Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland

Abstract

Increasing atmospheric carbon dioxide (CO2 ) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO2 will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO2 from root and rhizomicrobial respiration in intact plant-soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca. 0.4 kg C m(-2) ; Q10 = 3.5), whereas mineralisation of older SOC without plants increased with a Q10 of only 1.7 when subject to increases in ambient soil temperature. Subsequent (14) C dating of respired CO2 indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca. 8 years or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils but also suggest that the dual biological and physical effects of CO2 on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C.

Keywords: SOM; acclimation; carbon cycle; climate change; mineralisation; priming; soil organic matter; soil respiration.

Figure 1

Soil temperature and belowground respiration in the field experiment with ¹⁴C content of CO₂ respired in field and laboratory. Soil temperature and belowground respiration for control and warmed swards are shown in the upper and middle panels, respectively. Values for the ¹⁴C content of respired CO₂ are shown on the lower panel. All values are mean ± SEM; n = 3.

Figure 2

Response of belowground respiration to temperature in plots with and without plants. Values are individual measurements in the field for the entire 80 weeks of the experiment. Data from both warmed and control treatments of swards are included. Thus, seasonal changes in belowground respiration driven by photosynthesis are included where plants were present (open circles). The fitted line is: y = 0.0412e^(0.0535x); Q₁₀ = e^{(0.0535 × 10)}; r² = 0.421; n = 58 for soil without plants (filled circles) and y = 0.0573e^(0.1524x); Q₁₀ = e^{(0.1524 × 10)}; r² = 0.831; n = 252 for soil with plants.

Figure 3

Seasonal variation in soil temperature and respiration in soils without plants. Values are mean ± SEM; n = 3.