oa The Retention Capacity of Thawing Permafrost Inhibits the Export of Arctic Soil Dissolved Organic Carbon

Permafrost thawing in the Arctic is destabilizing the vast carbon pool stored in the frozen ground, causing more export of dissolved organic carbon (DOC) into fluvial systems. However, mechanisms of DOC transport within soil columns are largely unknown, impeding the effective estimation of DOC export from soils. We conducted leaching experiments on active layer organic soils and underlying permafrost mineral soils to study DOC transport and alteration during vertical percolation through thawed permafrost. We demonstrate that mineral permafrost exhibits high soil carbon loss as DOC (3.27–11.42%), comparable in magnitude to OC loss from microbial respiration in incubation studies. However, a large proportion of this carbon release may be compensated by the retention of active layer soil DOC in underlying mineral soils (11.17–46.42%), suggesting an “internal” carbon sink within soil columns. Meanwhile, this sink can function as a “filter” that preferentially retains aromatic components. This internal process may not only confound the study of active layer DOC transport but also modify the chemical composition and lability of both thawed permafrost and exported soil. Our research highlights the importance of studying DOC movement and transformations within soil columns to improve understanding of carbon cycling in soils and fluvial systems.