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DIVISION S-2-SOIL CHEMISTRY

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

^a Dep. of Agronomy, Louisiana State Univ. Agricultural Center, 104 Madison Sturgis Hall, Baton Rouge, LA 70803 USA
^b USDA-ARS, Southern Weed Sci. Unit, P.O. Box 350, Stoneville, MS 38776 USA

lgaston{at}agctr.lsu.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Potential surface water and groundwater contaminants include herbicides that are applied postemergence. Although applied to the plant canopy, a portion of any application reaches the soil either directly or via subsequent foliar washoff. This study examined sorption, degradation, and mobility of the postemergence herbicide acifluorfen (5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid) in Dundee silty clay loam (fine-silty, mixed, thermic, Aeric Ochraqualf) taken from conventional till (CT) and no-till (NT) field plots. Homogeneous surface and subsurface samples were used in the sorption and degradation studies; intact soil columns (30 cm long and 10 cm diam.) were used in the mobility study. Batch sorption isotherms were nonlinear (Freundlich model) and sorption paralleled organic C (OC) content. All tillage by depth combinations of soil exhibited a time-dependent approach to sorption equilibrium that was well described by a two-site equilibrium–kinetic model. Acifluorfen degradation followed first-order kinetics. No more than about 6% of applied ¹⁴C-acifluorfen was mineralized by 49-d incubation. Extracts of incubated soil gave little indication of degradation products; however, ¹⁴C did accumulate in an unextractable fraction. Degradation was faster in the surface soils compared to subsurface soils and faster in CT surface soil compared to NT surface soil. Tillage did not affect acifluorfen degradation in subsurface samples. Elution of Br pulses from the intact soil columns under steady-state, unsaturated flow indicated preferential water flow. Nonequilibrium transport of Br was well described using a two-region, mobile–immobile water model. Inclusion of sorption kinetics in the transport model rather than assuming equilibrium sorption led to improved predictions of acifluorfen retardation. Column effluent contained negligible concentrations of acifluorfen degradation products and, as in the incubation study, an unextractable residue developed in the soil columns. However, unlike results from the incubation study, a greater fraction of applied acifluorfen was apparently bound and there was also evidence of extractable degradation products. Furthermore, first-order rate constants obtained from the batch study underestimated acifluorfen degradation during transport. Faster acifluorfen degradation in the soil columns may have been due to poorer aeration compared to the batch systems.

Abbreviations: CT, conventional till • HPLC, high pressure liquid chromatography • NT, no-till • OC, organic carbon • TLC, thin-layer chromatography

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
THE FATE AND TRANSPORT OF POSTEMERGENCE HERBICIDES

in the soil environment have received little attention. Yet these compounds come in contact with the soil, directly or as foliar washoff (Reddy et al., 1994), and are subject to leaching and runoff. Acifluorfen (5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid) is a nitrodiphenyl ether postemergence herbicide (applied as the Na salt) that is widely used for the control of certain broadleaf weeds in soybean and peanut crops.

It exhibits a low pK_a of 3.5 (Roy et al., 1983) and is highly dissociated at typical soil pHs. Despite charge repulsion effects, acifluorfen is sorbed by soil or soil constituents (Pusino et al., 1991; Ruggiero et al., 1992; Pusino et al., 1993; Gennari et al., 1994b; Nègre et al., 1995; Locke et al., 1997). Although the extent of sorption in soil is generally proportional to OC content (Gennari et al., 1994b; Nègre et al., 1995; Locke et al., 1997), sorption likely involves processes other than partitioning between aqueous and organic matter phases. In particular, acifluorfen forms complexes with divalent and trivalent cations (Pusino et al., 1991; Pusino et al., 1993) that may be sorbed or precipitated. Complex formation and subsequent sorption may partially account for increased acifluorfen sorption with decreasing soil pH or increasing cation exchange capacity (Pusino et al., 1993; Gennari et al., 1994b; Locke et al., 1997). Also, acifluorfen sorption is a nonequilibrium, time-dependent process (Locke et al., 1997). Thus, the mobility of acifluorfen in soil is affected by the rate as well as the maximum extent of sorption.

Photodegradation of nitrodiphenyl ethers may involve several different reactions including nitroreduction and hydrolysis of the ether linkage (Ruzo et al., 1980). Depending on ring substituents, dechlorination, decarboxymethylation (Ruzo et al., 1980), or, in the case of acifluorfen, decarboxylation may occur (Pusino and Gessa, 1991). Aside from photodegradation, chemical degradation of nitrodiphenyl ethers may be possible. In particular, Ohyama and Kuwatsuka (1983) found that bifenox (methyl 5-[2,4-dichlorophenoxy]-2-nitrobenzoate) underwent nitroreduction in sterilized soil. In sterile microbial culture media (Andreoni et al., 1994), however, acifluorfen does not undergo degradation. Similar to degradation data for bifenox (Ohyama and Kuwatsuka, 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 cometabolic process (Andreoni et al., 1994), certain- bacterial strains are capable of metabolizing the herbicide (Fortina et al., 1996). Degradation products of acifluorfen isolated from microbial cultures include aminoacifluorfen, 5-([2-chloro-4-(trifluoromethyl)phenyl]oxy)-2-aminobenzamide, and 5-([2-chloro-4-(trifluoromethyl)phenyl]oxy)-2-(acetylamino)benzoic acid (Gennari et al., 1994a). Aminoacifluorfen has been recovered from soil treated with acifluorfen that was incubated for 6 d (Locke et al., 1997).

The primary objective of the present study was to quantify the degradation and sorption of acifluorfen in a Mississippi Delta soil and determine whether acifluorfen fate and mobility in this soil may be accurately described on the basis of these underlying processes. Sorption (isotherm and kinetics) and degradation were examined using batch systems. Intact, water-unsaturated soil columns were used to generate data on acifluorfen mobility. Because microenvironmental conditions within such soil columns may more closely match those in field soil than do conditions in batch (aerobic or anaerobic) degradation systems, data for acifluorfen mobility were also used to estimate degradation rates. A secondary objective was to assess the effects of 4 yr of no tillage (NT), compared to conventional tillage (CT), on acifluorfen degradation, sorption, and mobility.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Soil
Dundee silty clay loam soil (fine-silty, mixed, thermic, Aeric Ochraqualf) was taken from CT and NT plots (subject to cotton [Gossypium hirsutum] cropping management) following spring cultivation (tillage operations are listed in Gaston et al. [1996]). No acifluorfen had been applied to this site for four or more years and the soils contained no detectable acifluorfen (extraction and analysis described later).

Samples included intact columns of soil (PVC pipe 10.2 cm i.d. and 30.0 cm long) and bulk soil. Collection of intact soil cores is described in Gaston and Locke (1996). Bulk samples were collected from the 0- to 10-, 10- to 20-, and 20- to 30-cm depths. Bulk soil used in the sorption experiments was air-dried, ground, mixed, and sieved (<2 mm), whereas soil for the degradation experiment was mixed and stored at 4°C until used. Chemical data for the 0- to 10-, 10- to 20-, and 20- to 30-cm depths of the CT and NT soils are given in Table 1 .

Sorption Kinetics
Triplicate 5-g (oven-dry equivalent) samples of 0- to 10-, 10- to 20-, and 20- to 30-cm depth CT or NT soil were shaken with 15 mL of 10.0 µM Na acifluorfen (containing 33 Bq mL^{-1 14}C-acifluorfen) in 0.01 M CaCl₂ for periods of time up to 32 h. Soil solution was separated from the suspension as above and sorption calculated by change in solution radioactivity.

Degradation
Twenty-five g (oven-dry equivalent) of the 0- to 10-cm CT, 0- to 10-cm NT, 20- to 30-cm CT, or 20- to 30-cm NT soils were transferred to biometer flasks (Bartha and Pramer, 1965) or 200-mL centrifuge bottles. Sufficient Na acifluorfen solution was added to each field-moist sample to supply 0.875 µmol acifluorfen (containing 5100 Bq ¹⁴C-acifluorfen) and elevate soil moisture content to 35% by weight. Ten milliliters of 1 M NaOH was added to the side arm of each biometer flask and the flasks closed to the atmosphere. Samples in the centrifuge bottles were extracted (procedure discussed below) within 30 min after application. Soil samples were incubated at 25°C in the dark. After 7, 14, 21, 28, 35, and 49 d, soil was quantitatively transferred to 200-mL centrifuge bottles and extracted for acifluorfen and metabolites. Each soil by sampling time combination was replicated three times. The NaOH solutions were removed from all remaining biometer flasks semiweekly and fresh NaOH added. Aliquots were analyzed for ¹⁴CO₂ by liquid scintillation counting.

Soil was extracted using 50 mL of a 60:40 methanol:water solution that was shaken for 24 h. Suspensions were centrifuged (12 min at 8000 g), then supernatants decanted. A 1-mL aliquot was withdrawn for ¹⁴C analysis and the remainder saved. The soil was extracted a second time for 3 h and then centrifuged. Supernatants were analyzed for ¹⁴C and combined with the prior samples. The entrained extractant was allowed to evaporate before the air-dry soil was removed, ground, and duplicate 0.3-g samples combusted (Packard Oxidizer 306, Packard Instruments) to determine unextractable ¹⁴C (corrected for contribution from the entrained solution).

Methanol in supernatants was evaporated under vacuum at 45°C (Savant Speedvac SS3, Savant Instruments, Farmingdale, NY). The resulting aqueous concentrates were acidified to pH 2 with 1 M HCl and passed through C₁₈ solid-phase extraction columns (J.T. Baker, Phillipsburg, NJ). Polar metabolites in the aqueous effluent were measured as ¹⁴C. Sorbed acifluorfen and any moderately polar metabolites were eluted from the C₁₈ columns with 3 mL methanol; these concentrated samples were then analyzed using an HPLC system (Waters, Milford, MA) consisting of a Maxima controler, 510 pump, 710B autosampler, and 490 UV detector. An Alltima column (Alltech, Deerfield, IL) and 60:40 acetonitrile:water (pH 3.2) eluant were used. Further details on the HPLC method are given in Locke et al. (1997). The system also included an in-line liquid scintillation counter ("-Ram detector, INUS Systems, Tampa, FL). Detection limit for acifluorfen was <0.1 µM (50-µL injection volume).

Concentrated samples were also analyzed by thin-layer chromatography (TLC). Fifty-microliter aliquots were spotted on preadsorbent silica gel plates (20 x 20 cm, 250-µm gel, Whatman, Clifton, NJ), and developed to 10 cm using toluene:ethyl acetate:acetic acid:water (100:100:2:1, volume). The distribution of ¹⁴C was revealed using a Bioscan System 200 imaging scanner (Bioscan, Washington, DC).

Mobility Study
The system and procedure used to establish steady-state unsaturated flow through the intact soil columns have been previously described (Gaston and Locke, 1996). Briefly, the bottom of each soil column was covered with a porous glass membrane (11.0 cm diam., Whatman) and sealed with a PVC end-plate assembly that drained into a vacuum flask. A headspace extension attached to the top of the 30-cm PVC pipe opened to a CO₂ trap and supported a sprinkler head. A layer of acid-washed gravel protected the soil surface from the impact of sprinkler drops.

Calcium chloride solution (0.01 M) was applied at constant intensity by using a metering pump (model G6 RH1, FMI, Oyster Bay, NY) and constant suction head (60 cm water) was maintained at the bottom. The application rate (1.56 cm d^-1, average for all columns) was less than the saturated conductivity of the least conductive soil columns (low end of range as determined in a preliminary study). Steady-state conditions were assumed to exist when the time-averaged change in mass of a soil column reached approximately zero.

Once apparent steady-state flow existed, the gravel was removed and 10.0-mL pulses of 1.0 M KBr, followed by 529 µM Na acifluorfen (containing 4475 Bq mL^-1, ¹⁴C-acifluorfen), were uniformly applied to the soil surface (pulse and sprinkler infiltration rates equal) using glass pipets. Gravel was then replaced and sprinkler infiltration continued.

Column effluent was sampled twice daily through about two cumulative pore volumes and one daily thereafter. Aliquots were saved for Br and ¹⁴C analyses. The remainder was pooled into a series of 10 samples, each including effluent for about a 4-d flow period. Bromide was determined using ion chromatography (DX-100, DIONEX, Sunnyvale, CA). The NaOH solutions were replaced weekly and activity of trapped ¹⁴CO₂ measured.

Pooled samples were concentrated before HPLC analysis. Two hundred mL of each aqueous fraction were acidified and passed through a C₁₈ solid-phase extraction column as described earlier. Aqueous effluent from extraction columns was checked for polar ¹⁴C metabolites. Acifluorfen and moderately polar metabolites were then eluted with 3 mL of methanol.

The leaching experiment was terminated 40 d after application of acifluorfen and the change in mass of each column measured. The leaching apparatus was disassembled, soil carefully pushed (from bottom up) out of the PVC casing, and sectioned into 5.0-cm increments. There was no apparent soil compaction during removal. About 50-g subsamples from each section were transferred to 250-mL centrifuge bottles for extraction of acifluorfen and degradation products. Volumetric water content and bulk density with depth were calculated using the remaining soil from each section, corrected for the subsample removed. Soil subsamples were extracted as in the degradation experiment, however, with 100 mL of extractant. Aliquots of supernatant were measured for ¹⁴C activity. The entrained extracting solution was allowed to evaporate and unextractable ¹⁴C determined from duplicate 0.3-g, air-dry, ground samples upon combustion. Methanol in supernatants was evaporated and aqueous solutions concentrated and analyzed by HPLC as described above.

Models
Batch Systems
Sorption kinetics were described by the two-site equilibrium–kinetic model that assumes instantaneous sorption at a fraction of sites (type 1) as

(1a)

where S₁ is sorbed concentration (µmol kg^-1), C is solution concentration (µM), and k_e (L kg^-1) and N are empirical constants. Sorption at remaining sites (type 2) was described by Nth-order kinetics as

(1b)

where k_f (L kg^-1 d^-1) and k_r (d^-1) are forward and reverse rate constants, respectively. Since S = S₁ + S₂ and k_e + k_f/k_r = K_F (L kg^-1), Eq. 1a and 1b reduce to the Freundlich model, S = K_FC^N at equilibrium.

The first-order kinetic model was used to describe acifluorfen degradation as

(2)

where M is substrate mass (µmol) and k_d (d^-1) is the degradation rate constant.

Transport Systems
The mobility of acifluorfen under steady-state flow through the intact soil columns was described using the two-region model presented by van Genuchten and Wagenet (1989), which was adapted to account for variability in soil properties with depth (Gaston and Locke, 1996) and sorption kinetics:

(3a)

(3b)

where subscripts M and IM refer to mobile and immobile water regions, respectively; subscripts 1 and 2 refer to equilibrium- and kinetic-type sorption sites, respectively; subscript j denotes the j^th of N depth increments; is volumetric water content; is bulk density (Mg m^-3); D is the dispersion coefficient (cm² d^-1); v is pore water velocity (cm d^-1); is the mass transfer coefficient for diffusion between mobile and immobile water regions (d^-1); t is time (d); and z is depth (cm). The functions F and G describe acifluorfen degradation in the mobile and immobile water regions, respectively.

Boundary and initial conditions were

(3c)

(3d)

(3e)

(3f)

(3g)

(3h)

where C₀ is concentration of the input pulse (µmol L^-1), L is column length (cm), and t_p is pulse duration (d).

The two-site sorption model for mobile and immobile water regions was

(4a)

(4b)

(4c)

(4d)

where f_j is the fraction of sorption sites in the mobile water region of depth increment j. This fraction was assumed proportional to the fraction of mobile water, f_j = _M,j/_j.

If sorption kinetics are ignored, Eq. 4a–4d reduce to corresponding Freundlich models for mobile and immobile water regions written as

(4e)

(4f)

No attempt was made to distinguish between degradation occurring in solution and possibly in the sorbed phase; rather, these processes were lumped, so that

(5a)

(5b)

where k_M,j and k_IM,j (d^-1) are first-order degradation rate constants for the two regions in depth increment j.

Additionally, _M,j was assumed to be directly proportional to total volumetric water content, , and D_j and _j directly proportional to pore water velocity, . Justifications for these assumptions have been discussed in Gaston and Locke (1996). The parameters , , and were determined from tracer elution curves. Bulk density _j and total volumetric water content _j were determined from masses of column sections before and after drying.

Approximate solutions were generated using an implicit finite difference method. A least-squares procedure (van Genuchten, 1981), coupled with either the transport model (Eq. [3]–[5]) or batch sorption kinetics model (Eq. [2a] and [2b]), was used for parameter estimation.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Sorption Isotherms
Freundlich models for acifluorfen sorption (Fig. 1) indicated greater sorption in the Dundee NT compared with the CT surface soil. This is consistent with the slightly greater OC content of the NT soil (Table 1). In general, the extent of acifluorfen sorption paralleled OC content, decreasing with increasing depth below the soil surface. Also, greater sorption in the CT subsurface, compared with the NT subsurface (10 to 20 cm and 20 to 30 cm), was consistent with slightly higher OC content in the CT soil samples. However, differences due to tillage at the three depths were sufficiently small and uncertainty in Freundlich parameters (Table 2) sufficiently high that models for sorption in CT and NT soil at any depth were not significantly different (Taylor series approximation method of Hinds and Milliken [1987]; data not shown). On the other hand, comparison of sorption at each initial acifluorfen concentration (Table 3) shows that sorption was generally greater in the NT surface soil than in the corresponding CT soil and that sorption was generally greater in the subsurface CT soils than in corresponding NT soils.

View larger version (27K): [in this window] [in a new window]   Fig. 1 Acifluorfen sorption isotherms for three depth increments in Dundee conventional till (CT) and no-till (NT) soil. Curves show best-fits of the Freundlich model to each experimental isotherm

View larger version (14K): [in this window] [in a new window]   Fig. 2 Acifluorfen sorption kinetics for: (A) 0–10 cm, (B) 10–20 cm, and (C) 20–30 cm depth increments in Dundee conventional till (CT) and no-till (NT) soil. Curves show best-fits of the two-site equilibrium–kinetic model to each time-course data set. All data are relative to equilibrium sorption

Slow mineralization of acifluorfen was consistent with negligible concentration of extractable degradation intermediates. Despite the generally slow rate of evolution of ring-¹⁴C as radio-labeled CO₂, the data revealed differences among the four soils due to tillage and depth below the soil surface (Table 4). Mineralization was generally faster in the surface soils than in the corresponding subsurface soils and faster in the 0- to 10-cm CT soil than in the corresponding NT surface soil. Relative mineralization among the four soils was consistent with extent of acifluorfen degradation and accumulation of unextractable ¹⁴C (Table 4).

Acifluorfen degradation was generally greater in the surface soils for incubation times beyond 14 d (Table 4). By Day 21, degradation was generally greater in the surface CT than in the respective NT soil. There was no effect due to tillage in the subsurface soils. Acifluorfen degradation was described by first-order kinetics (Eq. [2]), as shown in Fig. 3 . The modeled data indicate faster degradation in the CT surface soil than in corresponding NT soil, faster degradation in the surface soils than in subsurface soils, but no difference due to tillage in the subsurface soils. Rate constants are given in Table 5 .

View larger version (20K): [in this window] [in a new window]   Fig. 3 Degradation kinetics of acifluorfen in surface and subsurface Dundee conventional till (CT) and no-till (NT) soil. Lines show best-fits of the first-order model to each experimental data set

Preferential Flow through Intact Soil Columns
All Br elution curves indicate some degree of preferential flow. Examples in Fig. 4 for columns CT 1 and NT 2 show a range of Br peak concentrations from about 0.2 to 0.4 pore volumes earlier, respectively, than expected in the absence of preferential flow. The mobile–immobile water model (Eq. [3], without sorption or degradation) was capable of providing good descriptions of Br elution in all cases. Optimized parameters are given in Table 6 . Measured variation in soil bulk density and water content with depth below the surface of each soil column is shown in Table 7 . Small values (Table 6) indicate slow mass transfer between mobile and immobile water regions.

View larger version (16K): [in this window] [in a new window]   Fig. 4 Example Br elution curves from two intact soil columns showing the effect of preferential water flow. Curves are best-fits of the two-region mobile–immobile water model

View this table: [in this window] [in a new window]   Table 7 Volumetric water content () and bulk density () at various depths in the Dundee soil columns

View larger version (13K): [in this window] [in a new window]   Fig. 5 Effect of tillage on elution of acifluorfen pulses applied to the surface of intact columns of Dundee conventional till (CT) and no-till (NT) soil. Average relative concentrations are shown

Simulation Results
Bromide elution curves indicated that water flow through the intact soil columns bypassed a portion of the total pore water volume (Fig. 4). Results of the batch sorption study showed that acifluorfen sorption in Dundee CT and NT soils is time-dependent (Fig. 2a–2c). Thus, it seems possible that sorption kinetics as well as preferential water flow might affect acifluorfen transport. Figure 6 shows ¹⁴C-acifluorfen effluent concentrations for column CT 1 compared to simulations assuming either instantaneous sorption equilibrium (Eq. [4e]–[4f]) or time-dependent sorption (Eq. [4a]–[4d]). Sorption parameters given in Table 2 were used. Inclusion of sorption kinetics led to a better description of acifluorfen retardation (Fig. 6). However, the fairly slow pore water velocity (Table 5) and long solute residence time favored close approach to sorption equilibrium; ignoring sorption kinetics resulted in <10% error in predicted retardation (Fig. 6).

View larger version (17K): [in this window] [in a new window]   Fig. 6 Predictions of acifluorfen elution from intact conventional till (CT) soil column 1 obtained when allowing for sorption kinetics or assuming sorption equilibrium compared with measured effluent concentrations. All concentrations in a curve are relative to the maximum concentration in that data set

View larger version (17K): [in this window] [in a new window]   Fig. 7 (A) Average predicted and measured effluent concentrations of acifluorfen from the intact soil columns. (B) Average predicted and measured concentrations of residual 14C (unextractable and extractable, exclusive of acifluorfen) in three depth increments of the soil columns

To quantify the magnitude of error in batch degradation rate constants that was responsible for the discrepancies between predicted and measured data (Fig. 7a and 7b), rate constants were adjusted to fit the acifluorfen effluent concentration and residual ¹⁴C data. First-order degradation was assumed and rate constants for the top and bottom 10-cm segments were optimized to obtain best-fits to the experimental data. The rate constant for the middle segment was assumed to be the average of these values. Also, based on the results of Gaston and Locke (1996), there was little to be gained by trying to estimate different rate constants for mobile and immobile water regions because mass transfer between these regions was slow (Table 6). Calculated first-order degradation rate constants for the four soil columns are listed in Table 5.

Estimated rate constants were often nearly an order of magnitude greater than the corresponding values obtained from the biometer flask study. Consistent with the batch data, however, rate constants for the surface soil were generally greater than those for the 20- to 30-cm depth. Adequacy of fits to the NT 1 column data are shown in Fig. 8a and 8b . The mass of acifluorfen displaced through the soil column was accurately described (Fig. 8a) and distribution of residual ¹⁴C better estimated (Fig. 8b) than when batch degradation data were used.

View larger version (21K): [in this window] [in a new window]   Fig. 8 (A) Simulation of acifluorfen elution from intact and no-till (NT) soil column 1 generated using best-fit values for kd in the 0–10 and 20–30 cm depths compared with measured effluent concentrations. (B) Distribution of residual 14C (unextractable and extractable, exclusive of acifluorfen) with depth obtained using best-fit values for kd compared with measured residual 14C

Because intact soil columns are approximate models for undisturbed soil, the distribution of k_d values with depth may be similar to field values under similar moisture conditions (wet, though unsaturated, with an average 0.07 void space, as calculated from the data in Table 7, assuming soil particle density of 2.65 Mg m^-3). Therefore, k_ds obtained from the mobility study may tend toward the high end of the possible range in values for the Dundee soils. On the other hand, aeration in the biometer flasks may have been artificially high with respect to field soil at similar water content (particularly below the soil surface). Thus, k_ds from the batch systems, though obtained at 35% moisture, may be more indicative of acifluorfen degradation under drier, more aerated conditions.

	Summary

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 
Effects of tillage on acifluorfen sorption isotherms in the Dundee CT and NT soils were related to amount and distribution of OC. Higher content of OC in the 0- to 10-cm NT led to greater acifluorfen sorption than in the surface CT soil, but more OC in the subsurface CT soils resulted in greater sorption than in the corresponding NT soils. Sorption isotherms for all six tillage–depth combinations were nonlinear. Approach to equilibrium was time-dependent and conformed to the two-site equilibrium–kinetic model. Kinetics were rapid (>70% of total sorption occurring almost instantaneously and, typically, more than 90% within 24 h) but revealed no clear trend with respect to tillage or OC content. Nevertheless, inclusion of sorption kinetics in the transport model for acifluorfen mobility gave more accurate predictions of retardation than assumption of instantaneous sorption equilibrium.

Acifluorfen degradation was more rapid in topsoil (0- to 10-cm) than in subsoil (20- to 30-cm). The first-order rate constant was larger for the CT topsoil, but there was no difference between CT and NT subsoils in rate of acifluorfen degradation. The incubation study revealed slow rates of acifluorfen mineralization and transformation to unextractable forms of ¹⁴C. There was little evidence for extractable degradation products. Analysis of effluent from the intact soil columns also indicated negligible concentration of degradation products. Thus, intermediate products of acifluorfen are apparently highly sorbed. However, unlike the incubation study, extracts of soil column segments did contain degradation products, particularly extracts of the soil surface segments. Discrepancy between these results likely reflects greater rates of acifluorfen degradation in the intact soil columns.

Although use of batch degradation rate constants in the transport model underpredicted the extent of degradation, the model was capable of describing effluent concentration of acifluorfen and distribution of residual ¹⁴C if degradation rate constants were optimized. Best-fit first-order rate constants were several-fold larger than rate constants calculated from the incubation study. Since acifluorfen degradation is faster under anaerobic conditions, more rapid degradation in the soil columns may have been promoted by poorer aeration than in the batch systems. Thus, degradation rate constants obtained from the mobility study may tend toward the high end of a possible range of values.Schmidt Simkins Alexander 1985

Received for publication August 12, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary REFERENCES

 

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