Components of Ecosystem Respiration in a Forest Exposed to Elevated CO₂

Duke Forest	Oak Ridge Experiment

Terrestrial ecosystems exchange about 120Gt of carbon (C) with the atmosphere, through the processes of photosynthesis (leading to gross primary productivity, GPP) and ecosystem respiration (Re). Increasing evidence indicates that raising atmospheric CO₂ enhances carbon uptake in most ecosystems, however, responses of Re and its components to elevated CO₂ are unclear. Net ecosystem productivity (NEP), is determined as the net balance between GPP and Re and is often a sensitive predictor of functional ecosystem properties. Unfortunately, the extent to which NEP will respond to rising CO₂ concentration is still unresolved due, largely, to our inability to reliably determine Re. Accordingly, the proposed research addresses the following questions:

1. to what extent does Re control sequestration of atmospheric carbon in forested ecosystems in a high CO₂ world?; and
2. what are the components of Re that exert the greatest leverage in determining the direction and magnitude of C sequestration as CO₂ concentration rises?

To accomplish this we propose to use a novel dual isotope approach combined with continuous measurements of soil respiration to estimate the components of Re in an intact loblolly pine-dominated forest exposed to elevated CO₂ using Free-air CO₂ Enrichment (FACE). The approaches used here allow direct estimates of the components of Re that cannot be accurately estimated using traditional methods. We will use the depleted ¹³C signature of the fumigation CO₂ coupled with the ¹⁸O/¹⁶O composition of soil respired CO₂ to distinguish and evaluate the effects of CO₂ on rhizosphere autotrophic respiration and microbial heterotrophic respiration. At ambient conditions, we will compare our annual estimates of the combined autotrophic and heterotrophic respiratory fluxes with those derived from eddy covariance measurements of CO₂ fluxes. Estimates of Re and the resulting NEP will be compared with direct measurements of NEP using changes in biomass and soil C. By using a variety of methods we will be able to quantify how elevated CO₂ affects Re and its components, and the feedbacks of the identified responses on forest NEP, thereby assessing the ability of this forest to sequester atmospheric carbon. Our main collaborators for this project are Bill Schlesinger (Duke University), Roser Matamala (Argonne National Lab), Rich Norby (ORNL) and Yiqi Luo (University of Oklahoma).