PARENT SESSION
Contributed Oral Session 41: Biogeochemistry: Climate Effects and Physiology
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 515 A, Level 5, Palais des congrès de Montréal

Soil respiration under elevated tropospheric carbon dioxide and ozone in northern hardwood forests.

Pregitzer, Kurt¹, Loya, Wendy¹, ¹ Ecosystem Science Center, Houghton, MI, USA

ABSTRACT- Human activity has increased the concentration of carbon dioxide (CO₂) and ozone (O₃) in the Earth's troposphere, and each of these trace gases has the potential to modify photosynthesis and plant growth across broad geographic regions, albeit in diametrically opposite ways. At the present time, we do not have the ability to predict the interactive effects of atmospheric CO₂ and O₃ on soil respiration, one of the two largest fluxes of carbon in forest ecosystems. One might expect soil respiration to track plant growth and this has been the case for a variety of single factor experiments, where soil respiration is routinely stimulated by exposure to elevated CO₂. We have studied soil respiration for seven years in a factorial CO₂ by O₃ FACE field experiment in aspen and mixed aspen-birch stands near Rhinelander, Wisconsin, USA. In the first 5 years of the experiment, elevated CO₂ significantly increased soil respiration, elevated O₃ decreased soil respiration, and CO₂ x O₃ was not significantly different from the control. However, in the last two field seasons (2003-2004), rates of soil respiration in the CO₂ x O₃ treatment were significantly greater than those at elevated CO₂. Over seven years of treatment exposure, why have root and microbial respiration exhibited a transient response to the treatments? Why does soil respiration no longer simply track changes in ecosystem NPP? Measurements of the isotopic signature of soil CO₂ and soil carbon, and rates of root production and mortality, provide interesting insight to these questions.

Key words: FACE, global change, stable isotope, carbon flux