Mass and Energy Exchanges of a Soybean Canopy under Various Environmental Regimes1

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Abstract
The environment of the east central Great Plains of North America can be very extreme during the course of a growing season. A field study was thus conducted during the summer of 1979 to examine the exchange of mass and energy between the atmosphere and a soybean canopy [Glycine max (L.) Merrill] for various environmental conditions experienced in the east central Great Plants of North America. The crop was planted in a Typic Argiudoll (Sharpsburg silty, clay loam) soil. Measurements of mass and energy exchange rates were made using the Bowen‐ratio energy balance technique.Hot, clear days dominated by sensible heat advection limited CO2 exchange but increased latent heat flux. As a result, the CO2‐water flux ratio (CWFR), a measure of water‐use efficiency, was low. Cloudy days suppressed both CO2 and latent heat flux. This effect caused CWFR to be greater. Optimal conditions for photosynthesis and CWFR occurred in the absence of sensible heat advection on clear days with moderate temperatures.Our measurements indicate that a developed soybean canopy (LAI of 4.1) did not become light saturated at photosynthetically active radiation (PAR) levels exceeding 400 Wm−2. Optimal air temperatures for CO2 exchange ranged between 29 and 32 C. Higher temperatures led to a reduction in CO2, exchange. The CWFR was found to be dependent on both net radiation and sensible heat advection.