Aged and bound herbicide residues in soil and their bioavailability. Part 2: Uptake of aged and non-extractable (bound) [carbonyl-14C]methabenzthiazuron residues by maize.

[Carbonyl-14C]methabenzthiazuron (MBT) was applied to growing winter wheat in an outdoor lysimeter. The amount applied corresponded to 4 kg Tribunil/ha. 140 days after application the 0-2.5 cm soil layer was removed from the lysimeter. This soil contained about 40% of the applied radioactivity. Using 0,01 M CaCl2 solution or organic solvents, the extractable residues were removed from the soil. The bioavailability of the non-extractable as well as aged residues remaining in the soil was investigated in standardized microecosystems containing 1.5 kg of dry soil. During a 4 weeks period the total uptake (4 maize plants/pot) amounted up to 3.6; 2.2; and 0.9% of the radioactivity from soils containing aged MBT residues, MBT residues non-extractable with 0.01 M CaCl2 or MBT residues non-extractable with organic solvents, respectively. About 20% of the radioactivity found in maize leaves represented chromatographically characterized parent compound. At the end of the plant experiment the soil was extracted again with 0.01 M CaCl2 and with organic solvents. The soil extracts and also the organic phases obtained from the aqueous fulvic acid solution contained unchanged parent compound.