Decoupling of plant carbon and nitrogen under elevated CO 2 and nitrogen addition in a typical alpine ecosystem

Aims Vegetation in high-altitude regions is hypothesized to be more responsive to increasing atmospheric CO 2 concentrations due to low CO 2 partial pressure. However, the underlying mechanisms driving this response at an ecosystem scale are poorly understood. We aimed to explore the plant carbon (C) and nitrogen (N) relationships and biomass allocation in response to elevated CO 2 and N addition in a Tibetan meadow. Methods A 5-year manipulation experiment was conducted in an alpine meadow (4585 m above sea level) to explore the responses of plant carbon (C), nitrogen (N), and biomass dynamics, as well as their allocation schemes, to elevated CO 2 (from 380 ppm to 480 ppm) and N fertilization. Results Elevated CO 2 alone significantly enhanced aboveground plant biomass by 98%, exhibiting a stronger CO 2 fertilization effect than the global average level (20%) for grasslands. Elevated CO 2 favored N accumulation in aboveground parts despite the declined concentration. Nitrogen fertilization alleviated the N constraints on CO 2 fertilization effects, which strengthened C sequestration capacity for the aboveground plant tissues. Moreover, our results indicate a decoupling between C and N cycles in alpine ecosystems under elevated CO 2 , especially in the N-enrichment environments. Conclusions Overall, this study shows a high sensitivity of aboveground plant biomass and decoupled C-N relationships under elevated CO 2 and N fertilization for high-elevation alpine ecosystems, highlighting the need to incorporate altitude effects into Earth System Models in predicting C cycle feedbacks to climate changes.