Nitrates and heavy metals in agrocenoses with prolonged use of organic fertilizers

In field experiments on sod-podzolic soils, the agroecological effectiveness of long-term action of cattle manure of various humidity was studied. It is shown that the use of semi-liquid bedless and litter manure, when optimizing doses and combinations with mineral fertilizers, improved soil fertility, increased the productivity of crops and crop rotations, ensured the environmental safety of agrocenoses, reduced the risks of accumulation of nitrates and heavy metals in soil and plants. The annual application of semi-liquid manure for 15 years to sod-podzolic heavy loamy soil in the forage crop rotation in the variant of the organo-mineral fertilizer system with a content of 240 kg N/ha ensured their high efficiency – 9.8 t c. u./ ha, which was 96% more than the control. The resulting feed was characterized by high protein nutrition, and the nitrate content corresponded to the norms of feeding farm animals. The use of increasing doses of semi-liquid manure increased the productivity of forage crop rotation from 7.28 to 10.3 tons of grain/ha in variants using a single (N120) to a fourfold dose (N580). Increasing the dose of manure to N600 proved ineffective. Depending on the dose of manure, the nitrate content in the layers of the soil profile changed. Nitrates accumulated most intensively (3.7 times more than in the control) in a layer of 0–100 cm of soil in the variant with the maximum dose of manure (N600). At the same time, there was no excess of nitrate content in the soil above the MPC (130 mg NO₃^–/kg). A correlation has been established between the accumulation of nitrates and the increase in nitrogen doses introduced with manure in soil layers, especially in the upper 0–100 and 100–200 cm, with correlation coefficients r = 0.73 and 0.60. With prolonged use of litter manure on sod-podzolic light loamy soil, its combination with mineral fertilizers at a dose of N180 was effective, where the annual harvest of grain units averaged 3.1 tons per hectare over 37 years, or exceeded the control by 38.5%. At the same time, vegetable products were of high quality – the grain contained up to 14.2% protein, potato tubers – up to 13% starch at an acceptable level of nitrate content. The content of heavy metals (Cd, Pb, Zn, Cu, Ni, Hg) and arsenic, as well as nitrates in the soil, depended on the intensity of fertilization with litter manure, but generally corresponded to acceptable standards.