Responses of net ecosystem CO2 exchange to nitrogen fertilization in experimentally manipulated grassland ecosystems
Due to past limitations in experimental technology, canopy function has generally been inferred from leaf properties through scaling and/or indirect measurements. The development of a facility (EcoCELLs) at the Desert Research Institute has now made it possible to directly measure canopy gas exchange. In this experiment, sunflowers (Helianthus annus) were planted in the EcoCELLs and grown under ambient (399 fLmol mol-l) and elevated (746 fLmol mol-l) CO2 concentrations. We continuously measured carbon flux during canopy development from which canopy quantum yield (4>c) was estimated. The results indicated that the total daily carbon flux was similar between elevated and ambient CO2 treatments in the early stage of canopy development. After the canopy closed, carbon flux under elevated CO2 averaged 53% higher than that under ambient CO2. Assimilation/incident irradiance (All) curves of leaves at different canopy positions were used to estimate leaf quantum yields (4>L), and All curves of canopies at late development stages were used to estimate 4>c. Elevated CO2 enhanced 4>L by 24%. There was little difference in 4>L at different canopy positions, averaging 0.0542 at ambient CO2 and 0.0671 at elevated CO2. Canopy quantum yield (4>c) was higher by 32% at elevated than ambient CO2. It increased with canopy development and was strongly correlated with leaf area index (LAI) by 4>c = 0.0094 LAI/(0.0829 + 0.1137 LAI) at ambient CO2 and 4>c = 0.01382 LAI/(0.1129 + 0.1224 LAI) at elevated CO2. In addition, the curvilinear relationship between radiation and canopy carbon fluxes suggests that canopy radiation use efficiency (CRUE) varied with radiation availability. The variability in 4>c and CRUE with canopy development and light levels warrants further research on the notion drawn from earlier work that CRUE in non-stressed conditions is relatively constant. @2000 Elsevier Science B .V; All rights reserved.