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Abstract

We examined soil and vegetation N isotopic composition (’15N) 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 15N. Over a 7-month sampling period, Larrea ’15N values increased from 5.7-0.1‰ to 9.0-1.1‰ with elevated CO2; under ambient conditions, ’15N values of shrubs increased from 4.9-0.3‰ to 6.6-0.7‰. No difference was found in soil ’15N under elevated and ambient CO2. Soil ’15N values under the drought deciduous shrubs Lycium spp. were greatest (7.2-0.3‰), and soil under the C4 perennial bunchgrass Pleuraphis rigida had the lowest values (4.5-0.2‰). Several mechanisms could explain the enrichment in 15N 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 N cycle, and that plant ’15N may be a useful tool for interpreting changes in the N cycle in numerous ecosystems.