From Stable to Labile: Forest Expansion Drives Changes in Alpine Soil Carbon Fractions

Abstract

Forests are expected to continue to expand into alpine ecosystems as temperatures increase. While forest expansion typically increases aboveground carbon storage, the consequences for belowground soil carbon storage and stability remain unclear. To address this, we combined a field‐based study in the eastern Himalaya of Bhutan with a meta‐analysis examining how forest expansion influences alpine soil organic carbon (SOC) and its fractions. We sampled 120 surface soils (0–15 cm) and excavated 20 soil pits (0–100 cm) across a forest‐ecotone‐alpine gradient on Chelela Mountain in western Bhutan. We quantified total SOC, particulate organic carbon (POC), mineral‐associated organic carbon (MAOC), and MAOC:POC ratio. Locally, forest expansion was associated with higher total SOC, especially in the ecotonal transition zone and in deeper soil layers. Ecotone soils had the greatest SOC, driven largely by MAOC, linked to greater root biomass and higher clay and silt content, whereas POC peaked in the mature forest soils. Although alpine soils stored less total SOC because of shallower soils, they contained a higher proportion of stable carbon, reflected in the highest MAOC:POC ratios. Globally, forest expansion did not significantly change SOC or POC, but significantly reduced MAOC (−28.4%) and the MAOC:POC ratio (−18.3%). These responses varied across ecosystems, where broadleaf expansion decreased SOC via reduced POC, while coniferous and shrub expansion reduced MAOC:POC ratios. Collectively, these findings suggest that forest expansion may shift soil carbon storage from stable mineral‐associated forms to more labile particulate fractions, potentially reducing the long‐term stability of carbon stored in alpine ecosystems.