Soil carbon as a blind spot in tropical rainforest restoration.

Following years of degradation, tropical forests are now a key focus for active ecosystem restoration, which involves strategies such as tree planting to maximize aboveground biomass. The impact of active restoration on soil carbon storage has, however, been overlooked in restoration management and carbon offsetting projects despite its substantial contribution to ecosystem carbon budgets. In this review, we examine how active restoration in tropical rainforests can affect soil carbon storage from all major tropical regions (i.e., Southeast Asia, Central Africa, and Neotropics) and formulate testable research avenues for specific pathways of carbon gain and loss. We investigate direct and indirect impacts of active forest restoration on soil carbon related to forest structure and tree planting. We conclude that increases in aboveground biomass and biodiversity, along with the enhancement of trees with particular mycorrhizal fungal partners, could promote soil carbon storage. Silvicultural site preparation measures preceding tree planting are unlikely to significantly affect soil carbon storage, despite altering forest structure and nutrient availability. Conversely, invasive species management could decrease soil carbon storage, but this remains one of the least studied pathways in tropical rainforest restoration. The substantial amount of carbon stored in soil means net changes in soil carbon storage bear significant implications for carbon cycling and offsetting initiatives, even if changes following active restoration measures may be small. We encourage targeted belowground field campaigns in tropical regions where data are particularly sparse to test these pathways and provide much needed information for carbon offsetting programs and restoration management.