Effect of concentration, sorption, and microbial biomass on degradation of the herbicide fluometuron in surface and subsurface soils

The variation in adsorption and degradation of fluometuron was determined in soil collected from different depths (0-120 cm) of a Beulah silt loam soil profile. Microbial demethylation was the initial degradation step in both surface and subsurface soils, resulting in accumulations of demethylfluometuron (DMFM). After addition of 1500 ng/g of soil of fluometuron, degradation was adequately described by first-order kinetics. However, these kinetics did not describe degradation after addition of 85 ng/g of soil of [ super(14)C-trifluoromethyl]fluometuron. At this lower concentration, degradation followed a lag period in the subsurface soil. Degradation rates decreased as depth increased, but rates were not correlated with adsorption coefficients (K sub(d)). Declines in microbial biomass and respiration with depth in the profile contributed to the reduced degradation rates in subsurface zones. Adsorption was primarily a first-order process, with K sub(d) values positively correlating with soil organic matter content. Approximately 56% of adsorbed fluometuron was desorbed in a single equilibration step, suggesting that a large fraction of adsorbed fluometuron was available for degradation. If fluometuron moves into subsurface soils, the decreased adsorption and degradation rates observed would increase potential leaching for fluometuron and its metabolites.