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