Elevated CO does not stimulate carbon sink in a semi-arid grassland.

Elevated CO is widely accepted to enhance terrestrial carbon sink, especially in arid and semi-arid regions. However, great uncertainties exist for the CO fertilisation effects, particularly when its interactions with other global change factors are considered. A four-factor (CO , temperature, precipitation and nitrogen) experiment revealed that elevated CO did not affect either gross ecosystem productivity or ecosystem respiration, and consequently resulted in no changes of net ecosystem productivity in a semi-arid grassland despite whether temperature, precipitation and nitrogen were elevated or not. The observations could be primarily attributable to the offset of ecosystem carbon uptake by enhanced soil carbon release under CO enrichment. Our findings indicate that arid and semi-arid ecosystems may not be sensitive to CO enrichment as previously expected and highlight the urgent need to incorporate this mechanism into most IPCC carbon-cycle models for convincing projection of terrestrial carbon sink and its feedback to climate change.