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.