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

Continuous half-hourly measurements of soil CO2 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 (theta). Daily mean R-s varied from a minimum of 0.1 mu mol m(-2) s(-1) in February to a maximum of 9.2 mu mol 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 degrees C (R-s10) and the temperature sensitivity of R-s (Q(10Rs)) calculated at the seasonal time scale were 3.8 mu mol m(-2) s(-1) and 3.8, respectively. Temperature normalization of daily mean R-s (RsN) revealed that theta in the 0-15 cm soil layer was the secondary variable accounting for the temporal variation of R-s during the growing season. Daily RsN showed two distinctive phases with respect to soil water field capacity in the 0-15 cm layer (theta(fc), similar to 0.30 m(3) m(-3)): (1) RsN was strongly reduced when theta decreased below theta(fc), which reflected a reduction in microbial decomposition, and (2) R-sN slightly decreased when theta increased above theta(fc), which reflected a restriction of CO2 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 mu mol m(-2) s(-1) and 5.5 to 1.3, respectively. On average, daily nighttime R-s10 (4.5 mu mol 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 theta in the 0-15 cm soil layer, with a tendency of low R-s10 and Q(10Rs) values at low theta. Overall, the use of seasonal R-s10 and Q(10Rs) parameters led to an overestimation of daily ranges of half-hourly R-s (Delta R-s) during drought conditions, which supported findings that the short-term temperature sensitivity of R-s was lower during periods of low theta. The use of daily nighttime R-s10 and Q(10Rs) parameters greatly helped at simulating Delta R-s during these periods but did not improve the estimation of half-hourly R-s throughout the year as it could not account for the diurnal hysteresis effect. (c) 2006 Elsevier B.V. All rights reserved.