Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem
|Title||Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Authors||Heikoop, Jeffrey M., Heather M. Throckmorton, Brent D. Newman, George B. Perkins, Colleen Iversen, Taniya Roy Chowdhury, Vladimir E. Romanovsky, David E. Graham, Richard J. Norby, Cathy J. Wilson, and Stan Wullschleger|
|Journal||Journal of Geophysical Research: Biogeosciences|
|Pagination||1000 - 1017|
The nitrate (NO3−) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO3− derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO3− signal with δ15N averaging −4.8 ± 1.0‰ (standard error of the mean) and δ18O averaging 70.2 ± 1.7‰. In active layer pore waters, NO3− primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high-centered polygons. The average δ15N and δ18O of NO3− from high-centered polygons were 0.5 ± 1.1‰ and −4.1 ± 0.6‰, respectively. When compared to the δ15N of reduced nitrogen (N) sources, and the δ18O of soil pore waters, it was evident that NO3− in high-centered polygons was primarily from microbial nitrification. Permafrost NO3− had δ15N ranging from approximately −6‰ to 10‰, similar to atmospheric and microbial NO3−, and highly variable δ18O ranging from approximately −2‰ to 38‰. Permafrost ice wedges contained a significant atmospheric component of NO3−, while permafrost textural ice contained a greater proportion of microbially derived NO3−. Large-scale permafrost thaw in this environment would release NO3− with a δ18O signature intermediate to that of atmospheric and microbial NO3. Consequently, while atmospheric and microbial sources can be readily distinguished by the NO3− dual isotope technique in tundra environments, attribution of NO3− from thawing permafrost will not be straightforward. The NO3− isotopic signature, however, appears useful in identifying NO3− sources in extant permafrost ice.