Prolonged warming leads to carbon depletion and increases nutrient availability in alpine soils

Alpine ecosystems, shaped by cold temperatures and prolonged snow cover, are warming twice as fast as lowlands, making them particularly vulnerable to climate change. This rapid warming alters ecosystem functioning by increasing soil temperatures and shifting snow regimes, leading to shorter snow cover periods and longer growing seasons. Such changes impact soil organic matter (SOM), which regulates carbon storage and soil fertility through microbial mineralization. We investigated long-term SOM changes following experimental warming in alpine grasslands. By transplanting soil plots downslope (2470 m a.s.l. to 1920 m a.s.l.), we increased mean annual temperature by 3 °C and extended the growing season by 77 days. Seven years later, we analyzed soil nutrients content, SOM characteristics (chemistry, labile pools, and thermal stability) and conducted discrete flux measurements to determine net ecosystem exchange (NEE). Soil incubations assessed microbial traits and their acclimation to warming. Our results indicate that, even after seven years, alpine soil microbial activities showed limited acclimation to warming, contributing to SOM destabilization and soil nutrients enrichment by boosting mineralization. Indeed, warmed plots acted as carbon sources, with an 18 % decrease in SOC stocks and increased NEE. Carbon losses exceeded C gains from plant productivity, primarily depleting labile pools. This may create a positive feedback loop between carbon cycling and climate warming. These findings highlight the long-term consequences of temperature increases and snow regime shifts on alpine ecosystem functioning and suggest that soil carbon losses in warming mountain environments may continue over time. • Long-term transplant experiment simulating 3 °C warming in an alpine ecosystem. • Alpine soil microbial communities remained not fully acclimated to warmer conditions. • Warming depletes labile carbon while enhancing soil fertility. • Carbon loss driven by both temperature increase and phenological changes. • Ecosystem properties remain unadjusted after 7 years of warming.