Fate of C-14-ring-labeled 2,4-D, 2,4-dichlorophenol and 4-chlorophenol during straw composting

Experiments were carried out to study the transformation of C-14-ring-labeled 2,4-D and the two related chlorophenols 4-chlorophenol (4-CP) and 2,4-dichlorophenol (4-DCP) during straw composting under controlled laboratory conditions. Incubation under sterile and nonsterile conditions was done to evaluate the relative importance of the biotic and abiotic processes. Pre-composted straw was treated with the three chemicals. The availability of the different chemicals was monitored during incubations as well as their degradation. Under nonsterile conditions, the mineralization of both chlorophenols reached 20% of the applied compounds, whereas it was 52% for 2,4-D. Transitory water-soluble metabolites of 2,4-D and chlorophenols were formed but they disappeared rapidly. After 21 days, 21% of the 2,4-D and 38% of the 2,4-DCP was stabilized as nonextractable (bound) residues under nonsterile conditions. Bound residues of both chemicals were negligible under sterile conditions. Availability of chemicals as estimated by water extraction decreased during incubation proportionally to mineralization and to the formation of bound residues. The increase in immobilization of the chemical residues was stronger under nonsterile conditions than under sterile conditions. Under nonsterile conditions 71% of the 4-CP was recovered as bound residues, whereas under sterile conditions 30% of the applied 4-CP formed bound residues after formaldehyde addition and only 8% with autoclaved straw. Global microbial activity decreased in the presence of the chlorophenols probably due to their toxic effect. These data indicate that the biological activity associated with straw transformation during composting stimulates the depletion of 2,4-D and chlorophenols by mineralization and by formation of bound residues.