Interaction Effect between Elevated CO₂ and Fertilization on Biomass, Gas Exchange and C/N Ratio of European Beech (<em>Fagus sylvatica</em> L.)

The effects of elevated CO<inf>2</inf> and interaction effects between elevated CO<inf>2</inf> and nutrient supplies on growth and the C/N ratio of European beech (Fagus sylvatica L.) saplings were studied. One-year-old beech saplings were grown in a greenhouse at ambient (385 ppm) and elevated CO<inf>2</inf> (770 ppm/950 ppm), with or without fertilization for two growing seasons. In this study, emphasis is placed on the combined fertilization including phosphorus, potassium and nitrogen with two level of elevated CO<inf>2</inf>. The fertilized plants grown under elevated CO<inf>2</inf> had the highest net leaf photosynthesis rate (Ac). The saplings grown under elevated CO<inf>2</inf> had a significantly lower stomatal conductance (gs) than saplings grown under ambient air. No interaction effect was found between elevated CO<inf>2</inf> and fertilization on Ac. A interaction effect between CO<inf>2</inf> and fertilization, as well as between date and fertilization and between date and CO<inf>2</inf> was detected on gs. Leaf chlorophyll content index (CCI) and leaf nitrogen content were strongly positively correlated to each other and both of them decreased under elevated CO<inf>2</inf>. At the end of both growing seasons, stem dry weight was greater under elevated CO<inf>2</inf> and root dry weight was not affected by different treatments. No interaction effect was detected between elevated CO<inf>2</inf> and nutrient supplies on the dry weight of different plant tissues (stems and roots). However, elevated CO<inf>2</inf> caused a significant decrease in the nitrogen content of plant tissues. Nitrogen reduction in the leaves under elevated CO<inf>2</inf> was about 10% and distinctly higher than in the stem and root. The interaction effect of elevated CO<inf>2</inf> and fertilization on C/N ratio in plants tissues was significant. The results led to the conclusion that photosynthesis and the C/N ratio increased while stomatal conductance and leaf nitrogen content decreased under elevated CO<inf>2</inf> and nutrient-limited conditions. In general, under nutrient-limited conditions, the plant responses to elevated CO<inf>2</inf> were decreased.