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Soil Science Society of America Journal 67:747-755 (2003)
© 2003 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Fluometuron Sorption and Degradation in Cores of Silt Loam Soil from Different Tillage and Cover Crop Systems

L. A. Gaston*,a, D. J. Boquetb and M. A. Boschc

a Dep. of Agronomy, LSU AgCenter, Baton Rouge, LA 70803
b Northeast Research Station, LSU AgCenter, Winnsboro, LA 71295
c Dep. of Agricultural Chemistry, LSU Ag Center, Baton Rouge, LA 70803

* Corresponding author (lagaston{at}agctr.lsu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Fluometuron [N,N-dimethyl- N'-[3-(trifluoromethyl)phenyl] urea], a herbicide used on cotton (Gossypium hirsutum), is fairly mobile in soil. This study quantified effects of tillage (conventional-till [CT] and no-till [NT]) and cover crop (native vegetation, hairy vetch [Vicia villosa] and wheat [Triticum aestivum]) on fluometuron sorption and degradation in intact cores (10-cm diam. by approximately 7.5 cm long) of Gigger (fine-silty, mixed, thermic Typic Fragiudalfs) soil. Batch sorption was well described by Freundlich isotherms. Sorption generally increased with soil organic C and was greater in NT, than in CT, 0- to 3-cm soil. No-till soil had more earthworms and arthropods, suggesting greater physical heterogeneity and potential physical nonequilibrium during transport. Tracer elution from slightly unsaturated (-0.1 bar) cores did not show preferential flow. First-order degradation rate constants were obtained by fitting a convective-dispersive/diffusive transport model to effluent fluometuron concentrations from seven simulated rains. Degradation was faster in NT, than in CT, native soil (kds = 0.09 and 0.04 d-1). Tillage did not affect degradation in the vetch (kds = 0.07 d-1) or wheat (kds = 0.8 d-1) soils. Degradation in CT and NT vetch cores was faster than in an earlier batch study. To the extent intact cores better represent field conditions than homogeneous soil, degradation in cores may more accurately reflect degradation in the field.

Abbreviations: CT, conventional till • NT, no-till


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
FLUOMETURON (Ciba-Geigy Corp., Greensboro, NC) is a widely used herbicide on cotton in the Southeastern USA. It is subject to runoff into surface waters (Zablotowicz et al., 2000b) and potentially to rapid downward movement (Essington et al., 1995). However, effects of soil management on processes that control its mobility are not fully understood. In general, higher organic C at or near the soil surface under NT, compared with CT, management (Blevins et al., 1983; Dalal, 1989; Havlin et al., 1990; Wood et al., 1991) leads to greater sorption of most herbicides, including fluometuron (Brown et al., 1994; Locke et al., 1995). Increased soil organic C from hairy vetch, ryegrass (Lolium multiflorum Lam.) or wheat cover crops also increases herbicide sorption (Brown et al., 1994; Reddy et al., 1997; Wilson et al., 1998).

The effects of tillage and cover crops on herbicide degradation, however, are much less clear. Studies by Locke et al. (1996), Reddy et al. (1995), and Locke and Harper (1991), for example, found negligible difference because of tillage practice in degradation rates of several herbicides. Other studies have found somewhat faster herbicide degradation in CT than NT soil (Brown et al., 1994; Gaston and Locke, 1996; 2000). Locke et al. (1995) found that degradation of fluometuron was faster in soil planted with ryegrass than in soil without it. However, Brown et al. (1994) found that hairy vetch, in combination with NT, slowed fluometuron degradation relative to CT soil with no cover crop.

Faster degradation in CT soil (Brown et al., 1994; Gaston and Locke, 1996; 2000) is surprising given that surface NT soil is expected to support larger microbial populations (Follett and Schimel, 1989; Zablotowicz et al., 2000a). Slower rate of degradation with lower population of microorganisms (Mueller et al., 1992) seems more likely, provided the total mass of herbicide is accessible for biodegradation. However, sorption may limit the bioavailability of an herbicide. Thus, greater sorption in NT soil may partially account for negligible difference in degradation rate because of tillage (Zablotowicz et al., 2000a) or faster rate in CT soil (Gaston and Locke, 2000; Brown et al., 1994).

Besides tillage effects on herbicide sorption and degradation, physical differences between CT and NT soils, particularly macroporosity and preferential flow, may affect herbicide mobility (Isensee et al., 1990; Gish et al., 1991). Development and persistence of root and worm biopores under NT is expected to increase preferential flow under saturated conditions (Edwards et al., 1988; Hall et al., 1989). Cover crops, in combination with NT, may also increase water entry and movement in soil (Gulick et al., 1994; Roberson et al., 1991). Preferential flow may also occur under unsaturated conditions (Seyfried and Rao, 1987; Jardine et al., 1993; Gaston and Locke, 1996; 2000), however, the effect of tillage is unclear (Gaston and Locke, 1998).

Thus, the objective of this study was to examine the influences of tillage and cover crop on fluometuron sorption and degradation in surface Gigger silt loam soil. Intact soil cores were used, and degradation behavior in these was compared with degradation previously determined using homogeneous samples of Gigger soil.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Soil
The Gigger soil came from long-term CT and NT cotton plots at the Northeast Research Station, Winnsboro, LA. Sections of polyvinyl chloride (PVC) (10-cm diam. by 10 cm long) were used to take intact cores of surface (0 to 7.5 cm) soil. Cores came from CT and NT plots with three different cover crops: (i) native winter vegetation (principally annual bluegrass [Poa annua], henbit [Lamium amplexicaule], shepherd's purse [Capsella bursa-pastoris], and Virginia pepperweed [Lepidium virginicum]), (ii) hairy vetch and iii) wheat. Additionally, bulk samples of 0- to 3- and 3- to 6-cm depth soil were collected. The intact cores were sealed in plastic and padded to limit jarring during transport. Two cores from different replicates of each tillage by cover crop combination were randomly selected for use in the fluometuron degradation–mobility study described below. These were refrigerated at 4°C until use. Remaining cores were used to estimate populations of earthworms and other invertebrates. Bulk soil samples were sieved (4 mm), thoroughly mixed and refrigerated at 4°C until microbial assays. A portion of each bulk sample was air-dried, ground, sieved (2 mm), and used to determine fluometuron sorption. Selected chemical data for these soils are given in Table 1.


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  		Table 1. The pH, organic C, and inorganic N of 0- to 3- and 3- to 6-cm Gigger soil from conventional-tillage and no-tillage plots with native vegetation, vetch, or wheat cover crops.

 
Soil Biological Activity
Microbial Activity
Microbial activity in the 0- to 3- and 3- to 6-cm depths of each tillage by cover crop combination was estimated by fluorescein diacetate (FDA) hydrolysis (Schnürer and Roswall, 1982). Respiration rate in the 0- to 3- and 3- to 6-cm depths of these soils was determined in a 10-d incubation study similar to the method described by Alexander (1982). Briefly, CO2 evolved from 20 g (oven-dry equivalent) samples of soil in 1-L Mason jars was trapped in 10 mL of 1.00 M NaOH and unreacted OH- titrated with HCl after precipitation of carbonate with Ba. Soil used in the FDA and respiration studies was adjusted to approximately -0.1 bar potential. Three replicates corresponding to field plots were used.

Earthworm and Arthropod Populations
Soil in cores not used for the fluometuron degradation and mobility study was removed and earthworms and arthropods enumerated by hand sorting (Lee, 1985). Duplicate cores from each replicate of the six tillages by cover crop combinations were used.

Fluometuron Sorption by Gigger Soil
Duplicate 3-g samples of soil (oven-dry equivalent) from the 0- to 3- and 3- to 6-cm depths of each tillage by cover crop combination were placed in glass centrifuge tubes and 6 mL of 34.4, 8.6, or 0.86 M fluometuron (containing 8 kBq uniformly ring-labeled 14C fluometuron) in 0.005 M CaCl2 background were added. The suspensions were shaken for 24 h, centrifuged for 10 min at 12 000 x g and 1-mL aliquots taken for liquid scintillation counting. Sorbed fluometuron was calculated by change in solution radioactivity.

Fluometuron Mobility in Soil Cores
Soil protruding from the bottoms of cores was gently broken away and depressions backfilled with acid-washed sand. Each core was capped with an airtight end plate fitted with a membrane to make an experimental unit similar to that described in Gaston and Locke (1996). Soil cores were drained into flasks connected to a regulated vacuum source (100 cm H2O) and wet with 0.005 M CaCl2 applied to the soil surface at a rate of 16 cm d-1 through a sprinkler head (Gaston and Locke, 1996). Once steady flow was achieved, sprinkler heads were removed and 10 mL of 378 µM fluometuron in 0.005 M CaCl2 were applied to the surface in a drop-wise manner at a rate equal to water application. To avoid interception and adsorption of fluometuron by surface organic residue, cotton and cover crop residue was removed from the NT cores before fluometuron was applied. After the fluometuron solution had apparently infiltrated the soil surface, the crop residue was replaced. To avoid possible deterioration of surface structure because of subsequent simulated rainfalls, the surface of the CT cores was covered with a layer of acid-washed gravel. Cores were transferred to airtight plastic containers to prevent evaporation and incubated at 25°C before beginning a series of seven simulated rainfall–leaching events with intermittent periods of incubation. A 0.005 M CaCl2 solution was used for simulated rain. Average duration of each rainfall was 0.33 d. The initial incubation lasted 3 d, followed by incubation periods of 16, 14, 14, 17, 17, and 26 d, totaling 109-d duration.

Unsaturated conditions were maintained in cores during each simulated rain by collecting effluent under 100 cm H2O vacuum. Effluent was collected as composite samples or in fractions (last simulated rain) from which aliquots were combined. Total volume of effluent was recorded and an approximately 100-mL aliquot passed through a conditioned (5 mL MeOH, followed by 10 mL of 0.005 M CaCl2) C18 solid phase extraction column. Fluometuron trapped on the column was eluted with 3 mL of MeOH and its concentration determined by high performance liquid chromatography (HPLC) (adapted from Mueller and Moorman, 1991). Briefly, fluometuron in a 20-µL injection volume was separated using a C18 column (150 by 4.6 mm id; 5 µm) and an isocratic 50:50, CH3CN/H2O mobile phase (1 mL min-1), then detected by UV at 240 nm.

Before the last simulated rainfall, the remaining crop residue (NT cores) or gravel (CT cores) was removed, and 10 mL of 1.00 M CaCl2 was applied dropwise to the soil surface. This pulse was eluted with 0.005 M of CaCl2, effluent was collected in fractions and Cl- concentration was determined by ion-specific electrode. The time-course of effluent Cl- concentration was used to estimate solute dispersion during flow as described below.

After the last simulated rainfall–leaching event, experimental units were disassembled, soil was pushed out of the PVC casing and cut into upper 3 cm and lower segments. Approximately 20-g subsamples were transferred to 125-mL Erlenmeyer flasks and extracted for residual fluometuron with 40 mL of a 80:20 MeOH/0.005 M CaCl2 solution by shaking for 24 h. The suspension was vacuum filtered. Soil remaining in the Erlenmeyer flask was removed with an additional 40-mL portion of extractant, and the filter cake was washed with an additional 20-mL of extractant. Methanol was rotary evaporated at 35°C, aqueous extracts were diluted to approximately 100 mL with 0.005 M CaCl2, and this solution was concentrated by solid phase extraction. Fluometuron was eluted from the C18 columns with 3 mL of MeOH and analyzed by HPLC. Volumetric water content ({theta}) in soil cores was determined from other subsamples. Bulk density ({rho}, Mg m-3) was calculated from soil core dimensions, total mass of air-dry soil and air-dry moisture content.

Estimating Fluometuron Degradation and Transport Parameters from Mobility Data
Assuming negligible evaporation during the incubation phases of the experiment, fluometuron elution can be modeled as a series of steady-state flow events, interrupted by periods of diffusive redistribution. Provided that preferential flow was not important, the model presented in Locke et al. (1994), adapted to include variability in soil properties with depth, can be applied.


[1a]
where the subscript refers to the ith of M depth increments, C is solution concentration (µM), S is adsorbed concentration (µmole kg-1), R is the retardation factor (dimensionless), D is dispersion coefficient (cm2 d-1), v is pore water velocity (cm d-1), z is distance (cm), t is time (d), and F(C, S) describes fluometuron degradation. The retardation factor depends on solute adsorption and is given by

[1b]
where sorption is described by a Freundlich isotherm, S = Kf CN. If degradation obeyed first-order kinetics, d(C+[{rho}/{theta}]S)/dt = - kd(C+[{rho}/{theta}]S), in which case F(C, S) = - kd(C+[{rho}/{theta}]S).

Appropriate boundary and initial conditions for periods of flow are,

[1c]

[1d]

[1e]

[1f]

[1g]
where tp is the duration (d) of pulse application of solute and L is core length (cm).

During periods of no flow, solute redistribution may occur by diffusion,

[2a]
where Doi is the diffusion coefficient (cm2 d-1). Boundary conditions are

[2b]

[2c]

[2d]

To be generally consistent with depths for which fluometuron sorption was determined, soil cores were assumed to consist of two depth increments, 0 to 3 cm and below. Also, to reduce the otherwise unwieldy number of parameters estimated from tracer and fluometuron elution curves, dispersion, Di, diffusion, Doi, and first-order degradation, kdi, coefficients were assumed the same in upper and lower depth increments. Previous work with short cores (Locke et al., 1994) found Do difficult to estimate (high uncertainty) from elution of Cl- under conditions of intermittent flow. Therefore, Do was jointly estimated with kd by fitting Eq. [1] and [2] to effluent fluometuron concentrations (given physical, L, {theta}i and {rho}i; chemical, Ki and Ni; and flow, D, vi and tp parameters; and durations of flow and no flow). Before estimating Do and kd, the dispersion coefficient, D, was estimated by fitting Eq. [1] to Cl- tracer elution data. The transport model (Eq. [1] and [2]) was expressed in implicit finite difference form and linked to a nonlinear least-squares procedure (van Genuchten, 1981) to obtain best-fit estimates of degradation and transport parameters.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Soil biological and chemical properties are presented first because these explain sorption and degradation data. Measured sorption and calculated transport parameters are then discussed since these are prerequisite to calculations of degradation rate from transport data. Finally, degradation in intact cores and bulk (batch) soil is compared.

Biological and Chemical Properties of Bulk Soil
Table 1 provides organic C, pH, and extractable inorganic N (NH+4 and NO-3) in the 0- to 3-and 3- to 6-cm depths of the six soils. Differences in pH were negligible at both depths; however, organic C was significantly higher in the surface NT compared with CT soil. Inorganic N was also higher in the NT surface soil, especially under vetch.

Table 2 gives FDA hydrolysis, soil respiration, and earthworm–arthropod data for the six tillage by cover crop treatment combinations. There were significant differences in FDA hydrolysis rate among the treatments in the 0- to 3-cm depth with NT vetch soil having the fastest rate and CT native soil the slowest. Hydrolysis was faster in NT surface soil than in CT surface soil (7.3 compared with 4.0 nmol g-1 h-1; p < 0.05), however, type of cover crop alone did not significantly affect FDA hydrolysis. There were no significant differences in FDA hydrolysis rates between treatments in the 3- to 6-cm depth. Difference in soil respiration in the surface soil because of tillage was small (NT was 43 µg C g-1 h-1 compared with CT at 31 µg C g-1 h-1; significant at p < 0.10) and cover crop had no affect. There were no significant differences in respiration rates in the 3- to 6-cm depth. Thus, only tillage significantly affected these indicators of microbial activity and the effect of tillage was limited to the surface 0- to 3-cm soil.


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  		Table 2. Influence of tillage and cover crop on FDA hydrolysis rate, microbial respiration, and numbers of earthworms and arthropods in Gigger soil.

 
Populations of earthworms and arthropods in the 0- to 6-cm depth Gigger soil were significantly higher in the NT soil cores. On the average there were 270 earthworms per square meter in the NT soil but only 70 m-2 in the CT soil (p < 0.05). Arthropods in NT cores outnumbered those in CT cores 520 to 200 m-2 (p < 0.05). Higher populations of these organisms in the NT soil suggest potentially greater infiltration and faster water movement than in the CT Gigger soil.

Fluometuron Sorption
Fluometuron sorption isotherms for the 0- to 3- and 3- to 6-cm depth soils are shown in Fig. 1 , respectively. In all cases, sorption was nonlinear and well described by Freundlich models (r2 > 0.999; parameters given in Table 3). Sorption was greater in NT than in CT surface soils (Fig. 1, top), consistent with higher organic C in the NT soils (Table 1). Cover crop had negligible effect on sorption in the NT surface soils. Among the CT surface soils, sorption was greatest in the wheat soil and least in the native vegetation soil. In contrast to the surface soils, differences in fluometuron sorption in the 3- to 6-cm depth because of tillage or cover crop were minor (Fig. 1, bottom).



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  		Fig. 1. Effects of tillage and cover crop on fluometuron sorption isotherm in 0- to 3-cm (top) and 3- to 6-cm (bottom) Gigger soil.

 

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  		Table 3. Freundlich parameters for fluometuron sorption in the 0- to 3- and 3- to 6-cm depths of Gigger soil from conventional-till (CT) and no-till (NT) soil with native vegetation, vetch, or wheat cover crop.

 
Transport Parameters
Concentrations of Cl- in effluent fractions from cores of CT and NT native soil are shown in Fig. 2 . Volume-averaged concentrations simulated using Eq. [1] with optimized values for dispersion coefficient, D, are also shown. Results were similar for the other 10 cores and indicated that unsaturated flow, irrespective of tillage or cover crop, was largely nonpreferential. Attempts to describe Cl- elution using a mobile–immobile water model (Gaston and Locke, 1996; adapted from van Genuchten and Wagenet, 1989) offered no better description of the data. This result is consistent with data by Wilson et al. (1998) who found little evidence for preferential flow through intact cores of Gigger soil. Values for the diffusion coefficient, Do, obtained from fluometuron effluent concentrations, ranged from approximately 0.1 to < 0.001 cm d-1 (average, 0.01 cm d-1), with standard errors >Do. Best-fit D and other physical parameters for each core are given in Table 4.



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  		Fig. 2. Elution of a Cl- tracer pulse from a replicate conventional till (CT) vetch (top) and no till (NT) vetch (bottom) core. Best-fits of Eq. [1] to average Cl- concentrations in effluent fractions are shown as step functions.

 

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  		Table 4. Transport parameters and first-order degradation rate constants for conventional till native (CTN), vetch (CTV), wheat (CTW), and no till native (NTN), vetch (NTV), wheat (NTW) cores. Values for D and kd were obtained by curve fitting. Batch first-order degradation rate constants from Gaston et al. (2000) are also given.

 
Fluometuron Degradation during Unsaturated Flow
Average concentrations of fluometuron in effluent from replicates of, respectively, CT and NT native, vetch and wheat cores produced by the seven simulated rainfalls are shown in Fig. 3 through 5 . Comparison of any CT data with the corresponding NT data shows that more fluometuron was displaced from the CT core. This result was because of less sorption in the 0- to 3-cm depth of the CT analog (Fig. 1, top; Table 3) and, in some cases, slower degradation in the CT core.



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  		Fig. 3. Elution of fluometuron from a replicate conventional till (CT) native vegetation (top) and no till (NT) native vegetation (bottom) core. Best-fits of Eq. [1] and [2] to average concentrations in leachate from each rain event are shown as step functions (optimized values for first-order degradation rate constants used).

 


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  		Fig. 5. Elution of fluometuron from a replicate conventional till (CT) wheat (top) and no till (NT) wheat (bottom) core. Best-fits of Eq. [1] and [2] to average concentrations in leachate from each rain event are shown as step functions (optimized values for first-order degradation rate constants used).

 


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  		Fig. 4. Elution of fluometuron from a replicate conventional till (CT) vetch (top) and no till (NT) vetch (bottom) core. Best-fits of Eq. [1] and [2] to average concentrations in leachate from each rain event are shown as step functions (optimized values for first-order degradation rate constants used).

 
Simulated loss of fluometuron (volume-averaged concentrations for each rainfall; as described by Eq. [1] and [2], with optimized kds, average Do = 0.01 cm d-1, and parameters in Tables 3 and 4) is also shown in Fig. 3 through 5. First-order degradation rate constants are given in Table 4. In general, the intermittent reactive flow model described fluometuron elution over the 109-d course of this study (average r2 = 0.97), lending support to calculated degradation constants.

The kds for the NT native soil were larger than those for the corresponding CT soil (Table 4). Within 95% confidence limits, however, kds for the NT and CT vetch soils were not different. Similarly, there was no difference between kds for the NT and CT wheat soils. Among the treatment soils, the greatest discrepancies between measured and fitted data occurred with the CT vetch and wheat (Fig. 4 and 5, respectively). Total losses of fluometuron were underestimated in these soils. Therefore, degradation rates for the CT vetch and wheat soils were likely overestimated.

Measured and simulated effluent concentrations of fluometuron approached zero in all cases (Fig. 3 to 5), but some extractable fluometuron remained in all soils except the NT wheat. Table 5 gives residual fluometuron in the upper 0- to 3-cm and lower 3- to approximately 7.5-cm core sections. Although the total residual fluometuron was small (average <0.03 of applied), essentially none would have remained if, as assumed, desorption had maintained nearly instantaneous equilibrium with solution concentrations. Thus, desorption kinetics may account for tailing in effluent concentrations relative to simulations (Fig. 4 and 5, top).


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  		Table 5. Fluometuron recovery from soil (by depth) in effluent and total.

 
Furthermore, all CT cores contained incorporated cover crop residues. Reddy et al. (1997) showed that cyanazine sorption by cover crop residues increased as the residue decomposed. Thus, fluometuron sorption in the CT cores may have increased with aging. Since the above ground biomass of wheat and vetch (5.0 and 2.2 Mg ha-1, respectively) was much greater than that of native vegetation in 1999 (0.7 Mg ha-1; Gaston et al., 2000), the effect would have been more pronounced in CT vetch and wheat cores.

Little difference in degradation rate because of tillage for the vetch and wheat soils may also reflect the effect of greater sorption in the NT soil. Zablotowicz et al. (2000a) have demonstrated that faster degradation of fluometuron in surface CT, compared with NT, Dundee (fine-silty, mixed, thermic Aeric Ochraqualfs) soil was because of higher fluometuron sorption and lower bioavailability in the NT soil. Their calculated rate constant for solution phase degradation was actually greater for the NT soil. Since sorption was greater in the surface NT soils than in the CT soils (Fig. 1a; Table 4), it can be inferred that solution phase degradation was faster in the NT, than in the CT, vetch and wheat soils.

Relative degradation rates were only partially consistent with FDA hydrolysis activity in the upper 3 cm (Tables 2 and 4). For example, hydrolysis and degradation were faster in the NT native than in the CT native soil. Also, there were no differences in hydrolysis or degradation due to tillage in the wheat soils. But, whereas FDA hydrolysis was faster in the NT vetch than in the CT vetch soil, degradation rates were the same.

Comparison with Batch Degradation Data
Table 4 also includes kds obtained in a previous batch study (Gaston et al., 2000). Results for CT and NT native and NT wheat cores were consistent with earlier results, but kds for cores of CT and NT vetch soils were about four times those from the batch study. Average kd for CT wheat cores was twice that from the batch study but uncertainty for one replicate core was high. Gaston et al. (2000) attributed slow degradation in the NT vetch soil to high concentration of available N. Gan et al. (1996) have demonstrated that addition of NH+4 slowed atrazine degradation. Unlike the static batch system, water flow through intact cores of vetch soils would leach soluble N (mostly NO-3, Table 1), favoring increased use of fluometuron by microbes as a N source. Vetch and wheat residue in the CT cores may have increased microbial populations and N demand, leading to faster degradation than in the batch study (crop residues were removed from CT soils; Gaston et al., 2000).

Although kds for the CT and NT native and NT wheat soils were similar for intact core and batch systems, kds for intact cores were averaged over the 0- to approximately 7.5-cm depth whereas those for the batch systems were for the upper 3-cm soil. Since degradation rate decreases with depth (Mueller et al., 1992; Zablotowicz et al., 2000a), degradation averaged over the upper approximately 7.5 cm may be slower than when averaged over the upper 3 cm. Thus, similar degradation rates in both systems suggests that degradation in the upper 3 cm of intact cores may have actually been faster. When separate kds for the upper 3 cm and underlying soil in cores were estimated by curve-fitting, agreement between core effluent concentrations and model simulations was somewhat improved (data not shown). Also, rate constants for the upper 3 cm were greater than either average rate constants for whole cores or those derived from the batch study. However, uncertainty in kds was high.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Biological activity was greater in NT than CT Gigger soil. However, there were no differences in biological activity because of cover crop alone. Fluometuron sorption was nonlinear and well described by the Freundlich model for all tillage by cover crop combinations. Greater sorption in the surface 0- to 3-cm NT soils than in the corresponding CT soils was consistent with greater organic C in the NT soils. There were no differences in sorption among treatment soils in the 3- to 6-cm depth. Fluometuron degradation was faster in NT native vegetation soil than in the corresponding CT soil, however, tillage had no effect on overall fluometuron degradation rate in the vetch and wheat soils. Based on lower sorption in the surface CT soils and slower degradation in the CT native vegetation soil, fluometuron herbicidal efficacy and potential for off-site movement may be greater in the CT systems.

First-order degradation rate constants for the CT and NT native vegetation and NT wheat cores were the same as kds obtained in an earlier batch degradation study. However, kds from soil cores were averaged over the 0- to approximately 7.5-cm depth, compared with 0- to 3-cm depth in the batch study. Given expected decrease in fluometuron degradation rate with increasing depth, degradation may have actually been faster in the cores. Degradation rate constants for the CT and NT vetch and CT wheat (possibly) soils were greater than obtained in the batch study. Faster degradation in the NT vetch cores may have been because of displacement of inorganic N by leaching. Presence of vetch or wheat residue in the CT cores may have simulated microbial activity and increased degradation.


   ACKNOWLEDGMENTS
 
The authors thank Dr. Martin Locke of the USDA-ARS Southern Weed Science Unit for providing the radiolabeled fluometuron and analytical support for the sorption study.

Received for publication August 24, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome