Fluxes of CO 2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO 2

Abstract

Increasing concentrations of atmospheric CO 2 may alter the carbon and water relations of prairie ecosystems. A C 4-dominated tallgrass prairie near Manhattan, KS, was exposed to 2 × ambient CO 2 concentrations using 4.5 m-diameter open-top chambers. Whole-chamber net CO 2 exchange (NCE) and evapotranspiration (ET) were continuously monitored in CO 2-enriched and ambient (no enrichment) plots over a 34-d period encompassing the time of peak biomass in July and August, 1993. Soil-surface CO 2 fluxes were measured with a portable surface chamber, and sap flow (water transport in xylem) in individual grass culms was monitored with heat balance techniques. Environmental measurements were used to determine the effect of CO 2 on the surface energy balance and canopy resistances to vapor flux. In 1993, frequent rainfall kept soil water near field capacity and minimized plant water stress. Over the 34-d measurement period, average daily NCE (canopy photosynthesis — soil and canopy respiration) was 9.3 g CO 2 m −2 in the ambient treatment adn 11.4 g CO 2 m −2 under CO 2 enrichment. However, differences in NCE were caused mainly by delayed senescence in the CO 2-enriched plots at the end of the growing season. At earlier stages of growth, elevated CO 2 had no effect on NCE. Soil-surface CO 2 fluxes typically ranged from 0.4 to 0.66 mg CO 2 m −2 s −1, but were slightly greater in the CO 2_enriched chambers. CO 2 enrichment reduced daily ET by 22%, reduced sap flow by 18%, and increased canopy resistance to vapor flux by 24 s m −1. Greater NCE and lower ET resulted in higher daytime water use efficiency (WUE) under CO 2 enrichment vs. ambient (9.84 vs. 7.26 g CO 2 kg −1 H 2O). However, record high precipitation during the 1993 season moderated the effect of WUE on plant growth, and elevated CO 2 had no effect on peak aboveground biomass. CO 2-induced stomatal closure also affected the energy balance of the surface by reducing latent heat flux ( LE ), thereby causing a consequent change in sensible heat flux ( H ). The daytime Bowen ratio ( H/LE) for the study period was near zero for the ambient treatment and 0.21 under CO 2 enrichment.