Effects of Atmospheric CO 2 Enrichment on Ecosystem CO 2 Exchange in a Nutrient and Water Limited Grassland

Abstract

We have completed 3 years of a study aimed at understanding the impact of elevated atmospheric CO2 on ecosystem properties of annual grasslands at the Jasper Ridge Biological Preserve, Stanford, CA, U.S.A. Measurements of net ecosystem CO2 uptake were made on intact grassland (on serpentine and sandstone derived soils grown in open-top chambers since December 1991). We measured CO2 exchange in the field with transparent Teflon-lined acrylic chambers coupled to an open gas exchange system. Net ecosystem CO2 uptake for both the high productivity sandstone and the low productivity serpentine grassland communities ranged from 2 to 11 umol m-2 ground s 1 in 1992 and 1993, similar to rates obtained with eddy covariance techniques on the sandstone and serpentine grasslands at Jasper Ridge in a previous study. There was a significant effect of elevated CO2 on net ecosystem CO2 uptake rate (40-48% increase in 1992 and 17-117% increase in 1993: ANOVA P = 0.018). Although elevated CO2 consistently enhanced net ecosystem CO2 uptake at the growth CO2 concentrations, acclimation occurred such that elevated C02-grown ecosystems had reduced rates of CO2 uptake relative to ambient C02-grown ecosystems at either ambient or elevated CO2 measurement concentrations of CO2. The reduction in ecosystem level photosynthetic capacity in elevated CO2 treatments was accompanied by decreased foliar ribulose-bis-phosphate carboxylase (rubisco) activity on a weight basis in the species dominant in both grassland communities. Decreases in rubisco activity resulted largely from increases in leaf mass per area in elevated CO2 plants. In general, net ecosystem CO2 uptake was positively correlated with peak biomass. However, the data suggest that biomass yield for a given level of net ecosystem CO2 uptake may be lower in elevated CO2 chambers, especially in the higher productivity sandstone community.