We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO2 efflux or soil respiration (RS) in an 18-year-old west-coast temperate Douglas-fir stand that experienced somewhat greater than normal summertime water deficit. For soil water content at the 4 cm depth (θ) >0.11 m3 m−3 (corresponding to a soil water matric potential of −2 MPa), RS was positively correlated to soil temperature at the 2 cm depth (TS). Below this value of θ, however, RS was largely decoupled from TS, and evapotranspiration, ecosystem respiration and gross primary productivity (GPP) began to decrease, dropping to about half of their maximum values when θ reached 0.07 m3 m−3. Soil water deficit substantially reduced RS sensitivity to temperature resulting in a Q10 significantly < 2. The absolute temperature sensitivity of RS (i.e. dRS/dTS) increased with θ up to 0.15 m3 m−3, above which it slowly declined. The value of dRS/dTS was nearly 0 for θ < 0.08 m3 m−3, thereby confirming that RS was largely unaffected by temperature under soil water stress conditions. Despite the possible effects of seasonality of photosynthesis, root activity and litterfall on RS, the observed decrease in its temperature sensitivity at low θ was consistent with the reduction in substrate availability due to a decrease in (a) microbial mobility, and diffusion of substrates and extracellular enzymes, and (b) the fraction of substrate that can react at high TS, which is associated with low θ. We found that an exponential (van’t Hoff type) model with Q10 and R10 dependent on only θ explained 92% of the variance in half-hourly values of RS, including the period with soil water stress conditions. We hypothesize that relating Q10 and R10 to θ not only accounted for the effects of TS on RS and its temperature sensitivity but also accounted for the seasonality of biotic (photosynthesis, root activity, and litterfall) and abiotic (soil moisture and temperature) controls and their interactions.
