Dissolved iron (dFe) in boreal rivers may play an important role in primary production in high-latitude oceans. However, iron behavior in soils and dFe discharge mechanism from soil to the rivers are poorly understood. To better understand iron dynamics on the watershed scale, we observed the seasonal changes in dFe and Dissolved Organic Carbon (DOC) concentrations in the river as well as dFe concentration in soil pore waters in permafrost watershed from May to October. During snowmelt season, high dFe production (1.38–4.70 mg L${}^{-1}$) was observed in surface soil pore waters. Correspondingly, riverine dFe and DOC concentrations increased to 1.10 mg L${}^{-1}$ and 32.3 mg L${}^{-1}$, and both were the highest in the year. After spring floods, riverine dFe and DOC concentrations decreased to 0.15 mg L${}^{-1}$ and 7.62 mg L${}^{-1}$, and dFe concentration in surface soil pore waters also decreased to 0.20–1.28 mg L${}^{-1}$. In late July, riverine dFe and DOC concentrations increased to 0.33 mg L${}^{-1}$ and 23.6 mg L${}^{-1}$ in response to heavy rainfall. In August and September, considerable increases in dFe concentrations (2.00–6.90 mg L${}^{-1}$) were observed in subsurface soil pore waters, probably because infiltrated rainwater developed reducing conditions. This dFe production was confirmed widely in permafrost wetlands in valley areas. Overall, permafrost wetlands in valley areas are hotspots of dFe production and greatly contribute to dFe and DOC discharge to rivers, especially during snowmelt and rainy seasons.

Primary production in the Sea of Okhotsk is largely supported by dissolved iron (dFe) transported by the Amur river, indicating the importance of dFe discharge from terrestrial environments. However, little is known about the mechanisms of dFe discharge into the Amur river, especially in terms of long-term change in dFe concentration. In the Amur river, extreme increase in dFe concentration was observed between 1995 and 1997, the cause of which remains unclear. As a cause of this iron anomaly, we considered the impact of permafrost degradation. To link the permafrost degradation to long-term variation in dFe concentration, we examined the changes in annual air temperature (Ta), accumulated temperature (AT), and net precipitation for three regions (northeast, south, and northwest) of the basin between 1960 and 2006. Ta and AT were relatively high in one out of every few years, and were especially high during 1988–1990 continuously. Net precipitation in late summer (July to September) has increased since 1977 and has stayed positive until 2006 throughout the basin. Most importantly, we found significant correlations between Ta and late summer dFe concentration with a 7-year lag (r = 0.54–0.69, p < 0.01), which indicate a close relationship between high Ta in year Y and increased late summer dFe concentration in year Y + 7. This correlation was the strongest in northeastern Amur basin where permafrost coverage is the highest. Similar 7-year lag correlation was also found between AT in the northeastern basin and late summer dFe concentration (r = 0.51, p < 0.01). Based on our findings, we propose the following hypothesis as a cause of iron anomaly. (1) Increased net precipitation since 1977 has increased soil moisture, which created suitable conditions for microbial dFe generation; (2) permafrost degradation during the warm years of 1988–1990 promoted iron bioavailability and led to the intensive dFe generation in the deeper part of the active layer; and (3) dFe took approximately 7 years to reach the rivers and extremely increased dFe concentration during 1995–1997. This is the first study to suggest the time-lagged impact of permafrost degradation on iron biogeochemistry in the Amur river basin.

Dissolved iron (dFe) transported by the Amur River greatly contributes to phytoplankton growth in the Sea of Okhotsk. Nevertheless, there has been little research on the source of dFe to rivers, especially in the Amur‐Mid Basin, which is situated in a sporadic permafrost area. In the Amur‐Mid Basin, permafrost generally exists in wetlands in flat valleys, and these permafrost wetlands could be a source of dFe to rivers. To assess the importance of permafrost wetlands for dFe export, we conducted a local survey on land and soil characteristics of wetlands, and moreover analyzed the chemical composition (dFe, dissolved organic carbon [DOC], pH, and electrical conductivity [EC]) of 24 rivers with different watershed sizes in summer. As a result of local survey, the thickly accumulated peat soils in the permafrost wetland were almost saturated and rich in organic matter from the surface to a greater depth near the permafrost table. In addition, the coverage of such permafrost wetlands in watersheds showed significant positive correlations with dFe (r² = 0.67, p = 1.7e−6) and DOC (r² = 0.48, p = 1.8e−4) and a negative correlation with EC (r² = 0.52, p = 7.7e−5). The dFe concentration was also correlated well with DOC concentration (r² = 0.68, p = 7.3e−7) but not correlated with pH and watershed area. These findings are the first to indicate that permafrost wetlands in the Amur‐Mid Basin considerably contribute to dFe and DOC export to rivers, and their coverage primarily determines riverine dFe and DOC concentrations in summer.

Permafrost degradation under a warming climate has altered hydrological and biogeochemical processes across the Arctic. Although increasing fluxes of weathering‐derived ions (e.g., Ca ²⁺ , Mg ²⁺ , and SO ₄ ²⁻ ) have been reported in Arctic rivers, the underlying mechanisms and hotspots within basins remain poorly understood due to limited analysis of environmental drivers such as climate and permafrost dynamics. We investigated long‐term trends (1980–2022) in Ca ²⁺ , Mg ²⁺ , and SO ₄ ²⁻ concentrations in the Kolyma River in northeastern Siberia, and examined basin‐wide changes in air temperature, precipitation, soil temperature, and active layer thickness. We found significant increases in ion concentrations, which were strongly correlated with rising subsurface soil temperatures ( r = 0.61) and active layer deepening ( r = 0.78) in the Yedoma‐rich Kolyma Lowland. These findings, along with the concentration ratios, suggest that sulfuric‐acid‐driven carbonate weathering has intensified in the deeper part of the active layer—where previously frozen minerals become newly exposed—thereby enhancing ion discharges to rivers. Record‐high concentrations were observed in 2020, when an extreme heatwave occurred, and produced exceptionally high subsurface soil temperatures (5.6°C; average 3.8 ± 0.6°C) during the thawed period (May–October) and active layer thicknesses (116.5 cm; average 100.3 ± 10.8 cm). These results underscore the sensitivity of Arctic river systems to heatwave‐induced permafrost degradation, which rapidly intensifies subsurface weathering and solute mobilization. Given the widespread distribution of Yedoma across the Arctic, similar responses may occur in other watersheds. Continued monitoring of water chemistry and permafrost dynamics is essential to understand changes in Arctic river biogeochemistry.

Dissolved iron (dFe) concentration in the Amur River increased considerably between 1995 and 1997, but the mechanisms of dFe discharge to the Amur River are poorly understood. We examined the long-term changes in climate variables in the Amur River basin between 1960 and 2000; net precipitation in late summer (July–September) has increased since 1977 throughout the basin; annual air temperature (Ta) was relatively high between 1988 and 1990. Most importantly, we found significant correlations between Ta and dFe concentration with a 7-year lag (r = 0.43–0.55, p < 0.01), which indicate a close relationship between high Ta in year Y and increased dFe concentration in year Y + 7. Correlation was the strongest in northeastern Amur basin where permafrost coverage is the highest. On the basis of our findings, we propose that (1) increased net precipitation since 1977 has increased soil moisture, which created favourable conditions for microbial iron reduction; (2) permafrost degradation during the warm years of 1988–1990 promoted iron bioavailability and led to the intensive generation of dFe in the deeper part of the active layer; and (3) dFe took approximately 7 years to reach the rivers and increased dFe concentration during 1995–1997.