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

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.