C and N isotopic fractionation along permafrost profiles: A theoretical framework and empirical tests across the Arctic

Deschamps, L., Paillassa, J., Lévesque, E. et Maire, V. (2026). C and N isotopic fractionation along permafrost profiles: A theoretical framework and empirical tests across the Arctic. Journal of Geophysical Research: Biogeosciences, 131 (7). e2025JG009082. ISSN 2169-8953 2169-8961 DOI 10.1029/2025JG009082

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Résumé

Abstract

Anticipating the decomposition of permafrost organic matter (OM) following thaw is critical for improving projections of climate-carbon feedbacks. Stable carbon (C) and nitrogen (N) isotope profiles provide integrated records of the environmental and biogeochemical processes that shaped OM during its accumulation and may therefore improve our understanding of the factors controlling its future vulnerability to decomposition. Here, we compiled isotope data from 122 soil units across 42 Arctic sites and used Bayesian hierarchical models to test whether relationships between stable isotope signatures and soil C/N ratios varied along a gradient of soil organic carbon (SOC) content. Nitrogen isotope fractionation remained consistent across the SOC gradient, with δ15N increasing as C/N decreased. In contrast, carbon isotope fractionation differed markedly between mineral and organic soils. In mineral soils, δ13C increased with decreasing C/N, indicating extensive microbial processing and recycling of OM during accumulation. In organic soils, δ13C decreased with decomposition, consistent with the preferential preservation of relatively 13C-depleted plant-derived compounds under waterlogged conditions. These contrasting isotope patterns reveal distinct OM accumulation pathways in mineral and organic permafrost soils. Our results provide a revised conceptual framework linking long-term OM accumulation pathways to the properties of permafrost OM exposed by thaw, improving the interpretation of Arctic soil isotope records and their implications for permafrost carbon vulnerability.

Plain Language Summary

Arctic permafrost stores enormous amounts of organic matter (OM) that could be released as carbon dioxide and methane as the climate warms. However, not all permafrost OM is equally vulnerable to decomposition after thawing. To understand why, we examined natural variations in the stable isotope signatures of carbon and nitrogen, which preserve information about how OM accumulates over thousands of years. We compiled data from 122 soil units across 42 Arctic sites and analyzed how isotope signatures changed as OM decomposed. Nitrogen isotope patterns were remarkably consistent across Arctic soils, whereas carbon isotope patterns differed strongly between mineral and organic soils. These contrasting patterns indicate that OM followed different accumulation pathways before becoming frozen. Mineral soils show evidence of extensive microbial processing, whereas organic soils mainly preserve plant-derived material that accumulated under waterlogged conditions. Rather than directly predicting future decomposition, our study provides a new framework for interpreting Arctic soil isotope records. By revealing how past environmental conditions shaped the composition of permafrost OM, our results help improve predictions of how Arctic carbon stocks may respond to permafrost thaw and future climate change.

Type de document:	Article
Date de dépôt:	08 oct. 2026 13:04
Dernière modification:	08 oct. 2026 13:04
Version du document déposé:	Version officielle de l'éditeur
URI:	https://depot-e.uqtr.ca/id/eprint/13155

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