MICROBIAL-DEGRADATION AND HUMIFICATION OF THE LAWN CARE PESTICIDE 2,4-DICHLOROPHENOXYACETIC ACID DURING THE COMPOSTING OF YARD TRIMMINGS

The fate of the widely used lawn care herbicide 2,4-dichlorophenoxyace tic acid (2,4-D) during the composting of yard trimmings consisting of primarily leaves and grass is an important unexplored question. In th is study, we determined the extent of 2,4-D mineralization, incorporat ion into humic matter, volatilization, and sorption during the compost ing of yard trimmings. Yard trimmings (2:1 [wt/wt] leaves-grass) were amended with C-14-ring-labeled 2,4-D (17 mg/kg of dry weight) and comp osted in a temperature-controlled laboratory scale compost system. Dur ing composting, thermophilic microbes were numerically dominant, reach ing a maximum of 2 x 10(11)/g. At the end of composting, 46% of the or ganic matter (OM) present in the yard trimmings was lost and the compo st was stable, with an oxygen uptake rate of 0.09 mg of O-2 per g of O M per h, and was well humified (humification index, 0.39). Mineralizat ion of the OM temporally paralleled mineralization of 2,4-D. In the fi nal compost, 47% of the added 2,4-D carbon was mineralized, about 23% was complexed with high-molecular-weight humic acids, and about 20% wa s not extractable (humin fraction). Less than 1% of the added C-14 was present in water expressed from the finished compost, suggesting a lo w potential for leaching of 2,4-D. Very little volatilization of 2,4-D occurred during composting. It is of interest that our results indica te active mineralization of 2,4-D at composting temperatures of 60 deg rees C because microbial 2,4-D degradation at thermophilic temperature s has not been previously documented.