Warming Weakens Soil Nitrogen Stabilization Pathways Driving Proportional Carbon Losses in Subarctic Ecosystems

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ABSTRACT Climate warming poses a significant threat to the nitrogen (N) and carbon (C) retention capacities of subarctic ecosystems, with cascading effects on soil nutrient cycling and long‐term ecosystem functioning. Here, we investigated the effects sustained soil warming on the temporal retention and stabilization of N in key ecosystem pools in a subarctic grassland performing a 15 N‐tracing experiment in different seasons. Our results reveal that warming reduced N retention across key soil pools, with the largest proportional losses occurring in the non‐extractable soil fraction, a critical long‐term reservoir of organic matter. These losses were driven by the depletion of organic compounds involved in ex vivo N stabilization and the weakening of in vivo stabilization mechanisms. Warming also decreased microbial and fine root biomass, limiting their ability to temporarily immobilize N during the snowmelt period, when soil N retention is most critical. In contrast, warming increased aboveground plant biomass and N uptake during the growing season, indicating a shift in resource allocation towards aboveground tissues. However, the increase in plant N uptake, both due to its magnitude (0.14% of N gained °C −1 ) and seasonality, was insufficient to offset the loss of N retention in the microbial biomass and fine roots (1.99% of N lost °C −1 ) and non‐extractable soil pools (1.7%–2.6% of N lost °C −1 ). As a consequence, we observed coupled and proportional C losses across all soil pools. These findings suggest that warming disrupts key pathways of soil N stabilization, leading to the “opening” of the N cycle and proportional, potentially irreversible, C losses from cold ecosystems.

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