PARENT SESSION
Poster Session # 13: Biogeochemistry, Photosynthesis, and Respiration.

Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Responses of autotrophic vs. heterotrophic soil respiration to elevated CO₂: Implications for soil C storage.

Taneva, Lina ¹, Matamala, Roser², Schlesinger, William ³, Gonzalez-Meler, Miquel¹, ¹ University of Illinois at Chicago, Chicago, IL² Argonne National Laboratory, Argonne, IL³ Duke University, Durham, NC

ABSTRACT- Soils are the largest active terrestrial carbon pool, with 2.5 times more C in the top meter of soil than is found in terrestrial vegetation. Through the process of soil respiration (R_S), soils contribute to an annual flux of CO₂ to the atmosphere that is 10 times greater than that of fossil fuel combustion. Because of the size of this flux, even small changes in the rate of R_S could have significant impacts on atmospheric CO₂ concentrations. Increasing evidence indicates that rising atmospheric CO₂ enhances carbon uptake in most ecosystems, highlighting terrestrial carbon sinks as an important factor in mitigating greenhouse gas emissions. However, soils have the potential to store part of the anthropogenic CO₂ only if a substantial proportion of the additional C fixed by plants grown under elevated CO₂ enters soil C pools that turn over slowly. If the additional C is allocated to relatively labile soil C pools, little net C storage may occur. We used stable isotopes to study the response of R_S and its components to elevated CO₂ in an intact loblolly pine-dominated forest in North Carolina under FACE. The depleted ¹³C signature of the fumigation gas is reflected in new photosynthetic tissue and can be followed into the different soil carbon pools. The ¹³C of soil-respired CO₂ can be used to separate R_S into its autotrophic and heterotrophic components, and its ¹⁸O can further partition heterotrophic respiration. Our data indicate that autotrophic respiration rises under elevated CO₂, suggesting that NEP in this forest may be limited at high CO₂ by increased root and rhizosphere activity.

Key words: elevated CO₂, C storage, soil respiration, stable isotopes