When Nitrogen Retention Fails: Carbon Losses in a Warming Arctic

High-latitude ecosystems play a central role in the global carbon cycle, yet their responses to climate warming remain one of the largest sources of uncertainty in Earth system projections. Most models assume that warming accelerates nitrogen (N) cycling, alleviates plant N limitation, and enhances vegetation productivity, thereby buffering warming-induced soil carbon (C) losses. This paradigm implies that nutrient feedbacks stabilize ecosystem C storage under rising temperatures. However, growing experimental evidence challenges this assumption.

In this talk, I synthesize results from long-term soil warming studies along natural geothermal gradients in subarctic ecosystems to reassess how warming reshapes C–N coupling, nutrient retention, and ecosystem resilience. Across years to decades of sustained warming, we observe large and proportional losses of soil C and N, despite increased microbial activity and N mineralization. Crucially, enhanced N availability does not translate into sustained plant growth or long-term ecosystem C gains.

Our findings reveal a mechanistic shift in ecosystem functioning under warming: increased microbial metabolic costs and carbon limitation reduce microbial biomass and weaken key nitrogen stabilization pathways. Microbial and fine-root N pools, critical short- and long-term N reservoirs in cold ecosystems, decline with warming, particularly during winter and snowmelt periods when plant uptake is low. Seasonal increases in plant N uptake during the growing season are too small and too transient to compensate for these losses. This leads to an effective “opening” of the N cycle, increased N losses, and stoichiometrically coupled soil C losses.

Although microbial communities eventually reorganize toward more conservative N cycling under long-term warming, this physiological adjustment stabilizes fluxes rather than restoring depleted soil C and N stocks. As a result, early warming-induced losses may be effectively irreversible, even under later ecosystem acclimation.

Taken together, these results suggest a fundamental conceptual shift: warming does not simply accelerate biogeochemical cycles but erodes the mechanisms that retain nutrients and carbon in high-latitude soils. This challenges the widespread assumption that nitrogen feedbacks buffer carbon losses and highlights the need to explicitly represent microbial physiology, nutrient retention, and seasonal asynchronies in Earth system models to improve predictions of carbon–climate feedbacks.