Total content and mobility of cadmium, cobalt and zinc in humus horizons of soils in the Republic of Tatarstan

Despite the fact that in recent decades there has been a decrease in anthropogenic emissions of heavy metals into the environment, the danger of technogenic pollution of soils is still acute in industrially developed countries. The combination of soil properties and components, such as pH, content of organic matter and clay minerals, carbonates and minerals with variable charge, determines the ability of soils to accumulate and retain heavy metals. The aim of this research was to assess the territory of the Republic of Tatarstan in relation to soil contamination with cadmium, cobalt and zinc, taking into account soil-geochemical interactions under different land use types. In the territory of the Republic of Tatarstan, located in the western part of the Russian Federation (55°20'36,1"N; 50°47'31,7"E), we collected 1170 topsoil samples. Sampling was conducted at a distance of at least 200 m from the nearest road from a depth of 0-20 cm. Samples are represented by zonal and nonzonal soil types of natural and agricultural types of land use (See Tables). Urban soils are united in the type of urbanozems. We determined the following parameters in soil samples: humus content, particle size distribution, and pH. Determination of the total content of Cd, Co, Zn, Mn and Fe was carried out after extraction using 5M HNO₃. The mobile forms of Cd, Co and Zn were extracted using acetate-ammonium buffer solution with ph 4.8. The final detection of heavy metals content was carried out by the atomic absorption method. Statistical data analysis was carried out using R Core Team package. Variogram analysis and spatial interpolation were performed using "gstat" package, and the method of regression trees using "tree" package. Maps were developed in QGIS geoinformation system. We estimated the regional background level from samples of the zonal soils of natural land use and amounted to 0.44±0.24 mg/kg for Cd, 10.4±3.6 mg/kg for Co and 43.3±12.0 mg/kg for Zn. Two geochemical zones of high heavy metal content are observed in the Republic of Tatarstan: the eastern part of Predkam'e and the northwest of Predvolzh'e (See Figures). The distribution of the pollution index (actual content / background level) according to land use types makes it possible to conclude that there is no significant anthropogenic contribution of the Cd, Co and Zn related to agricultural activity. Nevertheless, significant pollution with cadmium and zinc is observed for samples collected in the cities of Kazan, Naberezhnye Chelny and Almetyevsk. With respect to cobalt pollution, urban soils are characterized as uncontaminated. We assessed the structure of soil-geochemical interactions of metals in topsoil using the method of regression trees (See Figures). Cadmium shows a complex structure of geochemical interactions with a strong impact of soil type and land use type. In saturated alluvial meadow, soddy-carbonate and podzol soils siderophilic properties of Cd are pronounced. In the gray forest soil and urban soils Cd content increases with the humus content and pH increasing. The interaction structure of Co is the same for all soil types. In topsoil siderophilic and manganophilic properties of Co are manifested. The affinity of zinc to different carrier phases is strongly determined by the land use type: in natural and agricultural soil with low clay content the relation to Fe and Mn content is manifested; in the samples of urban soils the relation to content of physical clay and organic matter is observed. This article contains 4 Figures, 5 Tables and 31 References.

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