The objective of this study was to compare the carbon (C) budgets of two similar-aged boreal black spruce (Picea mariana [Mill.] BSP) forest communities: closed-canopy black spruce with feathermoss ground cover (BSFM) on moderately drained soils and open-canopy black spruce with Sphagnum ground cover (BSSP) on poorly drained soils. C content, soil surface carbon dioxide (CO 2 ) flux (R s ), heterotrophic respiration (R H ), net primary production (NPP), and net ecosystem production (NEP) were measured or estimated. The total C content for the two communities (BSFM, 113 Mg C ha -1 ; BSSP, 107 Mg C ha -1 ) did not differ significantly (P = 0.95). However, annual R s was significantly greater (P < 0.0001) for the BSFM (564 g C m -2 y -1 ) than the BSSP (319 g C m -2 y -1 ) community. The greater R H in the BSFM (440 g C m -2 y -1 ) than the BSSP community (264 g C m -2 y -1 ) and the higher peat C content for the BSSP (84 Mg C ha -1 ) than the BSFM (34 Mg C ha -1 ) community provided evidence that the BSSP community had a lower decomposition rate than the BSFM community. NEP was significantly (P = 0.04) more negative for the BSFM community (-222 ± 35 g C m -2 y -1 ) than the BSSP community (-128 ± 14 g C m -2 y -1 ), but both communities were C sources to the atmosphere. The results from this study illustrate the influence of small differences in edaphic conditions on ecosystem C accumulation.