Search

Soil nitrogen can alter storage and remobilization of carbon and nitrogen in forest trees and affect growth responses to elevated carbon dioxide concentration ([CO2]). We investigated these effects in oak saplings (Quercus robur L.) exposed for two years to ambient or twice ambient [CO2] in combination with low- (LN, 0.6 mmol N l−1) or high-nitrogen (HN, 6.1 mmol N l−1) fertilization. Autumn N retranslocation efficiency from senescing leaves was less in HN saplings than in LN saplings, but about 15% of sapling N was lost to the litter. During the dormant season, nonstructural carbohydrates made up 20 to 30% of the dry mass of perennial organs. Starch was stored mainly in large roots where it represented 35–46% of dry mass. Accumulation of starch increased in large roots in response to LN but was unaffected by elevated [CO2]. The HN treatment resulted in high concentrations of N-soluble compounds, and this effect was reduced by elevated [CO2], which decreased soluble protein N (−17%) and amino acid N (−37%) concentrations in the HN saplings. Carbon and N reserves were labeled with 13C and 15N, respectively, at the end of the first year. In the second year, about 20% of labeled C and 50% of labeled N was remobilized for spring growth in all treatments. At the end of leaf expansion, 50–60% of C in HN saplings originated from assimilation versus only 10–20% in LN saplings. In HN saplings only, N uptake occurred, and some newly assimilated N was allocated to new shoots. Through effects on the C and N content of perennial organs, elevated [CO2] and HN increased remobilization capacity, thereby supporting multiple shoot flushes, which increased leaf area and subsequent C acquisition in a positive feedback loop. [ABSTRACT FROM AUTHOR]