The stimulatory effect of elevated CO on soil respiration is unaffected by N addition.

Global atmospheric CO keeps rising and brings about significant effects on ecosystem carbon (C) cycling by altering C processes in soils. Soil C responses to elevated CO are highly uncertain, and how elevated CO interacts with other factors, such as nitrogen (N) availability, to influence soil C flux comprises an important source of this uncertainty, especially for those under-studied ecosystems. By conducting a manipulated CO concentration and N availability experiment on typical alpine grassland (4600 m asl), we combined the five-year in-situ measurement of soil respiration (SR) with an incubation experiment of microbial metabolic efficiency in the lab to explore the response of SR to elevated CO and N availability. The results showed that elevated CO at ambient N conditions and enriched N equally stimulated SR during the experimental period, whereas N supply had no significant effect. Elevated CO enhanced soil dissolved organic C and enzyme activity, while had marginal effects on microbial biomass and C use efficiency (CUE). Strengthened microbial activity dominated SR stimulation under elevated CO. Enriched N boosted enzyme activity and microbial CUE. N availability played divergent roles in mediating SR. The negliable regulation of N supply on elevated CO effects on SR was the offset consequences of the negative impacts of enhanced CUE and the positive contribution of heightened enzyme activity. Our findings suggest that rising CO would accelerate soil C cycling of the alpine grassland under various N regimes by stimulating microbial activity instead of lowering microbial metabolic efficiency. Such results are crucial for understanding the role of alpine ecosystems in the global C cycle.