Afforestation and reforestation support coupled gains in soil life and carbon storage worldwide

Forest planting is widely promoted as a nature-based climate solution for mitigating anthropogenic carbon (C) emissions. Whether different planting strategies lead to similar trajectories in C storage and how these affect soil microbial growth are poorly understood. Here we compile data on 1,158 paired afforestation/reforestation and reference sites across six continents. We find that afforestation has the potential to support greater soil microbial biomass and larger topsoil C storage across stand development spanning up to a century. Reforestation is associated with substantial recovery of soil C storage and soil microbial biomass, with both approaching original forest levels during the first three decades, but these gains are not sustained in older stands. Soil C and microbial biomass responses are generally coupled across prior land uses, climatic zones and planted tree species, with greater gains on degraded lands. Overall, our work provides evidence that afforestation and reforestation could contribute to soil C storage and support soil microbial growth under global change, offering critical insights for refining nature-based climate solutions.