Effects of elevated CO/sub 2/ and nutrient stress on nitrogen and phosphorus uptake and use efficiencies

Earlier they reported that non-nodulating Lee soybeans growing in complete nutrient solution showed decreased nutrient uptake efficiency (mg N or P/g root) at early pod fill when exposed to elevated CO/sub 2/. In order to look at CO/sub 2//nutrient interactions over time, plants were grown in growth chambers with nutrient solutions containing 10 mM N/1 mM P (controls) or 10 mM N/0.1 mM P (low P) or 0.5 mM N/1 mMP (low N) and exposed to either 350 or 700 ..mu..L/L CO/sub 2/. Uptake efficiencies for N and P of the control plants showed a pronounced positive response to high CO/sub 2/ in early growth stages, which diminished with time and then became negative. This trend was also present for the low N and low P plants. Nutrient use efficiency, however, was increased by day 7 (first harvest) by high CO/sub 2/ and the effect was sustained through the vegetative period. The N and P stresses produced contrasting responses: low P availability caused an increase in P use efficiency, whereas low N caused a decrease in N use efficiency. These effects resulted from P uptake beyond the requirement for growth, a pattern not observed for N uptake.