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

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 ¹⁵N-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.