Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO 2 efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration ( R s) on soil temperature ( T s) and water content ( θ). Daily mean R s varied from a minimum of 0.1 μmol m −2 s −1 in February to a maximum of 9.2 μmol m −2 s −1 in mid-July. Daily mean T s at the 2-cm depth was the primary variable accounting for the temporal variation of R s and no differences between Arrhenius and Q 10 response functions were found to describe the seasonal relationship. R s at 10 °C ( R s10) and the temperature sensitivity of R s ( Q 10Rs) calculated at the seasonal time scale were 3.8 μmol m −2 s −1 and 3.8, respectively. Temperature normalization of daily mean R s ( R sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R s during the growing season. Daily R sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer ( θ fc, ∼0.30 m 3 m −3): (1) R sN was strongly reduced when θ decreased below θ fc, which reflected a reduction in microbial decomposition, and (2) R sN slightly decreased when θ increased above θ fc, which reflected a restriction of CO 2 or O 2 transport in the soil profile. Diurnal variations of half-hourly R s were usually out of phase with T s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R s10 and Q 10Rs parameters calculated from half-hourly nighttime measurements of R s and T s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m −2 s −1 and 5.5 to 1.3, respectively. On average, daily nighttime R s10 (4.5 μmol m −2 s −1) and Q 10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R s10 and Q 10Rs values at low θ. Overall, the use of seasonal R s10 and Q 10Rs parameters led to an overestimation of daily ranges of half-hourly R s (Δ R s) during drought conditions, which supported findings that the short-term temperature sensitivity of R s was lower during periods of low θ. The use of daily nighttime R s10 and Q 10Rs parameters