Planting fast-growing hybrid poplars on a short rotation on marginal agricultural land in western Canada’s boreal region can provide a reliable fibre source for the forest industry as well as a potential sink for atmospheric carbon (C). In this study, we investigated 1) the impact of afforestation with hybrid poplar (Populus deltoides x Populus x petrowskyana var. Walker) on soil respiration over a period of four years (between 2005 and 2008), and 2) the dynamics of ecosystem C along a chronosequence (5, 8, 14, and 16 years old in 2009) of hybrid poplar plantations, to gain a better understanding of the potential to use such trees to sequester C. Our study showed that planting with hybrid poplars did not have an impact on soil respiration in the first year when compared to the agricultural system, but less CO₂ was respired in the hybrid poplar plantation five years after afforestation.  The changes in respiration were mostly attributed to the specific crop grown in a five-year barley-barley-alfalfa-alfalfa-alfalfa crop rotation in the agricultural system. Data from the chronosequence study indicated that the poplar plantation became a sink for C when the trees were around 6-7 years old; this result was dependent on interannual variation. A larger proportion of the total soil respiration was contributed by microbial organisms (heterotrophic respiration) as compared with the contribution by autotrophic respiration; in addition, the ratio of heterotrophic to total soil respiration had large interannual variation (about 0.63 in 2005 and 0.83 in 2009) which remained relatively stable across the chronosequence. This research also evaluated the key factors controlling soil respiration and the impact of afforestation and stand age on the sensitivity of soil respiration to temperature changes. All data indicate that planting hybrid poplars on former agricultural land was beneficial for mitigating climate change.