CHAMBER AND MICROMETEOROLOGICAL MEASUREMENTS OF CO2 AND H2O FLUXES FOR 3 C-4 GRASSES

Accurate measurements of surface fluxes of carbon dioxide (CO2) and wa ter (H2O) are important for several reasons and can be made using seve ral types of instrumentation. For three C-4 grasses-bermudagrass (Cyno don dactylon (L.) Pers.), a mixed species native tallgrass prairie, an d sorghum (Sorghum bicolor (L.) Moench.)-we measured evapotranspiratio n (ET) using a canopy chamber (CC) and Bowen ratio/energy balance (BRE B) instrumentation and we measured leaf CO2 uptake using a leaf chambe r (LC), and, after accounting for soil CO2 fluxes, we calculated leaf uptake using a CC and BREB instrumentation. In addition, soil CO2 flux es from bare soil were measured using a CC and soil chamber (SC). Meas urements were made on 4 and 5 May 1994 at the Blackland Research Cente r, Temple, TX, Flux of CO2 into the leaf was considered positive and w as expressed per unit ground area. Half-hour CC ET measurements were c onsistently and substantially greater than BREB measurements for all g rasses, perhaps because of increased soil evaporation due to greater t urbulence inside the CC. Leaf CO2 uptake measured using the three meth ods showed similar diurnal trends for all grasses (responding, primari ly, to changes in photosynthetic photon flux density), but consistentl y tended to be greatest for BREB measurements, The regression equation for LC CO2 uptake as a function of BREB uptake had a slope not statis tically different from 1.0, with large scatter likely because of limit ed leaf area sampled. CC CO2 uptake was consistently the least, partly because we may have underestimated soil CO2 flux in the CC, Half-hour soil CO2 fluxes from the CC were significantly greater (P < 0.05) tha n those from the SC for about two-thirds of the day on bare soil, perh aps because of large chamber ventilation rates. Differences of daytime soil CO2 fluxes averaged 0.07 mg m(-2) s(-1) (1.0 mg m(-2) s(-1) appr oximate to 22.7 mu mol m(-2) s(-1)). These results show the consistenc y, repeatability and, we believe, accuracy of leaf CO2 uptake and soil CO2 flux measurements made using all methods.