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Seasonal Carbon and Nitrogen Accumulation in Relation to Net Carbon Dioxide Exchange in a Carbon Dioxide-Enriched Soybean Canopy

Crop Simulation Res. Unit, Mississippi State, MS 39762

B. Acock^* and M. C. Acock

USDA-ARS-NRI, Systems Res. Lab., BARC-West, Beltsville, MD 20705

^* *Corresponding author

Crop modelers routinely equate net CO₂ exchange (CE) in crop canopies with biomass to simulate crop growth and productivity. This study was initiated to validate this relationship experimentally by monitoring CE during a whole-season C0₂-enrichment study on soybean [Glycine max (L.) Merr. cv. Forrest]. Dry weights of soybhean grown in sunlit plant growth chambers in C0₂ concentrations ([CO²]) of 330,450,600, or 800 µL L^– were sampled at 25 d after emergence (DAE) and after physiological maturity. Photosynthetic nates (P) and respiration rates were calculated from CE rates measured at 0.25-h intervals, day and night, throughout an entire season. The accuracy of CE measurements was tested by plotting gross P against [CO²] at 28,54, and 80 DAE (days when light flux density was at least 1300 µmol photons m^–2 s^–1). Gross P had the expected hyperbolic dependence on [CO²] with all estimated coefficients of determination >0.93. The net CO² required for producing various plant parts was calculated from measurements of dry weight and N content and from assumptions about carbohydrate, oil, mineral, and li,gnin content. The amount of C required to fix 1.0 g of N symbiotically has been reported to be anywhere from 2.5 to 19.4 g. In this sludy the relationship between CO² fixation and biomass was closest when calculations were based on the theoretical value of 2.0 g of C for each gram of N reduced for all [CO²] treatments except 800 µL L^–1, where a value of 4.0 g of C per gram of N fitted the data better.

Received for publication September 14, 1987.

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