A meta-analysis of afforestation impacts on soil greenhouse gas emissions

Afforestation is a natural climate solution and a key strategy to mitigate climate change. While tree planting primarily achieves this mitigation via above-ground carbon sequestration, soils also play a dual role as sources and sinks of greenhouse gases (GHG). Understanding afforestation impacts on soil GHG flux is essential for leveraging afforestation to combat global warming. In this research, we conducted a global meta-analysis of 157 studies to assess the effects of afforestation on soil GHG emissions across different prior land uses and to identify key emission drivers. Our results indicated that afforestation significantly reduced CO₂ emissions in former grasslands and deforested land and decreased CH₄ emissions across most prior land uses. However, soil N₂O flux was mostly unaffected by afforestation. The type of forest planted also influenced soil GHG emissions. Hardwood planting reduced CH₄ emissions, but no clear trends emerged for N₂O emissions from either softwood or hardwood forests. Tree planting density had no significant effect on GHG fluxes. GHG responses to afforestation also changed over time and were influenced by environmental factors. CO₂ emissions correlated positively with soil organic carbon, mean annual precipitation, C:N ratio, and soil temperature. N₂O flux increased with soil NO₃^- and microbial nitrogen and decreased with soil organic carbon and moisture. Additionally, soil microbial biomass carbon and soil organic carbon were positively correlated with CH₄ emissions. These findings highlight the importance of selecting tree species, site conditions, and environmental factors to optimize afforestation's GHG mitigation potential.