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Carbon Dioxide and the Photosynthesis of Fields Crops: A Metered Carbon Dioxide Release in Cotton Under Field Conditions¹

The benefits of increased carbon dioxide concentration in enclosed plant environments are well known, but there has been only limited experimentation with increasing carbon dioxide on a field scale. The purpose of this study was to artificially enrich a cotton (Gossypium hirsutum L.) crop with carbon dioxide using a known release rate and to determine the variation in canopy carbon dioxide concentration with respect to meteorological conditions.

Carbon dioxide was metered into the crop canopy while concentrations of carbon dioxide and other meteorological parameters in and above the canopy were measured. The crop had a leaf area index of 2.34, and the crop surface was aerodynamically rough. Turbulence caused considerable short-term fluctuation in concentrations, but concentrations of 450 to 500 ppm at three-fourths plant height were maintained with a release rate of 222.6 kg/ha/hour (198.6 Ib/acre/hour). Photosynthetic recovery of applied carbon dioxide was calculated from data obtained in the crop during its release and from data describing its behavior in a semiclosed plexiglass chamber just before the carbon dioxide release. Calculated recoveries ranged from 7 to 33% over a range of solar radiation levels from 205 to 1,095 W/m². The release produced unexpectedly high carbon dioxide concentrations 4 m above the soil surface (due to vertical movement), and these concentrations varied little with windspeeds up to 200 cm/sec at the 150-cm level. The daily net photosynthate production increased by an estimated 35%.

Key Words: Microclimate • CO₂ • fertilization • Turbulent transfer

² Research Soil Scienctists, USDA, Watkinsville, Ga. 30677; State College, Miss. 39762; Watkinsville, Ga.; and Raleigh, N.C. 27607, respectively.

Received for publication December 31, 1971.