Chamber and micrometeorological measurements of CO 2 and H 2O fluxes for three C 4 grasses

Abstract

Accurate measurements of surface fluxes of carbon dioxide (CO 2) and water (H 2O) are important for several reasons and can be made using several types of instrumentation. For three C 4 grasses—bermudagrass ( Cynodon dactylon (L.) Pers.), a mixed species native tallgrass prairie, and sorghum ( Sorghum bicolor (L.) Moench.)—we measured evapotranspiration (ET) using a canopy chamber (CC) and Bowen ratio/energy balance (BREB) instrumentation and we measured leaf CO 2 uptake using a leaf chamber (LC), and, after accounting for soil CO 2 fluxes, we calculated leaf uptake using a CC and BREB instrumentation. In addition, soil CO 2 fluxes from bare soil were measured using a CC and soil chamber (SC). Measurements were made on 4 and 5 May 1994 at the Blackland Research Center, Temple, TX. Flux of CO 2 into the leaf was considered positive and was expressed per unit ground area. Half-hour CC ET measurements were consistently and substantially greater than BREB measurements for all grasses, perhaps because of increased soil evaporation due to greater turbulence inside the CC. Leaf CO 2 uptake measured using the three methods showed similar diurnal trends for all grasses (responding, primarily, to changes in photosynthetic photon flux density), but consistently tended to be greatest for BREB measurements. The regression equation for LC CO 2 uptake as a function of BREB uptake had a slope not statistically different from 1.0, with large scatter likely because of limited leaf area sampled. CC CO 2 uptake was consistently the least, partly because we may have underestimated soil CO 2 flux in the CC. Half-hour soil CO 2 fluxes from the CC were significantly greater ( P < 0.05) than those from the SC for about two-thirds of the day on bare soil, perhaps because of large chamber ventilation rates. Differences of daytime soil CO 2 fluxes averaged 0.07 mg m −2 s −1 (1.0 mg m −2 s −1 ≈ 22.7 μ mol m −2 s −1). These results show the consistency, repeatability and, we believe, accuracy of leaf CO 2 uptake and soil CO 2 flux measurements made using all methods.