Deconvolution of Soil CO2 Efflux from Root, Litter, and SOM Components in a Ponderosa Pine Mesocosm Experiment Exposed to Elevated CO2 and O3

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 estimating 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 used 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 a `closed' chamber Ponderosa pine (Pinus ponderosa, Doug. Ex Laws.) mesocosm experiment exposed to elevated CO₂ and ozone. This approach uses the δ ¹³C and δ ¹⁸O 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. To enhance our ability to determine the relative contribution of the different sources: 1) Keeling plots were used to measure δ ¹³C and δ ¹⁸O signatures of surface CO₂ efflux, 2) mininert vials were used to measure signatures of root, soil, and litter respiration, and 3) the biomass-, volume- and respiration- weighted mean δ ¹⁸O signatures were calculated for roots versus soils across the evaporative gradient. Our results indicate that root and SOM respiration made up the bulk of CO₂ flux, root respiration was higher under elevated CO₂, and there was no effect of elevated ozone. Future experiments will determine the potential for using the dual isotope, three equation mixing model approach to determine the relative importance of root, litter, and SOM respiration under ambient CO₂ conditions.