Soil solution concentrations and chemical species of copper and zinc in a soil with a history of pig slurry application and plant cultivation

• Pig slurry applications increases the concentration of Cu and Zn in soil and solution.
• Soluble Cu and Zn occurs in complexed forms when the soil is cultivated.
• Black oat and maize accumulate more Zn compared to Cu in shoot biomass.
• Dissolved organic carbon is the main binder of heavy metals in the soil solution.

Successive pig slurry applications may increase soil copper (Cu) and zinc (Zn) concentrations and change the proportions of free chemical species in solution when combined with plant cultivation. The aim of this study was to assess the soluble, available, and total Cu and Zn concentrations and the distribution of their chemical species in the solution in a Hapludalf soil with a history of pig slurry application and plant cultivation. The study was conducted in undisturbed soil columns that originated from an 8-year-long experiment conducted at the experimental unit of the Federal University of Santa Maria in Santa Maria, southern Brazil. The soil was a Typic Hapludalf soil fertilized with pig slurry at rates of 0, 20, 40, and 80 m3 ha−1. The soil was collected from depth intervals of 0–0.05, 0.05–0.1, 0.1–0.2, 0.2–0.3, 0.3–0.4, and 0.4–0.6 m before and after cultivation with black oat and maize in a greenhouse to assess the total and available Cu and Zn concentrations and to extract the solution. The soil solution concentrations of the main cations, anions, and dissolved organic carbon (DOC) and pH were assessed. The distribution of Cu and Zn chemical species was assessed using the Visual Minteq software. The history of 21 pig slurry applications increased the concentration of Cu and Zn in surface soil intervals, but the concentration of Cu also increased in the soil solution at depth. The phytotoxicity caused by Cu and Zn may not occur even after several years of pig slurry application because the plants provide soil conditions in which chemical species complexed with dissolved organic carbon predominate and Cu and Zn in free forms are present only in small amounts.