Adsorption of fluometuron and norflurazon: effect of tillage and dissolved organic carbon

Adsorption is an important process that influences the fate and behavior of herbicides in soil. The herbicide adsorption process is influenced by both the dissolved and the solid soil organic carbon (DOC and SOC) content of soil. Both properties are affected by tillage management and biosolid utilization. This study was undertaken to examine the influence of tillage practice and various DOC sources and concentrations on the adsorption and desorption of fluometuron (N, N-dimethyl-N′-[3-(trifluoromethyl)phenyl] urea) and norflurazon (4-chloro-5(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone) in the Lexington silt loam (fine-silty, mixed, thermic, Typic Paleudalf). Batch adsorption/desorption studies were performed in the presence and absence of DOC extracted from poultry litter, dairy manure, and no-till surface soil. Herbicide adsorption is influenced strongly by soil organic carbon (SOC) content, with all soils having a greater affinity for norflurazon, relative to fluometuron. Average Koc values of 100 and 500 were determined for fluometuron and norflurazon. The influence of DOC on herbicide adsorption was a function of DOC source and concentration. Poultry litter DOC, characterized as having greater hydrophobicity and high-molecular-mass organic compound content, and dairy manure DOC (containing both low- and high-molecular-mass organic compounds) reduced herbicide adsorption. No-till DOC (hydrophilic with low molecular mass organic compounds) did not influence herbicide adsorption. In general, herbicide desorption was completely reversible. However, the equilibration period required to achieve reversibility varied with herbicide type, adsorption condition (DOC vs. no DOC), and DOC source and concentration in the desorption system. The reduction in herbicide adsorption in the presence of DOC could be attributed to the formation of soluble herbicide-DOC complexes. Further, herbicide desorption from mineral surfaces (low SOC soil) was observed to be more kinetically restricted relative to desorption from SOC (high SOC soil).