Stable isotopic evidence of enhanced export of microbially derived $${\text{NO}}_{3}^{ - }$$ NO 3 - following active layer slope disturbance in the Canadian High Arctic

Abstract

Permafrost disturbance is expected to alter nitrogen (N) export in High Arctic watersheds by enhancing loads of dissolved inorganic N (DIN), particularly nitrate (\({\text{NO}}_{3}^{ - }\)), by enabling nitrification and/or the mobilization of N previously sequestered in deeper permafrost soils. Using chemical, isotopic, and hydrologic measurements, we compare the seasonal evolution of concentrations and sources of \({\text{NO}}_{3}^{ - }\) in a stream draining an undisturbed catchment with one that drains a catchment that has been affected by active layer detachments (ALDs) at the Cape Bounty Arctic Watershed Observatory (74°54′N, 109°35′W) on Melville Island, Nunavut. Oxygen stable isotope values of \({\text{NO}}_{3}^{ - }\) (δ18O-\({\text{NO}}_{3}^{ - }\)) from streamwater indicate that \({\text{NO}}_{3}^{ - }\) in runoff predominantly originated from atmospheric sources in both catchments only during the first days of melt. In the undisturbed catchment, low \({\text{NO}}_{3}^{ - }\) concentrations and elevated streamwater δ18O-\({\text{NO}}_{3}^{ - }\) values relative to the disturbed catchment indicate that \({\text{NO}}_{3}^{ - }\) export was suppressed by sink mechanisms. In contrast, low δ18O-\({\text{NO}}_{3}^{ - }\) values and high \({\text{NO}}_{3}^{ - }\) concentrations in runoff from the disturbed catchment indicate that the supply of \({\text{NO}}_{3}^{ - }\) from microbial sources far outweighed sinks in the watershed. This research demonstrates that ALDs enhanced the export of microbially derived \({\text{NO}}_{3}^{ - }\) relative to undisturbed watersheds, and that this is likely a result of limited \({\text{NO}}_{3}^{ - }\) retention and enhanced nitrification in the mineral soils exposed in the scar zones of ALDs.