Differences in reactive nitrogen availability and N2O production between active layer and ice-rich Yedoma permafrost

Warming of the Arctic is causing permafrost thaw and the acceleration of the nitrogen (N) cycle. Permafrost thaw releases previously unavailable organic N reservoirs that are now available for decomposition. This is expected to increase the availability of inorganic N, which may ultimately lead to enhanced emissions of nitrous oxide (N₂O). This is particularly relevant for Yedoma, ice-rich permafrost sediments that store large amounts of N and are commonly found across large areas affected by permafrost. Despite the few previous studies investigating the effects of permafrost thaw on the N cycle and N₂O emissions, there is still poor understanding of the difference in N dynamics between the active and permafrost layers during and after thaw.

Here, we address this knowledge gap by conducting a N cycling study using 3 intact soil cores collected across the Baldwin Peninsula in northwest Alaska. These 2-m-long cores include the active layer, the interface between the active layer and permafrost, and more than one meter of ice-rich Yedoma permafrost. The study consisted of N₂O measurements during initial thawing of the soil, detailed depth profiling of extractable N, including ammonium (NH₄⁺), nitrate (NO₃^-), and total dissolved N (DN), right after the thaw, and a soil incubation experiment at 5 ^oC to determine N₂O production under oxic and anoxic conditions. Also, we included a treatment under anoxic conditions with acetylene inhibition to estimate the total denitrification, including N₂. Furthermore, we amended the soil with NO₃^- under anoxic conditions to investigate potential N₂O production and denitrification and to reveal possible NO₃^- limitation of these processes.

The permafrost layers presented an accumulation of NH₄⁺ content compared to the active layer, whereas NO₃^- was only found in the active layer and in minimal amounts. The active layer had the highest potential denitrification rate in the presence of NO₃^- and acetylene, but showed very low or negligible N₂O production when NO₃^- was not added. No N₂O production was observed in the permafrost layers in any of the treatments, even with the addition of NO₃^- or NO₃^- and acetylene, indicating that denitrification is not occurring. We suggestthat this lack of N₂O production and denitrification activity is due to microbial limitation. These results can help better understand the significance of permafrost N release during permafrost thaw to the Arctic ecosystem and its climate feedback.