Contrasting Trait-Mediated Mechanisms Shape Peatland Testate Amoeba Communities Under Long-Term Drying Across Fen-Bog Gradient.

Boreal peatlands store vast amounts of carbon and regulate regional hydrology, making their stability critical under accelerating climate change. This stability depends on community-level resilience and resistance shaped by the functional traits of organisms. Although climate-induced drying is already reshaping peatland communities, the trait-based mechanisms supporting stability in soil biota remain poorly resolved. We assessed stability in testate amoebae (TA) - a key group of the soil food web - by comparing functional trait patterns in ambient control plots with plots subjected to two decades of experimental water level drawdown (WLD) across three peatland types (rich fen, poor fen, and bog). In ambient conditions, null models revealed strengthened environmental filtering along the fertility gradient; nutrient-rich fen showed highest diversity and functional redundancy. Long-term WLD, however, intensified environmental filtering by reshaping the communities across all peatland types. Functional beta diversity revealed contrasting stability mechanisms: rich fen maintained functional stability through resilience, with major species turnover but minimal functional change, whereas bog communities retained function through resistance, relying on drought-adapted traits and showing minimal species turnover. Contrastingly, the poor fen lost functional stability, as low redundancy combined with dominance of wet-adapted traits led to both species and functional turnover. Under moderate drying, bogs are most likely to maintain functional stability, whereas fens, particularly poor fens, are more vulnerable. These trait-mediated differences indicate peatland type-specific functional thresholds with implications for predicting stability and carbon dynamics under future climates. Overall, continued warming increasingly compromises peatland soil biota and the ecosystem functions they mediate.