A systematic analysis and review of the impacts of afforestation on soil quality indicators as modified by climate zone, forest type and age

This global systematic analysis and review investigate the impacts of previous land use system, climate zone, forest type and forest age on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) stock, soil bulk density (BD) and pH at soil layers 0–20, 20–60 and 60–100 cm, following afforestation. Data came from 91 publications on SOC, TN and TP stock changes, covering different countries and climate zones. Overall, afforestation significantly increased SOC by 46%, 52% and 20% at 0–20, 20–60 and 60–100 cm depths, respectively. It also significantly increased shallower TN stocks by 28% and 22% at 0–20 and 20–60 cm depths, respectively, but had no overall impacts on TP. Previous land use system had the largest influence on SOC, TN and TP stock changes, with greater accumulations on barren land compared to cropland and grassland. Climate zone influenced SOC, TN and TP stock changes, with greater accumulations for moist cool than other climate zones. Broadleaf forests were better than coniferous forests for increasing SOC, TN and TP stocks of the investigated soil profile (0–100 cm). Afforestation for <20 years accumulated SOC and TN stocks only at the soil surface (0–20 cm), whilst afforestation for >20 years accumulated SOC and TN stocks to 100 cm soil depth. Changes to SOC and TN were positively correlated at depths down to 100 cm under all age groups, demonstrating that an increase TN could offset progressive N limitation, and maintains SOC accumulation as forests age. TP stock decreased significantly in topsoil (0–20 cm) for <20-year-old forest and did not change for >20-year-old forest, suggesting that it may become a limiting factor for carbon sequestration as forests age. Following afforestation, soil BD decreased alongside significant increases in SOC and TN stocks to 100 cm depth, but had no relationship with TP.