Data on the elemental composition ( mobile fractions and total content ) of soils in catena at the SE Valdai Hills , Russia

This study presents a dataset on seasonal soils sampling from September 2016 to May 2018 in the southern part of the Central Forest Reserve (SE Valdai Hills) within a catena with Endocalcaric Albic Glossic Stagnic Profondic Retisols (Cutanic, Loamic) and Albic Gleyic Histic Retisols (Cutanic, Loamic) under coniferous-deciduous forest (Tília cordáta, Pícea ábies, Ácer platanoídes) on loess-like loams underlain by carbonate moraine deposits. 152 soil samples were taken to 10 define total concentration of 67 chemical elements (ChEs), content of three mobile fractions (exchangeable, bound within organo-mineral complexes, bound with Fe and Mn hydroxides) of 69 ChEs and content of residual fraction, including macro elements (Al, Ca, Fe, K, Mg, Mn, Na, P, Ti, S, Si), heavy metals (Ba, Co, Cr, Cu, Ni, Pb, Rb, Sr, Th, U, V, Zn), trace elements (Ag, As, B, Be, Bi, Br, Cd, Cs, Ge, Hf, Li, Mo, Nb, Pd, Sb, Sc, Se, Sn, Ta, Te, Tl,W, Zr) and rare earth elements (Ce, Er, Eu, Gd, La, Lu, Nd, Pr, Sm, Tb, Tm, Dy, Ho, Y, Yb). We measured pH-value, total organic carbon content (TOC), 15 seven particle-size classes (<1, 5-1, 10-5, 50-10, 250-50, 500-250, 1000-500 μm), and basicity from carbonates. The dataset is available from Mendeley Data (http://dx.doi.org/10.17632/r29psg69z7.1, Enchilik et al., 2020) and will be further updated.


Introduction
The assessment of background state of landscapes, being basically provided in biosphere reserves, takes a special place in international programmes on the environment by UNESCO and UNEP. The analysis of geochemical structure of landscapes at different levels is an important part of background monitoring. Nowadays, the basic method is catenary. It is founded on detection of typical objects and studies of chemical compounds allocation in its components. The parameters on radial and 25 lateral allocation of elements in different parts of landscape are assessed on the example of model catenas which include the most spread elementary landscapes and its linkings.
The following changes are traced along the studied catena. Well drained summit position is a place where substances enter the ground basically from the atmosphere with wet and dry precipitation and migrate down the slope. Drainage weakens in upper footslope position and accumulation prevails. Drainage is generally affected by climatic and geological-40 geomorhological factors: rainfall and permeability respectively. Low permeability of parent rocks is characteristic for the territory of the reserve (Puzachenko et al., 2006) resulted in waterlogged conditions at the lower footslope position (soil profile 4) and a temporary watercourse, preferably after heavy rains. As a result, soil-moisture increases down the catena followed by the change in plant communities.

Physical and chemical properties measurement
In all samples, pH value was measured in suspension (static conditions) using a pH-meter "Expert-рН" (Russia) at the 70 Faculty of Geography of Lomonosov Moscow State University. Total organic carbon (TOC) content was determined in 120 samples using titrimetric method with phenylanthranilic acid (Reeuwijk, 2002). The particle-size distribution in soils was analyzed using laser diffraction technique and an 'Analizeter 22' equipment (Germany). Samples for the particle-size distribution analysis were pre-treated with 4% Na4P2O7. The Russian system of particle-size classes was used: G1clay (<1 μm), G2very fine silt (5-1), G3medium silt (10-5), G4 -сoarse silt (50-10), G5 -fine sand (250-50), G6medium 75 sand (500-250). All studied soils are loamy (containing >10% of <0,01 μm sized matter) with well-defined textural differentiation. Eluvial material is rich of silt fractions while clay fractions content is maximal in argic horizon and parent material. The content of clay fraction is also higher in parent material of middle-taiga landscapes of Karelia (Lukina et al., 2019).

Chemical composition measurement 80
Soil suspensions of NH4Ac, NH4Ac+1% EDTA, 1M HNO3 were used for mobile fraction extraction from soil subsample The content of elements is close to the average for Retisols of Eurasia (Semenkov et al., 2016) and agricultural soils of Northern Europe, which were formed on moraine deposits and parent material of Scandinavia (Reimann et al., 2018). The content of Cr, Cu, Fe, Ni and Sr is located on the lower edge of the average for Retisols of Eurasia while the content of Mn, Pb and Zn -on the higher edge of the average. The lowest content of Fe was found in umbric horizon of soils of summit and upper slope landscapes due to high pH. 95

Description of dataset and data availability
Statistical analysis was carried out using software package 'Statistica' and Microsoft Office Excel. Descriptive statistics of soil properties and ChEs distribution (mean, maximum, minimum, standard deviation, etс.) represented in table 2 (table S1).
This dataset is available from Mendeley Data at http://dx.doi.org/10.17632/r29psg69z7.1 (Enchilik et al., 2020). Descriptive statistics also characterized soil properties and ChEs distribution for nine replicates of horizons A and B within each 100 landscape position (table S3).
The significance of seasonal changes for soil proxies was assessed using Mann-Whitney U-test (table 3, table S2). Vertical distribution was characterized using R coefficient (Kasimov, Perelman, 1992) calculated as a ratio between the level of elements in soil horizons to the level of elements in parent material (table S4). Spatial distribution was characterized using L coefficient (Kasimov, Perelman, 1992) calculated as a relation between the level of elements in soil horizons of catena's 105 landscapes (upper slope, footslope) to the level of elements in soil horizons of the summit position (table S5). Significance of lateral differences was assessed using Sign Test, marked tests are significant at p<0,05 (table S6). Spearman's correlation analysis was used to calculate correlations between ChEs forms content and soil proxies. The differences were considered significant at p<0,05, p<0,01, p<0,001 (table S7.1, table S7.2). The calculations of relative error for elemental composition of soil and pure extraction solutions (of NH4Ac, NH4Ac+1% EDTA, 1M HNO3) were provided as well (table S8). 110

Conclusion
The soils of etalon south-taiga catena were studied to define conditions of migration, content, vertical and spatial allocation of total content and three mobile forms of its compounds: exchangeable ChEs fraction, bound within organo-mineral complexes ChEs fraction, bound with Fe and Mn hydroxides ChEs fraction as well as mobility of these metals.
Obtaining of fundamental knowledges on differentiation of metals in the soils of south-taiga catena is necessary to assess 115 migration and accumulation of elements in natural and technogenic landscapes. soils of southern taiga forests. Data is applicable for the assessment of contamination level of the elements with potential 120 toxicity. Data will help legislators to create health risk management plans. Elaborated data could be used for accurate identification of the sources of pollution and its migration routes as well as for more effective conservation and remediation of anthropogenically affected soils of southern taiga regions.
Iovcheva and E.V. Terskaya for assistance with laboratory work as well as to E.N. Aseeva, R.B. Sandlersky and Y.G. 125 Puzachenko for assistance in field work.

Conflict of interest.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.