Effect of long-term equilibration on the toxicity of molybdenum to soil organisms

To determine if long-term equilibration may alleviate molybdenum toxicity, earthworms, enchytraeids, collembolans and four plant species were exposed to three soils freshly spiked with Na2MoO4.2H2O and equilibrated for 6 or 11 months in the field with free drainage. Total Mo concentrations in soil decreased by leaching, most (up to 98%) in sandy soil and less (54–62%) in silty and clayey soils. Changes in residual Mo toxicity with time were inconclusive in sandy soil. In the other two soils, toxicity of residual total Mo was significantly reduced after 11 months equilibration with a median 5.5-fold increase in ED50s. Mo fixation in soil, i.e. the decrease of soil solution Mo concentrations at equivalent residual total soil Mo, was maximally a factor of 2.1 only. This experiment shows natural attenuation of molybdate ecotoxicity under field conditions is related to leaching of excess Mo and other ions as well as to slow ageing reactions.

► Three molybdate-spiked soils were equilibrated for 6 and 11 months outdoors.
► Mo chronic toxicity to earthworms, enchytraeids, Collembola and four plant species was assessed.
► Mo concentrations in all soils decreased due to leaching.
► Based on actual total Mo remaining in the soil, Mo toxicity decreased by a median factor of 5.5.
► Decreased Mo toxicity was due to leaching as well as slow ageing reactions.