Additional carbon sequestration potential of abandoned agricultural land afforestation in the boreal zone: A modelling approach

Agricultural land abandonment is a widespread phenomenon that generally results in C accumulation due to natural establishment of woody vegetation. However, whether afforestation of abandoned agricultural lands (AAL) can sequester more C than AAL naturally reverting to woodlands is unclear. In this study, we used the CBM-CFS3 model to compare the additional C sequestration potential of afforested AAL (AR) with a reference scenario where AAL naturally returns to forest (NR). Simulations were performed for stands located in Quebec's boreal zone (Canada) on podzolic soils. The AR scenario corresponded to stands afforested at a density of 2000 trees ha(-1) with one of five commonly planted species in the region, namely black spruce (BS), white spruce (WS), eastern white pine (EWP), jack pine (JP) and tamarack (TK). The NR scenario corresponded to stands naturally reverting to natural forests of one of five species naturally occurring in the region, namely BS, WS, balsam fir (BF), trembling aspen (TA) and white birch (WB). The yield tables used for NR were phased out by 5, 10, 15 and 20 years to simulate various dynamics of colonization by woody plants following agricultural abandonment. Net C accumulation in AR stands varies depending on the planted species, from 127 to 255 Mg C ha(-1) after 100 years with TK and WS, respectively. Net C accumulation in NR stands after 100 years ranges from 82 to 145 Mg C ha(-1) for BS and TA, respectively, but these values are sensitive to both tree density and colonization time following abandonment. In both scenarios, the soil C pool shrinks by 6 to 12 Mg C ha(-1) over the first 80-85 years, but the following soil C stock replenishment is faster in AR than in NR stands due to higher litter production (up to 50 Mg C ha(-1)). The additional C sequestration potential of AAL afforestation, i.e. the difference in net C accumulation between AR and NR stands, is the highest in stands afforested with WS and reaches a peak of 121-175 Mg C ha(-1) 80-87 years after afforestation depending on the NR scenario. Afterwards, additional C sequestration decreases due to a reduction in plantation growth combined with increasing natural regeneration growth. This modelling approach helps predict AAL additional C sequestration potential and suggests that AAL afforestation yields a more rapid C sequestration than natural regeneration, which could contribute to reaching net-zero emissions by 2050.