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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  R  s  (611 g C m −2  year −1 ), respectively. A gradual transition from  R  h -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 (θ) 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 θ was observed on either component of  R  s . Both components increased exponentially with soil temperature ( T  s ) during the growing season, but  R  r  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 belowground respiration processes but that photosynthate supply may control these processes in several ways.

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