Poorly Soluble and Mobile Forms of Heavy Metals in the Soils of the Volga Steppes

To this day, scientists argue about the extent and rate of accumulation of heavy metals and potentially toxic substances in the soil. Questions regarding the forms of heavy metals, their spatial distribution, solubility, plant availability, etc., are still relevant. It is known that the content of heavy metals is considerably influenced by the content and composition of soil organic matter, particle size distribution, soil pH and parent rocks, as well as redox conditions. Since 2007, the steppe soils of the Volga region have been monitored for the content of heavy metals in virgin and arable landscapes. In this work, the content of poorly soluble and mobile forms of Zn, Cu, Pb, Cd, Ni, Hg and As in 0–20 cm topsoil was determined in two types and eight subtypes of Chernozem and chestnut soils. Some differences were found in the accumulation of heavy metals between the soil types and subtypes which were studied. No changes were found in the content of poorly soluble heavy metals between the virgin and corresponding arable soil in the Volga steppes. The content of heavy metals in all the soils which were studied was below the maximum allowed concentrations. The relative content of the mobile forms of heavy metals as a percentage of the total varied from 7–25% for Cd and 0.5–2.2% for Zn and Cu. The accumulation of Pb, Cd, Ni and As in soils located near the large industrial zone showed an increasing trend. The maximum allowed concentrations of heavy metals were not exceeded in the soils studied. Also, agricultural soil management did not significantly increase the heavy metal concentrations.