Acifluorfen Sorption, Degradation, and Mobility in a Mississippi Delta Soil

repulsion effects, acifluorfen is sorbed by soil or soil constituents (Pusino et al., 1991; Ruggiero et al., 1992; Potential surface water and groundwater contaminants include herPusino et al., 1993; Gennari et al., 1994b; Nègre et al., bicides that are applied postemergence. Although applied to the plant canopy, a portion of any application reaches the soil either directly 1995; Locke et al., 1997). Although the extent of sorpor via subsequent foliar washoff. This study examined sorption, degration in soil is generally proportional to OC content dation, and mobility of the postemergence herbicide acifluorfen (5-[2(Gennari et al., 1994b; Nègre et al., 1995; Locke et al., chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid) in Dundee 1997), sorption likely involves processes other than parsilty clay loam (fine-silty, mixed, thermic, Aeric Ochraqualf) taken titioning between aqueous and organic matter phases. from conventional till (CT) and no-till (NT) field plots. Homogeneous In particular, acifluorfen forms complexes with divalent surface and subsurface samples were used in the sorption and degradaand trivalent cations (Pusino et al., 1991; Pusino et al., tion studies; intact soil columns (30 cm long and 10 cm diam.) were 1993) that may be sorbed or precipitated. Complex forused in the mobility study. Batch sorption isotherms were nonlinear mation and subsequent sorption may partially account (Freundlich model) and sorption paralleled organic C (OC) content. for increased acifluorfen sorption with decreasing soil All tillage by depth combinations of soil exhibited a time-dependent approach to sorption equilibrium that was well described by a twopH or increasing cation exchange capacity (Pusino et site equilibrium–kinetic model. Acifluorfen degradation followed al., 1993; Gennari et al., 1994b; Locke et al., 1997). first-order kinetics. No more than about 6% of applied 14C-acifluorfen Also, acifluorfen sorption is a nonequilibrium, timewas mineralized by 49-d incubation. Extracts of incubated soil gave dependent process (Locke et al., 1997). Thus, the mobillittle indication of degradation products; however, 14C did accumulate ity of acifluorfen in soil is affected by the rate as well in an unextractable fraction. Degradation was faster in the surface soils as the maximum extent of sorption. compared to subsurface soils and faster in CT surface soil compared Photodegradation of nitrodiphenyl ethers may into NT surface soil. Tillage did not affect acifluorfen degradation in volve several different reactions including nitroreducsubsurface samples. Elution of Br pulses from the intact soil columns tion and hydrolysis of the ether linkage (Ruzo et al., under steady-state, unsaturated flow indicated preferential water flow. 1980). Depending on ring substituents, dechlorination, Nonequilibrium transport of Br was well described using a two-region, mobile–immobile water model. Inclusion of sorption kinetics in the decarboxymethylation (Ruzo et al., 1980), or, in the case transport model rather than assuming equilibrium sorption led to of acifluorfen, decarboxylation may occur (Pusino and improved predictions of acifluorfen retardation. Column effluent conGessa, 1991). Aside from photodegradation, chemical tained negligible concentrations of acifluorfen degradation products degradation of nitrodiphenyl ethers may be possible. In and, as in the incubation study, an unextractable residue developed particular, Ohyama and Kuwatsuka (1983) found that in the soil columns. However, unlike results from the incubation study, bifenox (methyl 5-[2,4-dichlorophenoxy]-2-nitrobenzoa greater fraction of applied acifluorfen was apparently bound and ate) underwent nitroreduction in sterilized soil. In sterile there was also evidence of extractable degradation products. Furthermicrobial culture media (Andreoni et al., 1994), howmore, first-order rate constants obtained from the batch study underever, acifluorfen does not undergo degradation. Similar estimated acifluorfen degradation during transport. Faster acifluorfen to degradation data for bifenox (Ohyama and Kuwatdegradation in the soil columns may have been due to poorer aeration compared to the batch systems. suka, 1983), Andreoni et al. (1994) found that acifluorfen biodegradation in microbial cultures was more rapid under anaerobic rather than aerobic conditions. Although acifluorfen biodegradation may largely be a T fate and transport of postemergence herbicometabolic process (Andreoni et al., 1994), certaincides in the soil environment have received little bacterial strains are capable of metabolizing the herbiattention. Yet these compounds come in contact with cide (Fortina et al., 1996). Degradation products of the soil, directly or as foliar washoff (Reddy et al., 1994), acifluorfen isolated from microbial cultures include and are subject to leaching and runoff. Acifluorfen aminoacifluorfen, 5-([2-chloro-4-(trifluoromethyl)phe(5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobennyl]oxy)-2-aminobenzamide, and 5-([2-chloro-4-(trifluzoic acid) is a nitrodiphenyl ether postemergence herbioromethyl)phenyl]oxy)-2-(acetylamino)benzoic acid cide (applied as the Na salt) that is widely used for (Gennari et al., 1994a). Aminoacifluorfen has been rethe control of certain broadleaf weeds in soybean and covered from soil treated with acifluorfen that was incupeanut crops. bated for 6 d (Locke et al., 1997). It exhibits a low pKa of 3.5 (Roy et al., 1983) and is The primary objective of the present study was to highly dissociated at typical soil pHs. Despite charge quantify the degradation and sorption of acifluorfen in a Mississippi Delta soil and determine whether acifluorL.A. Gaston, Dep. of Agronomy, Louisiana State Univ. Agricultural fen fate and mobility in this soil may be accurately Center, 104 Madison Sturgis Hall, Baton Rouge, LA 70803; and described on the basis of these underlying processes. M.A. Locke, USDA-ARS, Southern Weed Sci. Unit, P.O. Box 350, Stoneville, MS 38776. Received 12 Aug. 1998. *Corresponding author Abbreviations: CT, conventional till; HPLC, high pressure liquid chro([email protected]). matography; NT, no-till; OC, organic carbon; TLC, thin-layer chromatography. Published in Soil Sci. Soc. Am. J. 64:112–121 (2000).