Submitted to: Ecosystems
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/17/2002
Publication Date: 12/1/2004
Citation: King, J., Mosier, A.R., Morgan, J.A., Lecain, D.R., Milchunas, D., Parton, W. 2004. Plant nitrogen dynamics in shortgrass steppe under elevated atmospheric carbon dioxide. Ecosystems. 7:147-160. Interpretive Summary: By the end of this century the concentration of atmospheric CO2 is expected to more than double. This continued increase in CO2 concentration is likely to have profound effects on crop and rangeland productivity. In many instances production may increase, others may not be noticeably affected, while others will be harmed. Since the impact of increasing CO2 concentrations had not been investigated in the short grass prairie an open top chamber study was initiated in 1997 at a shortgrass steppe site in northeastern Colorado, USA.. One of the questions to be resolved was the effect of increasing CO2 on forage quality in these grasslands. In this context we use above ground plant nitrogen concentration as a surrogate for the quality of grass injested by livestock. The effects of elevated atmospheric carbon dioxide (CO2) on plant nitrogen (N) concentrations may have important consequences on ecosystem N cycling and thus the sustainability of the system over the long term. Nine experimental plots were established: three opentop chambers with ambient CO2 levels (~365 µmol mol-1), three open-top chambers with twice-ambient CO2 levels (~720 µmol mol-1), and three unchambered control plots. The study revealed that above ground plant production increased by about 30% under elevated CO2. The forage produced contained significantly less total nitrogen and was less digestible than forage produced under ambient CO2 atmosphere. A trend to the production of lower quality forage became evident through the years of the study. Production of lower quality forage could result in slower system N cycling and lower forage production in the long term.
Technical Abstract: The effects of elevated atmospheric carbon dioxide (CO2) on plant nitrogen (N) concentrations may have important consequences on ecosystem N cycling. An elevated atmospheric CO2 study was initiated in 1997 at a shortgrass steppe site in northeastern Colorado, USA. Nine experimental plots were established: three opentop chambers with ambient CO2 levels (~365 µmol mol-1), three open-top chambers with twice-ambient CO2 levels (~720 µmol mol-1), and three unchambered control plots. Following three years of growing season CO2 treatment, the aboveground N concentration of plants grown under elevated atmospheric CO2 decreased, and the 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. Measurements of individual species at peak standing phytomass showed significant CO2 treatment effects on aboveground plant tissue N concentration and significant differences between species in N concentration, which suggest that changes in species composition under elevated CO2 will contribute to overall changes in nutrient cycling. The observed changes in aboveground plant N concentration could have important consequences on biogeochemical cycling rates and the long-term productivity of the shortgrass steppe as atmospheric CO2 concentrations increase.