Nine years of CO2 enrichment at the alpine treeline stimulates soil respiration but does not alter soil microbial communities

► Nine year long enrichment of alpine treelines with 13C-depleted CO2. ► Elevated CO2 persistently increased soil respiration. ► Negligible CO2 response of the biomass and community structure of microorganisms. ► Fungal phospholipid fatty acids incorporated largest amount of added 13C signal. ► Fungal DNA was affected by tree species but not by elevated CO2.