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In a phytotron experiment, we examined growth and rates of NO3- and PO43- uptake in seedlings of two desert C-3 shrubs (Larrea tridentata and Prosopis glandulosa) and a desert C-4 perennial grass (Bouteloua eriopoda) grown under CO2 partial pressures of 35 or 70 Pa. Plants were grown in soil but uptake studies were conducted on roots of intact seedlings placed in nutrient solutions containing both NO3- and PO43-. Elevated CO2 increased total biomass by 69 and 55% in Larrea and Prosopis seedlings and by 25% in Bouteloua. NO3- and PO43- uptake rates were more than doubled in Bouteloua at high compared to ambient CO2. In contrast, CO2 enrichment inhibited root NO3- uptake capacity in Larrea by about 55% without a significant effect on PO43- absorption rate; rates of NO3- and PO43- and uptake in Prosopis were insensitive to CO2 treatment. Elevated CO2 enhanced the proportion of biomass allocated to the fine roots in Bouteloua but markedly reduced this fraction in Larrea and Prosopis. Foliar N concentration of both shrubs decreased in response to elevated CO2, but was unaffected in Bouteloua. We suggest that compensatory changes in root size and activity are critical in determining interspecies variation in plant nutrient relations under high CO2.