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.