Comparing the composition of soil waters of West Siberian frozen mires sampled by different methods

The permafrost zone occupies around 30% of the West Siberian Plain. Permafrost bogs are the dominant type there and differ in structure and hydro-thermal regime from bogs of the "melted" area. A link between substance flows in rivers and processes occurring in interfluvial landscapes are soil waters providing transportation of hydrocarbon and other chemical elements into the river network. Evidently, under conditions of climate change the hydrochemistry of soil waters will act as a sensitive indicator of environment changes. It's known that the chemical properties of soil waters can vary depending on the chosen sampling method. The main aim of this research was to compare the native methods of sampling and then select the most optimal for the conditions of frozen bogs. The comparison was made using the materials obtained from peat oligotrophic permafrost soils in 2014-2015. We extracted all bog water samples on polygonal bogs in the southern tundra (key site "Tazovskiy"; 67°22'N, 78°39'E) and palsa bogs in the northern taiga (key site "Khanymey"; 63°47'N, 75°38'E). Samples were taken in three ways: 1) pressing of peat samples (titanium press); 2) vacuum filtration (porous ceramic cups, SDEC); 3) extraction of superpermafrost water. We considered methods of soil water sampling in pairs (superpermafrost water - water after vacuum filtration; water after pressing - water after vacuum filtration) and then we determined some parameters (рН, conductivity, DOC, DIC, SO₄, Cl, Na, Mg, Са, K, Fe, Al). Superpermafrost water is characterized by higher values of parameters in comparison with water after vacuum filtration (in 22 cases against 8). At the site "Tazovskiy" DOC content ranges from 42 mg/L in hollows to 97 mg/L at the polygon and 1.5 times more in superpermafrost water. There are no major differences for the key site "Khanymey" (z = -0.927; p = 0.354). The pH values do not depend on the landscape position in soil solution and are at 0.2-0.7 units higher than in superpermafrost water. The electrical conductivity of superpermafrost water is higher in all cases, especially on polygonal bog. The choice of extraction method has no effect on values of Ca and Mg on polygons and mounds. In hollows, different methods give the difference in Ca concentrations more than 1.5 times. Fe, Al, Na and K contents in superpermafrost water is generally higher than in soil solutions. These results show that values depend on the method and the position in the bog micro landscape; however, we did not identify any obvious differences between the methods. The second pair of methods (pressing - vacuum filtration) provides information about combined forms of the soil moisture and allows selecting the soil solution at a predetermined depth. The highest values of almost all parameters are found in soil solutions, selected by ceramic cups, regardless of the relief position and sampling depth, except for Fe, Al, K, Na. As for DOC, Ca, Mg, the difference at concentrations are observed both between methods and with the depth of soil profile in hollows and at the polygon. The comparison of means showed that statistically significant differences between the two studied and described above methods are observed in 61% of cases (in 44 of 72 pairs). The results show that for a correct interpretation of the data and comparison of the results from different research groups, the method of sampling peat soil water should be taken into account. Using vacuum filtration method is possible to study intraprofile heterogeneity. The superpermafrost water sampling is less expensive and suitable for studying the spatial variability of hydro-chemical properties.

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