PARENT SESSION
Symposium 24: Progressive Nitrogen Limitation of Plant and Ecosystem Response to Rising CO₂
Organized by: Y Luo
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Chatham Ballroom C.

Soil Nitrogen Cycling Under Elevated CO₂: A Synthesis of Forest FACE Experiments.

Zak, Donald^*,1, Holmes, William¹, Finzi, Adrien², Norby, Richard³, Schlesinger, William⁴, ¹ University of Michigan, Ann Arbor, Michigan² University of Boston, Boston, Massachusetts³ Oak Ridge National Laboratory, Oak Ridge, Tennessee⁴ Duke University, Durham, North Carolina

ABSTRACT- The extent to which greater NPP will be sustained as the atmospheric CO₂ concentration increases will depend, in part, on the long-term supply of N for plant growth. Over a two-year period, we used common field and laboratory methods to quantify microbial N, gross N mineralization, microbial N immobilization, and specific microbial N immobilization in three free-air CO₂ enrichment experiments (Duke Forest, Oak Ridge, Rhinelander). In these experiments, elevated atmospheric CO₂ has increased the input of above- and belowground litter production, which fuels heterotrophic metabolism in soil. Nonetheless, we found no effect of atmospheric CO₂ concentration on any microbial N cycling pool or process, indicating that greater litter production had not initially altered the microbial supply of N for plant growth. Thus, we have no evidence that changes in plant litter production under elevated CO₂ will initially slow soil N availability and produce a negative feedback on NPP. Understanding the time scale over which greater plant production modifies microbial N demand lies at the heart of our ability to predict long-term changes in soil N availability, and hence whether greater NPP will be sustained in a CO₂-enriched atmosphere.

Key words: atmospheric CO₂, global change, soil nitrogen cycling, negative feedback