Biophysical controls on rhizospheric and heterotrophic components of soil respiration in a boreal black spruce stand

We conducted a root-exclusion experiment in a 125-year-old boreal black spruce (Picea mariana (Mill.) BSP) stand in 2004 to quantify the physical and biological controls on temporal dynamics of the rhizospheric (R-r) and heterotrophic (R-h) components of soil respiration (R-s). Annual R-r, R-h and estimated moss respiration were 285, 269 and 57 g C m(-2) year(-1), respectively, which accounted for 47, 44 and 9% of Rs (6119 C m(-2) year(-1)), respectively. A gradual transition from Rh-dominated (winter, spring and fall) to R-r-dominated (summer) respiration was observed during the year. Soil thawing in spring and the subsequent increase in soil water content (theta) induced a small and sustained increase in R-h but had no effect on R-r. During the remainder of the growing season, no effect of theta was observed on either component of R-s. Both components increased exponentially with soil temperature (T-s) during the growing season, but Rr showed greater temperature sensitivity than R-h (Q(10) of 4.0 and 3.0, respectively). Temperature-normalized variations in R-r were highly correlated with eddy covariance estimates of gross ecosystem photosynthesis, and the correlation was greatest when R-r was lagged by 24 days. Within diurnal cycles, variations in T-s were highly coupled to variations in R-h but were-significantly decoupled from R-r. The patterns observed at both time scales strongly suggest that the flow of photosynthates to the rhizosphere is a key driver of below-ground respiration processes but that photosynthate supply may control these processes in several ways.