Use of Zinc Carbonate Spiking to Obtain Phytotoxicity Thresholds Comparable to Those in Field-Collected Soils

Several studies have reported the presence of smithsonite (ZnCO₃) in soils polluted by zinc mining. The present study aimed to determine upper critical threshold values of Zn phytotoxicity in a substrate spiked with ZnCO₃ and to compare them with those obtained in field-collected soils. We studied Zn toxicity to perennial ryegrass (Lolium perenne L.) grown in pots with unpolluted peat treated with increasing concentrations of ZnCO₃ that produced nominal total Zn concentrations of 0, 0.7, 1.3, 2.0, 2.6, and 3.3%. To keep constant near-neutral pH value in all the treatments, we used decreasing concentrations of dolomitic lime. In the treatment with total soil Zn of 3.3% (pH 6.8), the foliar Zn concentration of L. perenne was 1914 ± 211 mg kg^–1, falling into the range of 2400 ± 300 mg kg^–1 reported for Lolium species grown under similar laboratory conditions in a polluted soil (total soil Zn 5.4%, pH 7.3) collected near a Zn smelter. The value of 92 ± 98 mg kg^–1 was obtained for the median effective concentration (EC50) values of 0.01 M KNO₃-extractable Zn using the responses of shoot dry biomass, shoot length, and total pigments. This value falls within the range of 95 ± 46 mg kg^–1 reported in other studies for the EC50 values of salt-extractable Zn using field-collected soils. The application of ZnCO₃ for spiking was able to mimic foliar Zn concentrations of Lolium species observed in field-collected soils. The effective concentrations of soil Zn obtained in the present study are comparable to those obtained in field-collected soils. Future research should determine effective concentrations of metals using soils spiked with metal-containing compounds that mimic a real source of contamination. Environ Toxicol Chem 2020;39:1790–1796.