The carbon isotopic composition of heterotrophically-respired CO2 flux from soil organic matter

CO2 flux from soil to atmosphere combines CO2 derived from root metabolism (autotrophic respiration) and soil organic matter decomposition (heterotrophic respiration). A major challenge to biogeochemistry is the separation of soil respiration into these components and the evaluation of how each is controlled by environmental conditions and will respond to environmental change. Laboratory studies of the carbon isotopic composition of soil-respired CO2 produced during incubation provides an opportunity to separate heterotrophic respiration and study the controls on its isotopic composition during the course of long-term incubation experiments. We have developed and utilized methods using the carbon isotopic composition of laboratory respired CO2 to determine the relative contribution of different decomposing substrates to heterotrophic CO2 flux. We will present stable and radiocarbon isotope measurements of heterotrophic CO2 respiration from organic matter collected from soils over a wide range of environmental controls, and discuss how various factors may influence the contribution of different components to heterotrophic CO2 respiration. Incubation studies of soil organic matter from the Lower Mississippi Basin, and a moisture gradient in Alaska indicate that heterotrophic respiration progressively consumes older and more humified components of organic matter (at a decreased rate) in the absence of fresh input from biomass. Results of 4-year incubations of soil organic matter from a continental-wide data set from Australia indicate a pronounced difference in turnover time of C3- and C4-derived soil organic matter. These results have significant implications for modeling the soil to atmospheric flux, and responses of the size, isotopic composition and turnover time of the soil organic carbon pool to environmental change.