matter (δ15Nsoils) might offer an inexpensive and rapid alternative to direct denitrification measures. In addition, δ15Nsoils represent an integrated signal of denitrification over time, rather than the highly-fluctuating, instantaneous measures. However, a quantitative relationship between δ15Nsoils and wetland denitrification rates has yet to be established in the literature. Pilot studies to develop a δ15Nsoils indicator of denitrification were conducted in several different Ohio wetlands. Preliminary results show that δ15Nsoils are positively correlated to in-situ denitrification measurements collected within the same wetland. We will present details of this analysis. Our results suggest the potential for this method to be used as an estimate of denitrification across a broad array of wetland types. As such, we are currently evaluating δ15Nsoil collected from approximately 1000 wetland sites from across the conterminous United States that were sampled for the National Wetland Condition Assessment. δ15Nsoils range from -3.7 to 13.2‰, which represents an extremely wide range of N processing. By mapping δ15Nsoils across the US landscape, we suggest that it will be possible to identify areas important for reducing nitrate loading to the nation’s water resources.