NITROGEN AND PHOSPHORUS NUTRITIONAL INTERACTIONS IN A CO2 ENRICHED ENVIRONMENT

Nonnodulated soybean plants (Glycine max. [L.] Merr. 'Lee') were supplied with nutrient solutions containing growth limiting concentrations of N or P to examine effects on N- and P-uptake efficiencies (mg nutrient accumulated/gdw root) and utilization efficiencies in dry matter production (gdw2/mg nutrient). Nutritional treatments were imposed in aerial environments containing either 350 or 700-mu-L/L atmospheric CO2 to determine whether the nutrient interactions were modified when growth rates were altered. Nutrient-stress treatments decreased growth and N- and P-uptake and utilization efficiencies at 27 days after transplanting (DAT) and seed yield at maturity (98 DAT). Atmospheric CO2 enrichment increased growth and N- and P-utilization efficiencies at 27 DAT and seed yield in all nutritional treatments and did not affect N- and P-uptake efficiencies at 27 DAT. Parameter responses to nutrient stress at 27 DAT were not altered by atmospheric CO2 enrichment and vice versa. Nutrient-stress treatments lowered the relative seed yield response to atmospheric CO2 enrichment. Decreased total-N uptake by P-stressed plants was associated with both decreased root growth and N-uptake efficiency of the roots. Nitrogen-utilization efficiency was also decreased by P-stress. This response was associated with decreased plant growth as total-N uptake and plant growth were decreased to the same extent by P stress resulting in unaltered tissue N concentrations. In contrast, decreased total P-uptake by N-stressed plants was associated with a restriction in root growth as P-uptake efficiency of the roots was unaltered. This response was coupled with an increased root-to-shoot dry weight ratio; thus shoot and wholeplant growth were decreased to a much greater extent than total-P uptake which resulted in elevated P concentrations in the tissue. Therefore, P-utilization efficiency was markedly reduced by N stress.