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We examined soil and vegetation N isotopic composition (delta(15)N) and soil inorganic N availability in an intact Mojave desert ecosystem to evaluate potential effects of elevated atmospheric CO2 on N cycling. Vegetation from the dominant perennial shrub Larrea tridentata under elevated CO2 was enriched in N-15. Over a 7-month sampling period, Larrea delta(15)N values increased from 5.7 +/- 0.1parts per thousand to 9.0 +/- 1.1parts per thousand, with elevated CO2; under ambient conditions, delta(15)N values of shrubs increased from 4.9 +/- 0.3parts per thousand to 6.6 +/- 0.7parts per thousand. No difference was found in soil delta(15)N under elevated and ambient CO2. Soil delta(15)N values under the drought deciduous shrubs Lycium spp. were greatest (7.2 +/- 0.3parts per thousand), and soil under the C4 perennial bunchgrass Pleuraphis rigida had the lowest values (4.5 +/- 0.2parts per thousand,). Several mechanisms could explain the enrichment in N-15 of vegetation with elevated CO2. Results suggest that microbial activity has increased with elevated CO2, enriching pools of plant-available N and decreasing N availability. This hypothesis is supported by a significant reduction of plant-available N under elevated CO2. This indicates that exposure to elevated CO2 has resulted in significant perturbations to the soil 1 cycle, and that plant delta(15)N may be a useful tool for interpreting changes in the N cycle in numerous ecosystems.