PARENT SESSION
Oral Session #58: Elevated CO₂. Presiding: A. Finzi.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Madison Ballroom C.

Indications of increasing soil C storage under elevated CO₂ and O₃.

HOLMES, WILLIAM¹, ZAK, DONALD¹, ¹

ABSTRACT- Enhanced plant growth under elevated CO₂ could lead to increased storage of C in terrestrial ecosystems as CO₂ accumulates in the Earth's atmosphere. This effect may be dampened by elevated atmospheric O₃, which decreases photosynthesis and plant growth. Soils are substantial pools of C in most terrestrial ecosystems, but the degree of variability in soil C precludes detection of short-term changes in soil C storage. We used a ¹⁵N tracer technique to follow the flow of N from soil solution into microbial biomass and soil organic matter as a tool to quantify changes in soil C storage. We collected soil beneath Populus tremuloides, Betula papyrifera, and Acer saccharum growing under experimental atmospheric CO₂ and O₃ treatments in a FACE experiment in northern Wisconsin. Ozone had no effect on ¹⁵N recovery in microbial biomass or soil organic matter at ambient CO₂; O₃ effects at elevated CO₂ varied by species. Elevated CO₂ significantly increased ¹⁵N recovery in microbial biomass and soil organic matter. Recovery of ¹⁵N in these pools was significantly greater under Populus tremuloides and Betula papyrifera than under Acer saccharum. These results indicate greater amounts of N were incorporated into soil organic matter under elevated CO₂ and this effect was species specific. Because soil C:N did not differ among treatments, our results suggest that greater plant growth under elevated CO₂ has increased rates of soil organic matter formation.

KEY WORDS: CO₂, O₃, C, soil