Carbon-nitrogen interactions in CO2-enriched white oak: physiological and long-term perspectives

The responses of forest trees to atmospheric CO2 enrichment will depend in part on carbon-nutrient linkages. Insights into the possible long-term ecological consequences of CO2 enrichment can be gained from studying physiological responses in short-term experiments. One-year-old white oak (Quercus alba L.) seedlings were grown in unfertilized forest soil for 40 weeks in controlled-environment chambers with ambient (362 mu l l(-1)) or elevated (690 mu l l(-1)) CO2. As previously reported, seedling dry weight was 85% greater in the elevated CO2 environment, despite severe nitrogen deficiency in all seedlings. The increase in growth occurred without a concomitant increase in nitrogen uptake, indicating an increase in nitrogen-use efficiency in elevated CO2. The weight of new buds was greater in elevated CO2, suggesting that shoot growth in the next year would have been enhanced relative to that of seedlings in ambient CO2. However, there was less translocatable nitrogen in perennial woody tissue in elevated CO2; thus, further increases in nitrogen-use efficiency may not be possible. The leaves that abscised from seedlings in elevated CO2 contained higher amounts of soluble sugars and tannin and a lower amount of lignin compared with amounts in abscised leaves in ambient CO2. Based on lignin:N and lignin:P ratios, the rates of litter decomposition might not be greatly affected by CO2 enrichment, but the total amount of nitrogen returned to soil would be lower in elevated CO2,