The Impact of Fires on the Fractional Composition of Iron and Carbon Dynamics in the Cryogenic Soils of the Forest–Tundra of Western Siberia Under Changing Climate Conditions

Fires significantly influence the ecosystems of Western Siberia’s forest–tundra zone. Namely, they alter soil processes, including the transformation of different forms of iron and the redistribution of carbon flows. Recent climate change, associated with increased fire frequency, has had a long-term effect on the Arctic and sub-Arctic soil systems. Iron plays a key role in stabilizing organic carbon through the sorption and coagulation processes, yet the long-term changes in iron’s fractional composition under post-fire conditions remain insufficiently studied. This research investigates the impact of natural fires on the transformation of iron forms (amorphous, crystalline, and mobile), as well as on the dynamics of organic carbon in soils within the northern boundary of the forest–tundra natural zone in Western Siberia, between the Pur and Taz rivers. In our study, we have relied on granulometric and chemical analyses, magnetic susceptibility measurements, and iron fraction extractions. Our findings reveal that in post-fire areas, the depth of the seasonally thawed layer increases, accompanied by changes in the thermal and water regimes. This leads to reduced organic carbon content, particularly in intermediate horizons (5–30 cm), and the transformation of amorphous iron into a crystalline form. Crystallization growth is confirmed by increased magnetic susceptibility. Our results highlight the dual role of iron compounds: they contribute to the long-term stabilization of organic carbon, as well as causing its accelerated mineralization by affecting redox conditions. This study is crucial for understanding the biogeochemical processes associated with climate change and increasing fire frequency.