NITROGEN AND PHOSPHORUS DYNAMICS OF A TALLGRASS PRAIRIE ECOSYSTEM EXPOSED TO ELEVATED CARBON-DIOXIDE

A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO2 ConCentrations in open-top chambers and compared to unchambered a mbient CO2 during the entire growing season from 1989 through 1991. Do minant species were Andropogon gerardii (C4), A. scoparius (C4), Sorgh astrum nutans (C4) and Poa pratensis (C3). Nitrogen and phosphorus con centrations in A. gerardii, P. pratensis and dicotyledonous herbs abov e ground biomass were estimated by periodic sampling throughout the gr owing season in 1989 and 1990. In 1991, N and P concentrations in peak biomass were estimated by an early August harvest. N and P concentrat ions in root production as a function of treatment were estimated usin g root ingrowth bags that remained in place throughout the growing sea son. Total N and P in above- and belowground biomass were calculated a s products of concentration and peak biomass by species groups. N conc entration in A. gerardii and dicotyledonous herb aboveground biomass w as lower and total N higher in elevated CO2 plots than in ambient CO2 PlOts. N concentration in P. pratensis aboveground biomass was lower i n elevated CO2 plots than in ambient, but total N did not differ among treatments in 2 out of 3 years. In 1990, N concentration in root ingr owth bag biomass was lower and total N greater in elevated CO2 than in ambient CO2 plots. Root ingrowth bag biomass N concentration did not differ among treatments in 1991, but total N was greater in elevated C O2 plots than in ambient CO2 Plots. P concentration was lower under el evated CO2 compared to ambient in 1989, but did not differ substantial ly among treatments in 1990 or 1991. In all years, total P in abovegro und A. gerardii and root ingrowth bag biomass was greater under elevat ed CO2 than ambient. P concentration and total P in P. pratensis was s imilar among treatments.