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 (
Rs) on soil temperature (
Ts) and water content (
θ). Daily mean
Rs 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
Ts at the 2-cm depth was the primary variable accounting for the temporal variation of
Rs and no differences between Arrhenius and
Q10 response functions were found to describe the seasonal relationship.
Rs at 10 °C (
Rs10) and the temperature sensitivity of
Rs (
Q10Rs) calculated at the seasonal time scale were 3.8 μmol m
−2 s
−1 and 3.8, respectively. Temperature normalization of daily mean
Rs (
RsN) revealed that
θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of
Rs during the growing season. Daily
RsN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (
θfc, 0.30 m
3 m
−3): (1)
RsN was strongly reduced when
θ decreased below
θfc, which reflected a reduction in microbial decomposition, and (2)
RsN slightly decreased when
θ increased above
θfc, which reflected a restriction of CO
2 or O
2 transport in the soil profile.Diurnal variations of half-hourly
Rs were usually out of phase with
Ts at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime
Rs10 and
Q10Rs parameters calculated from half-hourly nighttime measurements of
Rs and
Ts 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
Rs10 (4.5 μmol m
−2 s
−1) and
Q10Rs (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
Rs10 and
Q10Rs values at low
θ. Overall, the use of seasonal
Rs10 and
Q10Rs parameters led to an overestimation of daily ranges of half-hourly
Rs (Δ
Rs) during drought conditions, which supported findings that the short-term temperature sensitivity of
Rs was lower during periods of low
θ. The use of daily nighttime
Rs10 and
Q10Rs parameters greatly helped at simulating Δ
Rs during these periods but did not improve the estimation of half-hourly
Rs throughout the year as it could not account for the diurnal hysteresis effect.
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