Plant Nitrogen Dynamics in Shortgrass Steppe under Elevated Atmospheric Carbon Dioxide

Abstract

The direct and indirect effects of increasing levels of atmospheric carbon dioxide (CO2) on plant nitrogen (N) content were studied in a shortgrass steppe ecosystem in northeastern Colorado, USA. Beginning in 1997 nine experimental plots were established: three open-top chambers with ambient CO2 levels (approximately 365 μmol mol−1), three open-top chambers with twice-ambient CO2 levels (approximately 720 μmol mol−1), and three unchambered control plots. After 3 years of growing-season CO2 treatment, the aboveground N concentration of plants grown under elevated atmospheric CO2 decreased, and the carbon–nitrogen (C:N) ratio increased. At the same time, increased aboveground biomass production under elevated atmospheric CO2 conditions increased the net transfer of N out of the soil of elevated-CO2 plots. Aboveground biomass production after simulated herbivory was also greater under elevated CO2 compared to ambient CO2. Surprisingly, no significant changes in belowground plant tissue N content were detected in response to elevated CO2. Measurements of individual species at peak standing phytomass showed significant effects of CO2 treatment on aboveground plant tissue N concentration and significant differences between species in N concentration, suggesting that changes in species composition under elevated CO2 will contribute to overall changes in nutrient cycling. Changes in plant N content, driven by changes in aboveground plant N concentration, could have important consequences for biogeochemical cycling rates and the long-term productivity of the shortgrass steppe as atmospheric CO2 concentrations increase.