Quantifying the carbon (C) balance of short-rotation woody crops is necessary for validating the C sequestration potential of these systems. We studied the changes in net ecosystem productivity (NEP) and ecosystem C storage 2–4 and 9–11 years after converting an agricultural land (planted to canola, Brassica napus L.) to hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantations in the Parkland region in central Alberta, Canada. The NEP across land uses ranged between 0 and 13 Mg C ha−1 year−1, while changes in C storage over two years (2006–2008) ranged between 1 and 7 Mg C ha−1 year−1 as biomass C and between −1 and 6 Mg C ha−1 year−1 as soil organic C. When agricultural land was converted to hybrid poplar plantations, soils under hybrid poplar plantations were initially large sources of C losing a total of 8 Mg C ha−1. As cultivation ceased and net primary productivity (and thus litter input) increased, the soil started to become a net C sink by year 2, reaching its pre-plantation level by year 7. At the ecosystem level, hybrid poplar plantations were a source of C in the first 2 years, due to the small contribution of plant biomass and litter relative to soil C loss. Thereafter, the ecosystem acted as a net C sink and reached its pre-plantation level by year 4. We conclude that growing hybrid poplars on rotations longer than 4 years in the study area would create a net C sink and converting agricultural land to fast-growing short-rotation woody crops has the potential for mitigating future climate change.Highlights► We determined NEP and C storage after converting an agricultural land to hybrid poplar plantations. ► The soil and the plantation system quickly became a net C sink after reforestation. ► Growing hybrid poplars longer than 4 years in the study area would create a net C sink. ► Afforestation with hybrid poplars has the potential for mitigating future climate change.