Abstract
Forest ecosystems are important sinks for rising concentrations of atmospheric CO2. In a previous data synthesis of four forest FACE experiments (1), forest net primary production (NPP) increased by 23 � 2% when the forests were grown under atmospheric concentrations of CO2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, more N must be taken up from the soil and/or the N already assimilated by trees must be used more efficiently to support high rates of forest productivity under elevated CO2. Biogeochemical models predict that increases in forest NPP under elevated CO2 in N-limited ecosystems result in a significant increase in N-use efficiency (NUE), and that additional uptake of N by trees under elevated CO2 is only possible in ecosystems where N is not limiting. Here, experimental evidence demonstrates that patterns of N uptake and NUE under elevated CO2 differed from that predicted by biogeochemical models. The uptake of N increased under elevated CO2 at the Rhinelander, Duke and Oak Ridge National Laboratory (ORNL) FACE sites, yet fertilization studies at the Duke and ORNL FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, NUE increased under elevated CO2 only at the POP-EUROFACE site where fertilization studies showed that N was not limiting to tree growth. In reviewing data from the forest FACE experiments, we suggest that some combination of increasing fine root production, increased rates of soil organic matter (SOM) decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO2 at the forest FACE sites. To accurately forecast the response of forest ecosystems to rising concentrations of atmospheric CO2, biogeochemical models must be reformulated to allow C transfers belowground that result in additional N uptake under elevated CO2.
