Effects of Afforestation on Soil Carbon and Nitrogen Accumulation Depend on Initial Soil Nitrogen Status

Abstract Long‐term carbon (C) sequestration in terrestrial vegetation and soil is mediated by soil nitrogen (N) supply. Afforestation is regarded as a global‐scale solution to climate change; thus, resolving the role of N in either facilitating or reducing the long‐term C benefits of this practice has essential implications to maximize its C sink potential. The impacts of afforestation on soil C, N, and their stoichiometric ratio have been widely explored but what regulates these impacts remains unclear at regional and global scales. In this study, we conducted an intensive field sampling investigation including 610 pairs of afforested and control plots in northern China and extensively compiled a global data set containing 211 afforested‐control pairs worldwide to evaluate responses of soil N concentrations and C:N ratios to afforestation and further explored their major regulator. We identified a soil N threshold, the inflection point where afforestation changes from increasing to decreasing soil C and N, which was 0.86 (95% CI: 0.81–0.91) kg N m −2 in 0–1 m depth. Changes in soil C:N ratios with afforestation were mediated by initial relative abundance of soil C and N and types of mycorrhiza associated with planted trees. Increases in soil C:N were mostly driven by trees with ectomycorrhizal associations but did not change for those associated with arbuscular mycorrhizal fungi. These results provide a data‐based understanding on soil C and N dynamics following afforestation and its underlying mechanisms and further highlight the importance of site selection based on initial soil properties in future afforestation.