PARENT SESSION
Poster Session 32: Biogeochemistry
Thursday, August 11, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Will nitrogen limitation preclude a sustained productivity response of a deciduous forest to elevated concentrations of atmospheric CO₂?

Iversen, Colleen^*,1, Norby, Richard², Gunderson, Carla², ¹ University of Tennessee, Knoxville, Tennessee² Oak Ridge National Laboratory, Oak Ridge, Tennessee

ABSTRACT- Forest response to CO₂ enrichment of the atmosphere has long been thought to be moderated by soil nitrogen (N) availability. Increases in forest productivity and demand for N in response to CO₂-enrichment have been predicted to exacerbate any existing N limitation within the ecosystem, thereby creating a negative feedback. Elevated [CO₂] is predicted to decrease the available N in an ecosystem via N allocation to long-lived biomass pools, or immobilization in increased soil carbon stocks. Existing N-limitation, in concert with these processes, may lead to an inevitable negative feedback to ecosystem response to elevated [CO₂]. N-limitation is generally measured as a positive productivity response to fertilization. We fertilized a sweetgum stand adjacent to the Oak Ridge National Laboratory (ORNL) Free-Air Carbon Enrichment (FACE) experiment with a one-time application of 200 kg ha^-1 N as urea to examine whether the sweetgum trees in the ORNL research park are limited by the availability of N. Fertilization increased green-leaf N concentration ([N]) an average of 18% throughout the canopy, and resulted in higher light-saturated photosynthetic rates, significantly so at the top of the canopy. Leaf [N] in the fertilized plots exceeded the critical value for 90% of potential stem production, and fertilization increased stem production (basal area increment) by an average of 30%. Green-leaf [N] in the FACE stand does not exceed this threshold, and stem productivity has shown a limited response to elevated [CO₂]. Leaf litter [N] tracked green-leaf [N], and litterfall mass increased with fertilization. We conclude that this sweetgum stand is limited by the availability of N, and it is likely that the sweetgum trees in the adjacent FACE experiment are also limited by N. Existing soil N-limitation is predicted to have consequences for sustained forest productivity, carbon allocation, and carbon storage in response to elevated [CO₂]. However, compensatory ecosystem responses, including changing C: N ratios, increased fine-root proliferation and faster N cycling may alleviate the pressure of N-limitation.

Key words: nitrogen limitation, elevated [CO₂], sweetgum, fertilization