ELEVATED ATMOSPHERIC CARBON-DIOXIDE EFFECTS ON SORGHUM AND SOYBEAN NUTRIENT STATUS

Increasing atmospheric carbon dioxide (CO2) concentration could have s ignificant implications on technologies for managing plant nutrition t o sustain crop productivity in the future. Soybean (Glycine max [L.] M err.) (C3 species) and grain sorghum (Sorghum bicolor [L.] Moench) (C4 species) were grown in a replicated split-plot design using open-top field chambers under ambient (357 mu mol/mol) and elevated (705 mu mol /mol) atmospheric CO2. At anthesis, leaf disks were taken from upper m ature leaves of soybean and from the third leaf below the head of sorg hum for analysis of plant nutrients. Leaf greenness was measured with a Minolta SPAD-502 chlorophyll meter. Concentrations of chlorophylls a and b and specific leaf weight were also measured. Above-ground dry m atter and seed yield were determined at maturiry. Seed yield of sorghu m increased 17.5% and soybean seed yield increased 34.7% with elevated CO2. There were no differences in extractable chlorophyll concentrati on or chlorophyll meter readings due to CO2 treatment, but meter readi ngs were reduced 6% when sorghum was grown in chambers as compared in the open. Leaf nitrogen (N) concentration of soybean decreased from 54 .5 to 39.1 g/kg at the higher CO2 concentration. Neither the chambers nor CO2 had an effect on concentrations of other plant nutrients in ei ther species. Further work under field conditions is needed to determi ne if current critical values for tissue N in crops, especially C3 cro ps, should be adjusted for future increases in atmospheric CO2 concent ration.