Influence of increased atmospheric CO(2) concentration on quality of plant material and litter decomposition.

Nitrogen (N) and lignin concentrations in plant tissues and litter of plants grown in greenhouses or open-top chambers in elevated atmospheric CO(2) concentration 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 CO(2)-springs with that of the same species growing at a control site. In the short-term studies, elevated CO(2) caused significant decreases in tissue N concentration and the extent of the decrease varied with species. Nitrogen amendment of the soil lessened the CO(2)-enrichment effect. Lignin concentration was modified by elevated CO(2) 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 CO(2)-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 CO(2) and in ambient air indicated that CO(2) enrichment can cause both enhancements and decreases of carbon mineralization. We conclude that (1) long-term responses to elevated CO(2) 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 CO(2) 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.