SOIL CARBON-DIOXIDE FLUXES IN NATURAL AND CO2-ENRICHED SYSTEMS

Carbon dioxide fluxes between the soil and atmosphere were determined on the 1991 free-air carbon dioxide enrichment (FACE) experiment at th e Maricopa Agricultural Center, Maricopa, Arizona. The study was condu cted on drip-irrigated cotton in conjunction with other physical and p hysiological measurements. Fluxes were measured with a 1.6 l closed-ch amber static sampling system. The main treatment for the open-air rele ase study had two levels of CO2-ambient air with CO2 concentration of approximately 370 mumol mol-1 (control) and CO2-enriched air with CO2 concentration of approximately 550 mumol mol-1 (FACE). The enrichment was made over the daylight hours (05:00-19:00 h). Two quantities of wa ter application, 'wet' (1050 mm) and 'dry' (790 mm), were superimposed on the two CO2 levels. The observed soil CO2 fluxes ranged from 2 to 8 mumol m-2 s-1 over the cultivation period. The CO2 fluxes were signi ficantly higher in the FACE than in the control plots, and also higher for the wet than for the dry irrigation level. In addition, an intera ction between CO2 and water levels was present. The CO2 enrichment eff ect on soil CO2 flux remained for approximately 4 weeks after the enri chment was ended. A detailed study on the procedure for determining fl ux indicated that some of the random and inconsistent flux values obse rved in the field could be attributed to a high CO2 concentration pres ent in the first of the two gas samples taken to estimate flux.