Studies have been carried out in the southern forest-steppe of the Republic of Bashkortostan on agrochernozem with a heavy loamy granulometric composition, high humus content, a close to neutral reaction of the environment, and a high cation exchange capacity with a predominance of calcium. Sewage sludge from the Ufa wastewater treatment plants were applied at a dose of 30 t/ha and plowed in the top 20-cm layer of soil. Samples have been taken from the depths of 0–5, 5–20, and 20–40 cm six months after application. The addition of sewage sludge leads to increase in the content of humus, mineral nitrogen (up to 2.5 times), available phosphorus (up to 30%), and exchangeable potassium (up to 12%), as well as neutralization of the soil solution. At the same time, the content of mobile forms of Mn, Cu, Zn, Co, Pb, and Cd does not exceed 16.6% of the MPC; the content of gross forms of As, Zn, Pb, Hg, Cd, Cr, Co, Cu, and Mn does not exceed 51% of MPC; and the total chemical indicator of the heavy metal content is “acceptable” (2.6–10.2). Only the concentration of Ni, which in the control variant is 0.75 MPC, increases to 0.8 MPC. Consequently, the Ni content is limiting factor for application of sewage sludge and is a key element for monitoring the content of heavy metals in soils.
Melentyeva et al. (Mon,) studied this question.