Afforestation of marginal land has been recommended by the Intergovernmental Panel on Climate Change (IPCC) as a means of enhancing the capacity of soil to act as a carbon (C) sink. However, the success of this practice is variable and depends on many factors, including the type of land management used. In this study, we quantified and compared the C stocks in two highly productive agricultural soils afforested with poplar 10 years before the study and in adjacent soils still used for agricultural purposes. One of the agricultural soils was sown with grass and maize in a crop rotation system and the other was alternately sown with maize (6 months) and left fallow (6 months). In all soils, we estimated the C pools associated with the live biomass (including the C extracted by harvesting in the cropped soils in the 10 years since afforestation), dead biomass, amount of fertiliser added (quantifying all inputs of C added as fertiliser in the cropped soils since the start of afforestation) and the soil organic matter (0–100 cm), to enable estimation of the total C stocks in the ecosystem. Afforestation had different effects on the various C pools. Thus, although afforestation enhanced the C sink capacity of the ecosystem when carried out on agricultural land, the degree to which this occurred varied according to previous land use. The increase in C stocks that occurred from afforestation was lower in the plots employing maize-grass rotation, compared to those with maize-fallow rotation. The ecosystem C balance was quite similar in both afforested plots (247.4 and 233.0 Mg C ha−1), while it was quite different in the cultivated plots (182.3 Mg C ha−1 in Laraño; 73.9 Mg C ha−1 in A Barca). Although at both sites the C balance was higher in the afforested plot than in the cultivated plot, the differences were much smaller in Laraño (65.1 Mg C ha−1 higher in the forested plot than in the cultivated plot) than in A Barca (159.1 Mg C ha−1 higher in the forested plot than in the cultivated plot). Our results from highly productive agricultural land are similar to previous findings on marginal land.