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 the Maricopa Agricultural Center, Maricopa, Arizona. The study was conducted on drip-irrigated cotton in conjunction with other physical and physiological measurements. Fluxes were measured with a 1.6 1 closedchamber static sampling system. The main treatment for the open-air release study had two levels of CO2--ambient air with CO2 concentration of approximately 370 #mol mo1-1 (control) and CO2-enriched air with CO2 concentration of approximately 550 #tool tool -l (FACE). The enrichment was made over the daylight hours O5:00-19:00 h). Two quantities of water application, 'wet' (1050 mm) and 'dry' (790 mm), were superimposed on the two CO 2 levels. The observed soil CO2 fluxes ranged from 2 to 8 #mol m -2 s -1 over the cultivation period. The CO2 fluxes were significantly higher in the FACE than in the control plots, and also higher for the wet than for the dry irrigation level. In addition, an interaction between CO2 and water levels was present. The CO2 enrichment effect on soil CO2 flux remained for approximately 4 weeks after the enrichment was ended. A detailed study on the procedure for determining flux indicated that some of the random and inconsistent flux values observed in the field could be attributed to a high CO2 concentration present in the first of the two gas samples taken to estimate flux.