Abstract
N availability may constrain long-term increases in forest productivity and subsequent increases in C storage in response to CO2-fertilization. Fumigation with elevated [CO2] resulted in increased fine-root production in the sweetgum (Liquidambar styraciflua) Free-Air CO2-Enrichment (FACE) experiment at Oak Ridge National Laboratory (ORNL). Fine roots turn over quickly at ORNL FACE and C storage in sweetgum biomass was limited. To examine the premise that increased root production at ORNL FACE was a physiological response to N-limitation, we fertilized a sweetgum plantation adjacent to ORNL FACE on the Oak Ridge National Environmental Research Park (NERP). Annual additions of 200 kg ha-1 of N as urea in 2004 and 2005 increased inorganic soil N availability, which in turn increased stand net primary production (NPP) by approximately 25%. Fertilization increased leaf N concentration and canopy leaf area production, which facilitated a greater than 30% increase in stem production and shifted C partitioning aboveground. We conclude that sweetgum production on the Oak Ridge NERP is limited by soil N availability, and we suggest that N-limitation may have caused increased belowground partitioning in ORNL FACE. Current soil nutrient status and changes in soil N availability mediated by changes in forest C partitioning will shape future forest responses to elevated [CO2].
