We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO2 efflux or soil respiration (R-S) 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 (theta) > 0.11 m(3) m(-3) (corresponding to a soil water matric potential of -2 MPa), R-S was positively correlated to soil temperature at the 2 cm depth (T-S). Below this value of theta, however, R-S was largely decoupled from T-S, and evapotranspiration, ecosystem respiration and gross primary productivity (GPP) began to decrease, dropping to about half of their maximum values when theta reached 0.07 m(3) m(-3). Soil water deficit substantially reduced R-S sensitivity to temperature resulting in a Q(10) significantly < 2. The absolute temperature sensitivity of R-S (i.e. dR(S)/dT(S)) increased with theta up to 0.15 m(3) m(-3), above which it slowly declined. The value of dR(S)/dT(S) was nearly 0 for theta < 0.08 m(3) m(-3), thereby confirming that R-S was largely unaffected by temperature under soil water stress conditions. Despite the possible effects of seasonality of photosynthesis, root activity and litterfall on R-S, the observed decrease in its temperature sensitivity at low theta 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 T-S, which is associated with low theta. We found that an exponential (van't Hoff type) model with Q(10) and R-10 dependent on only theta explained 92% of the variance in half-hourly values of R-S, including the period with soil water stress conditions. We hypothesize that relating Q(10) and R-10 to theta not only accounted for the effects of T-S on R-S 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.
