Abstract: Atmospheric CO₂ concentrations are predicted to increase from approximately 350 μmol·mol^-1today to over 700 μmol·mol^-1in the late 21st century. In the future, elevated CO₂ levels are likely to have profound effects on precipitation. This change would seriously affect the desert ecosystem, altering the carbon and nitrogen allocations of desert plants and so leading to changes in ecosystem structure and function. Although many studies have examined the effects of precipitation and CO₂, the interactions between precipitation changes and CO₂ in desert plants have attracted little attention to date. In order to assess the possible effect of global climate change on desert ecosystems, a pot experiment was conducted to study the interaction of elevated CO₂ concentrations and changing precipitation with organic carbon, total nitrogen, C/N, organic carbon accumulation (absorption), and total nitrogen accumulation (absorption) in the roots, stems and leaves of Reaumuria soongorica, a dominant species on desert steppe in the arid regions of China. The main experiment included three CO₂ concentrations (350, 550 and 700 μmol·mol^-1) and five precipitation conditions (natural precipitation as control [W₀], precipitation minus 30% [-W₂], precipitation minus 15% [-W₁], precipitation plus 15% [+W₁], and precipitation plus 30% [+W₂]). The main results are as follows: 1) At the same precipitation treatment, organic carbon in root, stem and leaf were significantly increased and total nitrogen decreased by elevated CO₂. The rise of organic carbon contents in roots was highest at 13.33% under 700 μmol·mol^-1 CO₂ concentration and -W₂ precipitation. Total nitrogen contents in leaves were lowest at 56.31% under 700 μmol·mol^-1 CO₂ concentration and +W₁ precipitation. C/N, organic carbon and total nitrogen accumulation (absorption) in root, stem and leaf were significantly increased by elevated CO₂. 2) At the same CO₂ concentrations, organic carbon and total nitrogen in roots and stems significantly increased with raised precipitation levels. The largest increase in organic carbon in leaves was 11.56% under +W₂ precipitation and 550 μmol·mol^-1 CO₂ concentration. The largest decrease in total nitrogen in leaves was 40.16% under precipitation +W₂ and 700 μmol·mol^-1 CO₂ concentration. C/N, organic carbon and total nitrogen accumulation (absorption) in root, stem and leaf were affected significantly by precipitation. 3) Under the interactive effects of elevated CO₂ and changing precipitation conditions, organic carbon contents and C/N were allocated mostly to roots, total nitrogen, organic carbon accumulation and total nitrogen accumulation (absorption) mostly to leaves. These results suggest that in the future, with CO₂ concentration and precipitation changes, the growth states of R. soongorica will be decided by the interactive effects of CO₂ concentration and precipitation changes on carbon and nitrogen. Elevated CO₂ concentration can relieve the inhibition of drought on the carbon and nitrogen absorbed and used of R. soongorica, and moist conditions can reinforce the impact of these elements.

Key words: elevated CO₂ concentration, precipitation change, organic carbon, total nitrogen, Reaumuria soongorica