Evaluation of heavy metal impacts in soil systems based on nitrification

Heavy metals can adversely affect soil microbes. The effect of five spiked heavy metals in soil, Zn, Cu, Ni, Cd, and Pb, on nitrification was studied. Zinc showed the strongest inhibitory effect on nitrification followed by Cu and Ni. Lead and Cd did not affect nitrification. The best fraction that gave a common threshold for the soils studied was the water-soluble Zn and Cu. This approach did not applied to Ni. Three approximate ranges of nitrification inhibition were identified for Zn extracted with water at a weight ratio 40 to 1 with soil: 0 to 0.125 mg Zn L−1—no inhibition; 0.125 to 0.5 mg Zn L−1—partial inhibition; and over 0.5 mg Zn L−1—complete inhibition. More than 3.8 mg water-extractable Cu kg−1 soil inhibited nitrification, while less than 2 mg kg−1 was safe. The interval between these two limits was partially inhibitory. The weight ratio soil to water for Cu extraction was 1:10. Zinc was also nitrification inhibitor when added with spiked biosolids. Leached Zn concentrations that caused partial nitrification inhibition were 5 to 19 mg kg−1 and 6.6 to 10.5 mg kg −1 in a sandy loam and sand, respectively. Nitrification as a bottleneck in the N cycle influences further the heavy metal release from biosolids into the soil solution. Here, the inhibited nitrification was always associated with a reduced release of metals. Excessive Zn was thus an inhibitor not only of nitrification, but also of its own release into the soil solution. It appears that low concentrations of heavy metals in the solution of biosolid-amended soils are not always ecologically safe enough. They are safe in terms of phytotoxicity, but in a latent way can adversely affect the soil microbes. The ratio C:N, which controls the decomposition rates, resulted the key factor to explain the impacts of studied heavy metals in the biosolid-amended soils. On the other hand, factors such as exchange reactions, temperature, or even pH did not play any important role in the heavy metals release in biosolid-amended soils.