Earthworm colonization weakens vertical carbon stratification in alpine peat soils under climate-relevant degradation scenarios

Peatlands store a disproportionally large fraction of global soil carbon, yet their stability is increasingly threatened by climate-driven drying and degradation. One underexplored consequence of peatland drying is the potential colonization of soil fauna, such as earthworms, which have historically been absent from waterlogged peat soils. However, the implications of earthworm colonization for peatland carbon dynamics and vertical soil functioning remain poorly understood. Here, we used intact peat soil columns from alpine peatlands to investigate how increasing earthworm densities affect carbon pools, nitrogen availability, and microbial processes across two soil depths (0–10 cm and 10–20 cm). Earthworm treatments included a low-density and a high-density combination of epigeic and endogeic species, reflecting realistic colonization scenarios under peatland degradation. Earthworm addition substantially altered the vertical distribution of soil carbon. In control soils, total carbon and dissolved organic carbon exhibited pronounced depth stratification, whereas earthworm presence weakened or even reversed these depth patterns. Moreover, earthworms increased dissolved nitrogen concentrations and modified extracellular enzyme activities, indicating changes in nutrient cycling and microbial decomposition pathways. Integrated carbon stability indices further suggested a shift toward more decomposable carbon pools under earthworm treatments. Together, our results demonstrate that earthworm colonization can fundamentally reorganize vertical carbon distribution and biogeochemical functioning in peat soils. These findings highlight soil fauna as an overlooked but potentially critical mediator of peatland carbon destabilization under climate-driven degradation.