Nitrogen Release From Permafrost Thaw May Partially Offset Future Soil Carbon Losses

Permafrost ecosystems contain significant soil carbon stocks which could be partially decomposed and released into the atmosphere if permafrost thaws. However, their future response to climate change is uncertain due to the complex interactions between permafrost physics, hydrology, and the carbon and nitrogen cycles. In particular, the release of nitrogen from thawing permafrost is an overlooked feedback that could reduce the vegetation nitrogen limitation and enhance plant carbon uptake, affecting the future greenhouse gas balance in the Arctic. In this study, we use a new version of the Institut Pierre-Simon Laplace (IPSL) Earth system model, called IPSL-Perm-LandN, which includes an explicit representation of the nitrogen cycle and key permafrost physical and biogeochemical processes (e.g., soil freezing, insulation by soil organic matter, cryoturbation, vertically resolved soil biogeochemistry). We performed idealised climate change simulations to isolate the impacts of CO₂ fertilisation, climate change, and of increased nitrogen availability following permafrost thaw. In our model, this nitrogen-induced feedback offsets over 80% of the climate-induced permafrost soil carbon losses. It prevents an additional negative contribution of more than 10 PgC °C^-1 to the global carbon-climate feedback parameter $\gamma$ from permafrost ecosystems. Consequently, the permafrost region remains a carbon sink throughout the simulation, driven by the combined effects of CO₂ fertilisation and increased nitrogen availability following thaw. However, the future vegetation carbon uptake due to increased nitrogen availability has only been quantified in a few studies and may be overestimated by our model. Therefore, its strength remains highly uncertain and care must be taken not to underestimate the permafrost carbon-climate feedback when designing climate change mitigation strategies.