PARENT SESSION
Oral Session #53: Elevated CO₂: Communities, ecosystems, soils.
Presiding: G. Lin
Wednesday, August 7. 8:00 AM to 11:30 AM. Coconino Meeting Room, TCC.

Plant community structure and function is influenced by mycorrhizae, CO₂, and nitrogen.

JOHNSON, NANCY^*,1, WOLF, JULIE¹, KOCH, GEORGE¹, ROWLAND, DIANE², ¹ Northern Arizona University, Flagstaff, AZ² USDA-ARS, National Peanut Research Laboratory, Dawson, GA

ABSTRACT- Arbuscular mycorrhizal (AM) fungi are ubiquitous root symbionts that facilitate plant uptake of nutrients and water and impact plant community composition. Anthropogenic enrichment of CO₂ and biologically available N generally increase photosynthesis and are expected to influence mycorrhizal function. We established 48 greenhouse mesocosms (49 cm x 31 cm x 43 cm) to study AM effects on grassland plant community responses to CO₂ and N enrichment. Each mesocosm was planted with 14 plant species (three seedlings each) and maintained with or without AM inoculum, at ambient or elevated CO₂, and at high or low soil N. After one growing season, interactions among mycorrhizae, CO₂, and N significantly influenced plant community structure and function. Mycorrhizae increased species richness at elevated CO₂ but not at ambient CO₂. Aboveground production was highest, and plant diversity was lowest in the high N treatments. Elevated CO₂ increased average soil moisture, and mycorrhizae and N decreased it. Each of the eight treatment combinations generated significantly different plant communities. In general, legumes increased with mycorrhizae, C3 grasses increased with N enrichment, and C3 forbs increased with elevated CO₂. Contrary to expectations, CO₂ enrichment did not consistently increase mycorrhizal responsiveness of mycotrophic plants. Soil fertility and mycorrhizae influence the trajectory of plant community responses to CO₂ enrichment in complex ways; and, it is impossible to assess community scale responses without using community scale experiments.

KEY WORDS: mycorrhizae, community structure, elevated CO₂ , water relations