Interpreting the temperature sensitivity of soil CO2 efflux from forest soils

The Boreal Ecosystem Research and Monitoring Sites (BERMS) science team (now part of the Fluxnet Canada Research Network) has been making long-term measurements of soil CO₂ efflux (R_s) in a mature boreal aspen forest since 2001 to better understand the biophysical factors controlling the temporal variability of R_s. In this study, we used the continuous half-hourly measurements of R_s made with an automated chamber system to conduct an in-depth analysis of the dependence of R_s on soil temperature (T_s) at different time scales.

The amount of CO₂ respired from the soil annually was approximately 945, 785 and 920 g C m^-2 for 2001, 2002 and 2003, respectively. Although T_s exerted a significant control on the spring recovery of R_s, most of the annual variability in total R_s was attributed to variations in volumetric soil water content (θ). At the seasonal time scale, R_s increased exponentially with T_s. In 2001, the soil CO₂ efflux at 10^oC (R₁₀) and temperature sensitivity of R_s (Q₁₀) estimated from T_s at 2 cm were 3.9 μmol m^-2 s^-1 and 3.4, respectively. A residual analysis revealed that a strong reduction in R_s occurred when θ in the root zone dropped below 0.10 m³ m^-3. Separating the growing season R_s data set between high and low θ (based on the above threshold value) showed that both the R₁₀ (4.4 and 2.8, respectively) and Q₁₀ (3.1 and 2.5, respectively) were reduced during periods of low θ. To avoid the confounding effects of long-term changes in θ, T_s and phenology, values of R₁₀ and Q₁₀ were calculated daily during the growing season of 2001 using nighttime datasets of R_s and T_s at 2 cm. These daily Q₁₀ values slowly decreased from 3.1 in May to 1.9 in September. In contrast, the daily R₁₀ values closely followed the seasonal patterns of T_s and θ. This suggests that the biomass of biologically active soil, rather than its temperature sensitivity, responded to changes in environmental forcing. The results also showed that total CO₂ production occurred mostly in the shallow soil layers (above 5 cm) during spring. The gradual warming deep in the soil and the low θ near the soil surface late in the growing season increased the relative contribution of deep soil layers to total CO₂ production.