1 Introduction Historically, the Arctic land has served as a major carbon reservoir. However, this region is warming nearly four times faster than the rest of the globe (Rantanen et al. 2022) and facing considerable uncertainties in ongoing land changes, including permafrost thaw, thermokarst formation, shrubification, and mineralisation processes (Tank et al. 2023; Kaplan and New 2006). Arctic watersheds, which store roughly 50% of the global soil organic matter (Tarnocai et al. 2009; Amon et al.