• Adsorption/desorption;
  • Bioavailability;
  • Dose–response modeling;
  • Ecotoxicology;
  • Metal


Previous studies have shown that toxicity of cationic trace metals in soil is partially confounded by effects of the accompanying anions. A similar assessment is reported here for toxicity of an oxyanion, i.e., molybdate (MoOmath image), the soil toxicity of which is relatively unexplored. Solubility and toxicity were compared between the soluble sodium molybdate (Na2MoO4) and the sparingly soluble molybdenum trioxide (MoO3). Confounding effects of salinity were excluded by referencing the Na2MoO4 effect to that of sodium chloride (NaCl). The pH decrease from the acid MoO3 amendment was equally referenced to a hydrochloric (HCl) treatment or a lime-controlled MoO3 treatment. The concentrations of molybdenum (Mo) in soil solution or calcium chloride (CaCl2) 0.01 M extracts were only marginally affected by either MoO3 or Na2MoO4 as an Mo source after 10 to 13 days of equilibration. Effects of Mo on soil nitrification were fully confounded by associated changes in salinity or pH. Effects of Mo on growth of wheat seedlings (Triticum aestivum L) were more pronounced than those on nitrification, and toxicity thresholds were unaffected by the form of added Mo. The Mo thresholds for wheat growth were not confounded by pH or salinity at incipient toxicity. It is concluded that oxyanion toxicity might be confounded in relatively insensitive tests for which reference treatments should be included. Environ. Toxicol. Chem. 2010;29:1274–1278. © 2010 SETAC