Old Carbon on the Move: Landscape Drainage enhances lateral Carbon Export in an Arctic wet Tussock Tundra Ecosystem, Northeast Siberia

With global warming, permafrost landscapes are rapidly changing, including shifts in hydrology, vegetation, thaw, and carbon pathways; the thaw rate and depth determine whether stored carbon is released vertically into the atmosphere or transported laterally through the hydrosphere. Understanding the controls on lateral carbon export is crucial for quantifying Arctic carbon budgets and potential climate feedbacks. In this study, we investigated the seasonal dynamics of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and particulate organic carbon (POC) along with their isotopic signatures (Δ14C, δ13C) at a Kolyma floodplain site (NE Siberia), contrasting a long-term drained area (dry soil conditions) with a neighboring natural control area (wet). Our results revealed that DOC was the dominant carbon fraction mobilized laterally, followed by DIC and POC. The integrated carbon export over the season was approximately two-thirds higher in the drained areas than in the control area. However, these carbon fluxes varied strongly with water table depth, resulting in distinct seasonal patterns between the drainage and control areas. In the drained area, July drawdown dried previously saturated soils, favoring aerobic decomposition, whereas in the control area, persistently high water levels maintained anaerobic conditions, including methanogenesis. Isotopic analyses indicated a mid‑ to late‑season shift towards the export of older, deeper carbon, strongest in the drained area, implicating progressive active‑layer deepening as a critical driver. These spatial contrasts underscore the role of hydrological conditions in shaping the amount and quality of permafrost carbon export and how warming‑driven hydrologic change may alter Arctic carbon cycling.