Afforestation influences soil organic carbon (SOC) stocks in terms of magnitude, direction (gain, loss or no change) and duration. Due to data shortage at regional or national scales, global datasets have commonly been evaluated by meta-analysis to address the effects of land use changes on SOC. Here, we present a meta-analysis of relative change in SOC within the Northern European region, which has not been well represented in previous syntheses. Effect sizes were reported as log response ratios (RR) for each pair of agricultural (control) and afforested plots. To quantify effects of afforestation on SOC stocks in forest floors and mineral soils (0-10 cm and 0-20/30 cm), we tested the influence of different former land use classes, such as croplands and grasslands, forest age and forest type on SOC stock changes. Secondly, we investigated the influence of study design, since SOC stock estimates from control/afforested plots have an implicit spatial dependency according to the experimental approach used. This has not been accounted for in meta-analyses of afforested soils before. In addition, changes in SOC stocks (in 0-10 cm) were expressed on equivalent soil mass and fixed depth to estimate possible differences in effect size related to these two SOC accounting methods.
We found an overall positive effect of afforestation on SOC accumulation with age, supported by the oldest age classes (>30 years). This effect was enhanced when the forest floor was included in the total SOC stock, in particular for coniferous forests. Afforestation on former grasslands had a negative (RR= -0.06, for both soil depths) non-significant (p> 0.05) effect on SOC stocks. In croplands, effects depended on the soil layer and were significantly positive (RR= 0.14, p< 0.05) in 0-10 cm, while negative and nonsignificant (RR= -0.01, p>0.05) in the 0-20/30 cm layer. This suggests that cropland to forest conversion increases SOC stocks in top-soils within a decadal time scale. Results derived from a meta-regression of
cropland afforestation alone, showed that the effect size had a significant overall increase of 0.7% per year for the 0-20/30 cm and became 1.3% per year when forest floors were included. Failure to account for different study designs and their implicit spatial dependency resulted in an overestimation of the RR; however, there was no influence on the main trends. Lastly, effect sizes were underestimated if the SOC stocks were based on fixed depth rather than equivalent soil mass.
We conclude that significant SOC sequestration in Northern Europe occurs after cropland to forest rather than grassland to forest conversion and that C accumulation takes place after > 30 years since land use change.