Publications

Nitrogen (N) and lignin concentrations in plant tissues and litter of plants grown in greenhouses or open-top chambers in elevated atmospheric CO₂concentration were compared with those of plants grown in ambient air in short-term studies. We also compared the N concentration of plant material of Quercus ilex L. and Q. pubescens Willd. growing in the vicinity of natural CO₂-springs with that of the same species growing at a control site. In the short-term studies, elevated CO₂ caused significant decreases in tissue N concentration and the extent of the decrease varied with species. Nitrogen amendment of the soil lessened the CO₂-enrichment effect. Lignin concentration was modified by elevated CO₂ and the effect was species specific, but no general positive or negative trend was evident. A comparison of trees growing under natural conditions near a natural CO₂-spring and at a control site revealed no site differences in N concentration of the plant material. A comparison of published results on decomposition rates of litter produced in elevated atmospheric CO₂ and in ambient air indicated that CO₂ enrichment can cause both enhancements and decreases of carbon mineralization. We conclude that (1) long-term responses to elevated CO₂ could differ from the results obtained from short-term studies and that (2) biodiversity could be an important factor altering the sign of the feedback on atmospheric CO₂ concentration. We also discuss the implications of our finding of a long-term, inhibitory effect of the initial N concentration of litter on the decomposition rate of litter and its consequence on ecosystem feedback.