Organic matter, nutrients, and colloidal trace elements in pore waters, dispersed ice and aqueous leachates from mineral soils of the permafrost region

• Substantial pools of nutrients and trace metals in porewaters and dispersed ice of mineral soils. • The majority of trace metals are present in the form of organo-Fe and Al–Si colloids. • Maximum concentrations of inorganic solutes occur at a depth of 40–80 cm due to bi-directional freezing. • Dispersed ice in mineral soils represents a highly labile reservoir of organic and inorganic nutrients. • Organic and inorganic solutes are impoverished in mineral soil water and ice compared to those of peat soil. Under all considered climate warming scenarios, currently frozen mineral soils in permafrost regions may become a tipping point in the overall balance between land and coastal zone ecosystems. The thawing of dispersed ice in these soils is likely to liberate substantial amounts of organic carbon (OC), along with organic and inorganic macro- and micronutrients and trace elements. On one hand, this represents a potentially important feedback mechanism to climate warming, as the mobilized OC and nutrients may be exported to large rivers and ultimately to the Arctic Ocean’s coastal zone. On the other hand, toxic metals stored in the ice may also be leached into aquatic environments, posing ecological risks. To quantify the potential for solute mobilization from frozen soils to adjacent aquatic ecosystems, we sampled mineral soil cores from the continuous permafrost zone in the Western Siberian Lowland. In interstitial porewater, dispersed ice, and aqueous leachates, we analyzed 77 solutes—including organic and inorganic nutrients and trace metals—in both the dissolved (<0.22 µm) and low molecular weight (<3 kDa) fractions. In the porewaters and dispersed ice, maximum concentrations of aromatic OC, Fe, Al, and several insoluble elements were observed at depths of 40–90 cm, within the active layer. This accumulation may be linked to bidirectional freezing concentration processes, leading to the buildup of organo-Fe and Al colloids—carrying these insoluble elements—just above the active layer boundary. In addition to colloids, we also detected substantial quantities of sub-colloidal inorganic particles (0.22–3 µm), with Al–Si–Fe aggregates being the dominant components. We found comparable storage of nutrients (N, Si) and biologically important trace elements (Fe, Mn, Se) in the fluids of both peat and mineral soils. In contrast, most other major and trace organic and inorganic solutes were significantly enriched in peat porewater and dispersed ice compared to the interstitial aqueous phase of mineral soils. The concentrations of major and trace solutes in aqueous leachates of mineral soils (1:5) were 2 to 30 times lower than those in native porewater from the active layer. This discrepancy can be attributed to the periodic leaching of solutes from the active layer during the unfrozen period of the year. The global significance of these findings is that the initial thawing of permanently frozen mineral soils—those located below the current active layer—will have the greatest impact on solute removal from soil to aquatic systems. Overall, the dispersed ice within mineral horizons below the active layer serves as a highly labile reservoir of organic and inorganic nutrients. These solutes can be readily exported to adjacent surface waters as permafrost thaw and climate warming continue.