Will warming lead to an increased use of older soil organic carbon ( SOC ) by microbial communities, thereby inducing C losses from C ‐rich alpine soils? We studied soil microbial community composition, activity, and substrate use after 3 and 4 years of soil warming (+4 °C, 2007–2010) at the alpine treeline in S witzerland. The warming experiment was nested in a free air CO 2 enrichment experiment using depleted 13 CO 2 (δ 13...