Preprints
https://doi.org/10.5194/egusphere-2022-671
https://doi.org/10.5194/egusphere-2022-671
 
10 Aug 2022
10 Aug 2022
Status: this preprint is open for discussion.

Permafrost degradation and nitrogen cycling in Arctic rivers: Insights from stable nitrogen isotope studies

Adam Francis1, Raja S. Ganeshram1, Robyn E. Tuerena2, Robert G. M. Spencer3, Robert M. Holmes4, Jennifer A. Rogers3, and Claire Mahaffey5 Adam Francis et al.
  • 1School of Geosciences, University of Edinburgh, Edinburgh, UK
  • 2Scottish Association for Marine Science, Oban, UK
  • 3Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, Florida, USA
  • 4Woodwell Climate Research Center, Falmouth, Massachusetts, USA
  • 5Department of Earth, Ocean and Ecological Sciences, University of Liverpool, UK

Abstract. Across the Arctic, vast areas of permafrost are being degraded by climate change, which has the potential to release substantial quantities of nutrients, including nitrogen into large Arctic rivers. These rivers heavily influence the biogeochemistry of the Arctic Ocean, so it is important to understand the potential changes to rivers from permafrost degradation. This study utilised dissolved nitrogen species (nitrate and dissolved organic nitrogen (DON)) along with nitrogen isotope values (δ15N-NO3- and δ15N-DON) of samples collected from permafrost sites in the Kolyma River and the six largest Arctic rivers. Large inputs of DON and nitrate with a unique isotopically heavy δ15N signature were documented in the Kolyma, suggesting the occurrence of denitrification and highly invigorated nitrogen cycling in the Yedoma permafrost thaw zones along the Kolyma. We show evidence for permafrost derived DON being recycled to nitrate as it passes through the river, transferring the high 15N signature to nitrate. However, the potential to observe these thaw signals at the mouths of rivers depends on the spatial scale of thaw sites, permafrost degradation and recycling mechanisms. In contrast with the Kolyma, with near 100 % continuous permafrost extent, the Ob’ River, draining large areas of discontinuous and sporadic permafrost, shows large seasonal changes in both nitrate and DON isotopic signatures. During winter months, water percolating through peat soils records isotopically heavy denitrification signals in contrast with the lighter summer values when surface flow dominates. This early year denitrification signal was present to a degree in the Kolyma but the ability to relate seasonal nitrogen signals across Arctic Rivers to permafrost degradation could not be shown with this study. Other large rivers in the Arctic show different seasonal nitrogen trends. Based on nitrogen isotope values, the vast majority of nitrogen fluxes in the Arctic rivers is from fresh DON sourced from surface runoff through organic-rich top-soil and not from permafrost degradation. However, with future permafrost thaw, other Arctic rivers may begin to show nitrogen trends similar to the Ob’. Our study demonstrates that nitrogen inputs from permafrost thaw can be identified through nitrogen isotopes, but only on small spatial scales. Overall, nitrogen isotopes show potential for revealing integrated catchment wide nitrogen cycling processes.

Adam Francis et al.

Status: open (until 29 Sep 2022)

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Adam Francis et al.

Adam Francis et al.

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Short summary
Climate change is causing extensive permafrost degradation and nutrient releases into rivers with great ecological impacts on the Arctic Ocean. We focused on nitrogen (N) release from this degradation and associated cycling using N isotopes, an understudied area. Many N species are released at degradation sites with exchanges between species. N inputs from permafrost degradation and seasonal river nitrogen trends were identified using isotopes, helping to predict climate change impacts.