Copper and zinc aging in soils for a decade: changes in metal extractability and phytotoxicity

Environmental contextTrace metal toxicities to soils and plants depend on residence time in soils, a poorly understood phenomenon termed ‘aging’. Our research aimed to better understand long-term aging by measuring the solubility and toxicity of copper and zinc over a 10-year period after their addition to soils as soluble salts. We determined that, while metal solubility and toxicity did decrease in a decade, the highest levels of added metals (200 and 400mgkg–1) still had toxic effects on soybeans. AbstractTo assess long-term effects of field aging on Cu and Zn availability and phytotoxicity in soils, soils were spiked in the field using metal sulfate salts, and tested over 10 years for changes in total metals, salt-extractable (0.01M CaCl2) metals, Cu ion activity and phytoavailable metals using a soybean assay. Metal losses from the soils were generally small, with the coarse-textured (Arkport) soil having greater losses than the fine-textured (Hudson) soil. However, large reductions in salt-extractable metals occurred over the 10-year period, with most of this decline observed in the first several years following spiking. Copper ion activities decreased after 10 years of aging in all of the Cu-spiked soils, but remained high enough to be phytotoxic at metal loadings of 200 and 400mgkg–1. The soybean assay showed that Zn phytoavailability was significantly elevated in both soils at the loadings of 200 and 400mgkg–1 Zn. Higher plant tissue Cu was evident at additions of 200 and 400mgkg–1 Cu in the Arkport soil, but only at the 400mgkg–1 additions in the Hudson soil. Plant growth was significantly reduced at the 400mgkg–1 additions for both metals in both soils; growth inhibition at the 200mgkg–1 addition was also observed for both metals in both soils, but was not statistically significant for Zn. In summary, soils spiked with 200mgkg–1 (or more) of Cu or Zn salts express significant phytotoxicity after 10 years of field aging despite a shift of the metals into less labile forms.