Formation and fate of bound residues of [14C]benzene and [14C]chlorobenzenes in soil and plants.

Outdoor experiments with [14C]hexachlorobenzene, [14C]pentachlorobenzene, [14C]1,2,4-trichlorobenzene, and [14C]benzene in soil-crop systems indicate that the formation rate of bound residues in soil and plants, expressed as bound residues in percentage of total residue in the sample, decreases with increasing number of chlorine in the molecule and, thus, with increasing chemical stability. The time course of formation and fate of bound residues in soil and plants is characterized by a very slow decrease of residue levels in soil, indicating that biodegradation of bound residues hardly exceeds their reformation from the parent compound during one vegetation period, and by a decrease of residue levels in plants. The portion of bound residues as compared to the total residue increases with time, indicating that bound residues are more persistent than the parent compounds and their soluble metabolites; benzene is an exception. Cress plants, in general, contain less bound residues than do barley plants. Again, benzene is an exception. In deeper soil layers, soil-bound residues occur also. The ratio between bound and extractable residues does not differ to a larger extent between the soil layers.