Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem

• Published in: Journal of Geophysical Research: Biogeosciences
• Main Authors: Heikoop, Jeffrey M., Throckmorton, Heather M., Newman, Brent D., Perkins, George B., Iversen, Colleen M., Chowdhury, Taniya Roy, Romanovsky, Vladimir E., Graham, David E., Norby, Richard J., Wilson, Cathy J., Wullschleger, Stan D.
• Language: unknown
• Published: 2021
• Subjects: 58 GEOSCIENCES; Arctic; Barrow; Ice; permafrost; Tundra; wedge*; Alaska
• Online Access: http://www.osti.gov/servlets/purl/1213341; https://www.osti.gov/biblio/1213341; https://doi.org/10.1002/2014JG002883

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 at 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.