Effect of long-term application of pig slurry and NPK fertilizers on trace metal content in the soil

One of the goals of sustainable agricultural production is to avoid soil contamination by elements defined as trace metals (TMs). The aim of this study was to assess the long-term impact of the use of pig slurry (PS) and NPK mineral fertilizers on the soil content of cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn). In a 9-year crop rotation, PS was used three times only before root crops. The same four levels of NPK doses (N0P0K0, N1P1K1, N3P2K2, N4P2K2) were applied to both plots with and without PS. Soil samples were collected in early spring from topsoil (0–0.3 m) and subsoil (0.3–0.6 m). Three forms of TMs were determined in the soil: pseudo-total (Aqua regia); bioavailable (Mehlich 3 method) and readily bioavailable (mobile) forms (1 M NH4NO3). The tested factors did not have a significant impact on the Cd, Cu and Pb content, regardless of the form analyzed and the soil depth. PS application significantly increased the content of bioavailable forms of Zn regardless of the year, and the content of pseudo-total Zn only in the sugar beet year, i.e. after manure application. Increasing NPK doses increased the content of mobile Zn in the topsoil, especially in PS plots. A tendency to accumulate mobile forms of Cd and Pb was also observed on NPK-fertilized plots. Thus, long-term application of high NPK doses may increase the risk of contamination of the food chain with these metals. The content of mobile Cd and Zn was positively related to the content of total nitrogen in the soil and negatively related to pH.