Forest Expansion and Changes in Soil Phosphorus Fractions in the Eastern Himalaya

Forest expansion into alpine ecosystems is expected to accelerate under climate warming, yet its effects on soil phosphorus (P) dynamics remain poorly understood, particularly in the Himalayas. To address this, we examined how soil P fractions and pools change from forests to alpine ecosystems on Chelela Mountain, Bhutan. We collected 120 surface soil (0-15 cm) and 20 whole-profile (up to 100 cm) samples across the forests, ecotones, and alpine ecosystems, and applied Hedley fractionation to characterize P fractions. Both surface and whole-profile total P peaked in the ecotones; however, the lowest P shifted from the forests to the alpine ecosystems across the whole profile. This ecotonal peak was driven by greater inorganic (Pi), moderately labile, and residual P, particularly NaOH Pi and HCl Pi. In contrast, alpine ecosystems retained more organic P, potentially reflecting slower microbial decomposition under colder conditions, whereas forests showed tighter P cycling, likely due to stronger plant uptake. Shifts toward more inorganic and moderately labile P as forests expand into alpine ecosystems may create a positive feedback that further promotes woody encroachment into alpine ecosystems under climate warming. Overall, these changes could alter nutrient availability and vegetation dynamics, with potential consequences for alpine biodiversity, forage resources, and pastoral livelihoods that depend on these ecosystems.