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REFINING THE DUAL ISOTOPE APPROACH TO DETERMINE FIELD ESTIMATES OF LITTER, ROOT, AND SOM COMPONENTS OF SOIL CO2 EFFLUX
Citation:
Gregg, J W., D L. Phillips, M G. Johnson, C P. Andersen, AND P T. Rygiewicz. REFINING THE DUAL ISOTOPE APPROACH TO DETERMINE FIELD ESTIMATES OF LITTER, ROOT, AND SOM COMPONENTS OF SOIL CO2 EFFLUX. Presented at Linking the Complexity of Forest Canopies to Ecosystem and Landscape Function, IUFRO Canopy Processes, Corvallis, OR, July 16-17, 2001.
Description:
Stable isotopes have become an important tool for determining the relative importance of CO2 sources and sinks contributing to the global carbon budget. Of particular importance is the determination of the terrestrial CO2 flux which is difficult to decipher without determining the relative importance of autotrophic and heterotrophic respiration from below-ground sources. Whereas increased SOM respiration could indicate reduced C storage ultimately creating a stronger terrestrial CO2 source, increased autotrophic respiration could indicate greater NPP and therefore an overall stronger terrestrial sink. Here, we examine the potential for refining the dual isotope, three equation mixing model approach of Lin et al. 1999 to determine the relative importance of root, litter, and SOM respiration in the field. This approach uses the d13C and d18O signatures of surface CO2 efflux and the component litter, root and SOM fluxes to provide a system of three equations to solve for the three unknown source fluxes. Our initial objective was to determine whether it was theoretically possible to determine the relative importance of litter, root and SOM sources to the surface flux in a natural ecosystem where root and SOM d13C signatures differ by only 2?. Calculations with a statistical error propagation model (Phillips and Gregg 2001) indicated that the relative source contributions could be estimated ?8% (s.e.; for n = 10 and source s.d. = 0.25?). We then used a Ponderosa pine mesocosm experiment exposed to ambient and elevated CO2 systems to determine the importance of the d13C signatures derived from tank CO2 to decipher litter, root and SOM contributions to the surface CO2 efflux. To increase the likelihood of determining the relative contribution of the different sources under ambient conditions, we employed an array of techniques to increase the accuracy and precision of d13C and d18O signatures: 1) Keeling plots were used to measure the 13C and 18O signatures of surface CO2 efflux, 2) mininert vials were used to measure the signatures of root, soil, and litter respiration, and 3) the volume- and respiration- weighted mean d18O signatures of roots and soils were used across the evaporative gradient in d18O signatures with depth. Our results indicate that root and SOM respiration made up the bulk of the respired CO2 and that dual isotope, three equation mixing models could be used in natural ecosystems to separate the sources of terrestrial CO2 flux.