Soil tests have been widely developed to predict trace metal uptake by plants. The prediction of metal toxicity, however, has rarely been tested. The present study was set up to compare 8 established soil tests for diagnosing phytotoxicity in contaminated soils. Nine soils contaminated with Zn or Cu by metal mining, smelting, or processing were collected. Uncontaminated reference soils with similar soil properties were sampled, and series of increasing contamination were created by mixing each with the corresponding soil. In addition, each reference soil was spiked with either ZnCl2 or CuCl2 at several concentrations. Total metal toxicity to barley seedling growth in the field-contaminated soils was up to 30 times lower than that in corresponding spiked soils. Total metal (aqua regia-soluble) toxicity thresholds of 50% effective concentrations (EC50) varied by factors up to 260 (Zn) or 6 (Cu) among soils. For Zn, variations in EC50 thresholds decreased as aqua regia > 0.43 M HNO3 > 0.05 M ethylenediamine tetraacetic acid (EDTA) > 1 M NH4 NO3 > cobaltihexamine > diffusive gradients in thin films (DGT) > 0.001 M CaCl2 , suggesting that the last extraction is the most robust phytotoxicity index for Zn. The EDTA extraction was the most robust for Cu-contaminated soils. The isotopically exchangeable fraction of the total soil metal in the field-contaminated soils markedly explained the lower toxicity compared with spiked soils. The isotope exchange method can be used to translate soil metal limits derived from soils spiked with metal salts to site-specific soil metal limits.

The toxicity of trichloroethene (TCE) likely restricts microbial activity in close vicinity of a TCE dense nonaqueous phase liquid (DNAPL). This study examined the distribution of a dechlorinating community in relation to the distance from a TCE DNAPL using a diffusion-cell set-up. Subcultures of the KB-1(™) culture dechlorinating TCE to cis-dichloroethene and grown with either formate or lactate as electron donor were used to inoculate the diffusion-cells. 16S rRNA gene clone library analysis showed that both inocula consisted of dechlorinating bacteria similar to Geobacter lovleyi SZ and fermentative microorganisms related to Clostridium and Clostridiales. qPCR and RFLP analyses of pore water and sand samples showed a stratified microbial community composition in the diffusion-cells. Geobacter dominated where TCE was present, that is, in the lower 3 cm of the 5.5-cm-thick sand layer. Even at 0.5 cm distance from the DNAPL layer, Geobacter densities were two orders of magnitude higher than at inoculation, despite the expected TCE toxicity. In the upper 2.5 cm of the sand layer, where TCE was depleted, apparently fermenting populations prevailed, corresponding to Clostridium in some diffusion-cells. This analysis demonstrates that the microbial community composition in a source zone is related to the distance from the DNAPL.