Effect of nitrogen and phosphorus availability on the growth response ofEucalyptus grandisto high CO₂

The response of Eucalyptus grandis seedlings to elevated atmospheric CO 2 concentrations was examined by growing seedlings at either 340 or 660 n mol CO 2 mol ‐1 for 6 weeks. Graded increments of phosphorus and nitrogen fertilizers were added to a soil deficient in these nutrients to establish if the growth response to increasing nutrient availability was affected by CO 2 concentration. At 660 μmol CO 2 mol ‐1 , seedling dry weight was up to five times greater than at 340 μmol CO 2 mol ‐1 . The absolute response was largest when both nitrogen and phosphorus availability was high but the relative increase in dry weight was greatest at low phosphorus availability. At 340 μmol CO 2 mol ‐1 and high nitrogen availability, growth was stimulated by addition of phosphorus up to 76 mg kg 1 soil. Further additions of phosphorus had little effect. However, at 660 μmol CO 2 mol ‐1 , growth only began to plateau at a phosphorus addition rate of 920mg kg ‐1 soil. At 340 μmol CO 2 mol ‐1 and high phosphorus availability, increasing nitrogen from 40 to 160mg kg ‐1 soil had little effect on plant growth. At high CO 2 , growth reached a maximum at between 80 and 160mg nitrogen kg ‐1 soil. Total uptake of phosphorus was greater at high CO 2 concentration at all fertilizer addition rates, but nitrogen uptake was either lower or unchanged at high CO 2 concentration except at the highest nitrogen fertilizer rate. The shoot to root ratio was increased by CO 2 enrichment, primarily because the specific leaf weight was greater. The nitrogen and phosphorus concentration in the foliage was lower at elevated CO 2 concentration partly because of the higher specific leaf weight. These results indicate that critical foliar concentrations currently used to define nutritional status and fertilizer management may need to be reassessed as the atmospheric CO 2 concentration rises.