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Impacts of either elevated CO2 or drought stress on plant growth have been studied extensively, but interactive effects of these on plant carbon and nitrogen allocation is inadequately understood yet. In this study the response of the dominant desert shrub, Caragana intermedia Kuanget H.c.Fu, to the interaction of elevated CO2 (700 +/- 20 mu mol mol(-1)) and soil drought were determined in two large environmental growth chambers (18 m(2)). Elevated CO2 increased the allocation of biomass and carbon into roots and the ratio of carbon to nitrogen (C:N) as well as the leaf soluble sugar content, but decreased the allocation of biomass and carbon into leaves, leaf nitrogen and leaf soluble protein concentrations. Elevated CO2 significantly decreased the partitioning of nitrogen into leaves, but increased that into roots, especially under soil drought. Elevated CO2 significantly decreased the carbon isotope discrimination (Delta) in leaves, but increased them in roots, and the ratio of Delta values between root and leaf, indicating an increased allocation into below-ground parts. It is concluded that stimulation of plant growth by CO2 enrichment may be negated under soil drought, and under the future environment, elevated CO2 may partially offset the negative effects of enhanced drought by regulating the partitioning of carbon and nitrogen.