Effect of soil water stress on soil respiration and its temperature sensitivity in an 18‐year‐old temperate Douglas‐fir stand

We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO 2 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 ( θ ) > 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 θ , 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 θ 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. d R S /d T S ) increased with θ up to 0.15 m 3 m −3 , above which it slowly declined. The value of d R S /d T S was nearly 0 for θ < 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 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 T S , which is associated with low θ . We found that an exponential (van't Hoff type) model with Q 10 and R 10 dependent on only θ 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 θ 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.