Elevated CO2 decreases soil carbon stability in Tibetan Plateau

Abstract

The lack of ecosystem-scale CO2 enrichment experiments in alpine regions considerably restricts our ability to predict the feedback of the global carbon (C) cycle to climate change. Here we investigate soil C response in an experiment with 5-year CO2 enrichment and nitrogen (N) fertilization in a Tibetan meadow (4585 m above the sea level). We found that despite non-significant increase in bulk soil C pool, elevated CO2 dramatically altered the allocation of C in different soil fractions and soil mineralization potentials. By changing soil microbial composition and enhancing enzyme activities, elevated CO2 significantly accelerated soil organic matter (SOM) mineralization rates and stimulated the microbial utilization of ‘old C’ relative to that of ‘new C’. Furthermore, N fertilization under elevated CO2 altered the decomposition process, increased the fungi to bacteria ratio, and decreased the coarse particulate organic matter pool and enzyme activities, indicating that N fertilization counters the CO2 fertilization effect. Overall, our findings suggest a growing threat of elevated CO2 in reducing SOM stability, and highlight the key role of N availability in driving soil C turnover under elevated CO2.