Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas‐fir terracosms

Abstract

Although numerous studies indicate that increasing atmospheric CO 2 or temperature stimulate soil CO 2 efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO 2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO 2 and elevated temperature on these components of soil CO 2 efflux in Douglas‐fir terracosms. We measured both soil CO 2 efflux rates and the 13 C and 18 O isotopic compositions of soil CO 2 efflux in 12 sun‐lit and environmentally controlled terracosms with 4‐year‐old Douglas fir seedlings and reconstructed forest soils under two CO 2 concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO 2 efflux. In most cases, litter decomposition was the dominant component of soil CO 2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO 2 concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO 2, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual‐isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.