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

Abstract

Although numerous studies indicate that increasing atmospheric CO2 or temperature stimulate soil CO2 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 CO2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO2 and elevated temperature on these components of soil CO2 efflux in Douglas‐fir terracosms. We measured both soil CO2 efflux rates and the 13C and 18O isotopic compositions of soil CO2 efflux in 12 sun‐lit and environmentally controlled terracosms with 4‐year‐old Douglas fir seedlings and reconstructed forest soils under two CO2 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 CO2 efflux. In most cases, litter decomposition was the dominant component of soil CO2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO2 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 CO2, 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.

13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

“…Tu and Dawson 2005), or partition total soil respiration into root-derived and soil-derived components (e.g. Lin et al 1999) often use the δ 13 C values of organic carbon in total or each ecosystem component (leaf, stem, root, and litter) to estimate the δ 13 C values of respiration generated by total or each component. If the δ carbon cycle processes, such as rhizosphere respiration that includes both respiration of roots and respiration of rhizosphere microorganisms utilizing carbon substrates derived from live roots, is often crucial for the proper use and the reliability of isotope approaches (Werth and Kuzyakov 2010).…”

Measurement of Soil Respiration

“…Gansert 1994;Ryan et al 1996), excavating and directly measuring in a closed chamber in the field (Widén and Majdi 2000), excising roots in the laboratory (Burton et al 1998), trenching (Bowden et al 1993;Fisher and Gosz 1986;Boone et al 1998;Hart and Sollins 1998), girdling of trees (Högberg et al 2001), performing 14 C, 13 C, or 18 O studies (Horwath et al 1994;Swinnen et al 1994;Högberg and Ekblad 1996;Lin et al 1999;Hög-berg et al 2001), inhibiting one respiratory component with specific inhibitors or herbicides (Helal and Sauerbeck 1991;Nakane et al 1996), applying a controlled accumulation technique in the laboratory (Persson et al 1989), and enhancing one component over the other (Bowden et al 1993). Lin et al (1999) used stable isotopes, but their system was strongly influenced by the tank CO 2 with a very different carbon isotopic composition compared to ambient CO 2 . Isotopes have to be measured frequently if partitioning between microbial and root respiration is the objective, since activities change so fast seasonally.…”

Application of stable isotope techniques in studies of carbon and nitrogen biogeochemical cycles of ecosystem

“…Hagedorn et al (2003) estimated the influence of two forest soils (acidic loam and calcareous sand) on the net input of new C into soils under CO 2 ( 13 C-depleted CO 2 ) enrichment. The overall effects of CO 2 enrichment on soil C were small in both soils, and the potential of soils for C sequestration was limited as only a small fraction of new C input into soils would become long-term soil C. Moreover, some researches have successfully applied dual stable isotope ( 13 C and 18 O) technique to partition soil CO 2 efflux into three components (rhizosphere respiration, litter decomposition, and SOM oxidation), and found that the components responded differently to elevated CO 2 and elevated temperature (Lin et al, 1999;2001) …”