Evaluation of soil metal bioavailability estimates using two plant species (L. perenne and T. aestivum) grown in a range of agricultural soils treated with biosolids and metal salts

Few studies have quantified the accuracy of soil metal bioavailability assays using large datasets. A metaanalysis from experiments spanning 6 months to 13 years on 12 soil types, compared bioavailability estimate efficiencies for wheat and ryegrass. Treatments included biosolids metals, comparing total metal, Ca(NO3)2, EDTA, soil solution, DGT and free ion activity. The best correlations between soil metal bioavailability and shoot concentrations were for Ni using Ca(NO3)2 (r2 ¼ 0.72) which also provided the best estimate of Zn bioavailability (r2 ¼ 0.64). DGT provided the best estimate of Cd bioavailability, accounting for 49% of shoot Cd concentrations. There was no reliable descriptor of Cu bioavailability, with less than 35% of shoot Cu concentrations defined. Thus interpretation of data obtained from many soil metal bioavailability assays is unreliable and probably flawed, and there is little justification to look beyond Ca(NO3)2 for Ni and Zn, and DGT for Cd.