Content of heavy metals in agrochernozem of the Southern Cis-Urals after introduction of sewage sludge as a fertilizer

The study was conducted in the southern forest-steppe of the Republic of Bashkortostan on heavy loamy agrochernozem with a high content of humus, close to a neutral pH, high capacity of cation exchange with a predominance of calcium. Sewage sludge (SS) from Ufa municipal wastewater treatment plants was introduced at a dose of 30 t/ha in the top layer of 0–20 cm of soil. Samples were taken from depths of 0–5, 5–20 and 20–40 cm six months after application. The introduction of SS led to an increase in the content of humus, mineral nitrogen (up to 2.5 times), mobile phosphorus (up to 30%) and exchangeable potassium (up to 12%), as well as the neutralization of the soil solution. At the same time, the content of mobile forms of heavy metals (HM) Mn, Cu, Zn, Co, Pb and Cd did not exceed 16.6% of the model predictive control (MPC), the content of gross forms of As, Zn, Pb, Hg, Cd, Cr, Co, Cu and Mn did not exceed 51% of the MPC, the total chemical index of the HM content was “acceptable” (2.6–10.2). Only the concentration of Ni, which was 0.75 MPC in the background soil, increased to 0.8 MPC. Consequently, the nickel content was a limiting factor in the introduction of SS, and it was a key element for monitoring the content of HM in soils.