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.

Evidence of a strong soil organism effect on plant response to CO₂ and nitrogen enrichment.

KOCH, GEORGE^*,1, ROWLAND, DIANE¹, JONES, CHAS¹, JOHNSON, NANCY¹, ¹ Northern Arizona University, Flagstaff, AZ

ABSTRACT- Interactions between plants and soil organisms are influenced by relative carbon and nitrogen (N) limitation. This study compared effects of soil organisms on performance of a mycorrhizal grass (Agropyron repens) and a non-mycorhizal mustard (Berteroa incana), grown at ambient or elevated CO₂, and at high or low soil N. Plants were grown in soil with a full community of organisms including arbuscular mycorrhizal fungi (AMF) or a partial soil community (< 25 microns) consisting of bacteria and microfungi but no AMF. Biomass of both plant species was significantly reduced by the full soil community compared to the partial soil community. Added N significantly increased growth in all treatments, and significantly reduced, but did not eliminate, the growth stunting caused by full soil communities. Total plant N and phosphorus content was significantly lower with full compared to partial soil communities. In the mycorrhizal grass, elevated CO₂ increased leaf specific photosynthesis, yet had minor effects on the stunting caused by soil organisms. Soil respiration per unit root mass, and the ratio of soil respiration to photosynthesis, were significantly increased by the full soil community. Our findings suggest that soil organisms affect plant growth through competition for soil N and C sink activity, and that mycotrophic status of plants influences the relative importance of these two mechanisms. These results stress the importance of including soil organisms in CO₂ and N enrichment experiments.

KEY WORDS: elevated CO2 , mycorrhizae, nitrogen, respiration