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Soil Science

Issue: Volume 162(7),July 1997,pp 501-509

Copyright: © Williams & Wilkins 1997. All Rights Reserved.

Publication Type: [Article]

ISSN: 0038-075X

Accession: 00010694-199707000-00005

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TILLAGE AND COVER CROP EFFECTS ON CYANAZINE ADSORPTION AND DESORPTION KINETICS

Reddy, Krishna N.; Locke, Martin A.; Gaston, Lewis A.

Author Information

Southern Weed Science Lab., USDA-ARS, P.O. Box 350, Stoneville, MS38776. Dr Reddy is corresponding author. E-mail:

Received Nov. 18, 1996; accepted Jan. 23, 1997.

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Abstract
Accumulation of partially decomposed plant residues under no-tillage (NT) and cover crop management systems can affect herbicide fate in the soil. This study evaluated adsorption and desorption of cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} in soils and herbicide-killed Italian ryegrass (Lolium multiflorum Lam.) residues collected from a long-term conventional tillage (CT) and NT cotton field. The four cotton production systems included were CT and NT, each with and without ryegrass as a cover crop. Adsorption was determined by reacting 0.5 g of soil or ryegrass residue with 8 mL of 14C-cyanazine solution (five concentrations: 0.13 to 15.68 µmol L-1) for 48 h. The Freundlich Kf values were higher in NT than in CT soils and higher in soils from ryegrass cover crop than in soils from no cover crop. The Kf was higher in ryegrass residue (13.33) than in soils (1.77 to 2.94). The N values for soils (>0.90) and ryegrass residue (>0.95) indicated nearly linear adsorption. Time-course adsorption data analyzed by an equilibrium/kinetic model indicated that adsorption was rapid initially (within 1 h), followed by a slow increase in CT and NT soils from ryegrass plots. In contrast, adsorption achieved equilibrium within 48 h of reaction time in ryegrass residue. Cyanazine adsorption increased with increased decomposition of plant residues. The Kf for ryegrass residues sampled at 5 weeks after cotton planting was 17% higher than the residues sampled at 3 weeks before planting. The CaCl2-desorbable cyanazine in two consecutive 24-h cycles ranged from 77 to 88% in soils and from 46 to 47% of that adsorbed in ryegrass residues. Two additional 24-h desorptions with methanol removed most of the remaining cyanazine. Under field conditions, the plant residues on the soil surface in NT and cover crop systems can apparently intercept and temporarily retain cyanazine.
Crop production with no-tillage (NT) and cover crop systems is of current interest in the context of recent legislation that links farm subsidies with soil conservation practices and also because of renewed emphasis on integrated weed management programs to reduce herbicide use. No-tillage is the extreme form of conservation tillage wherein soil is left undisturbed and crop residues are distributed more or less uniformly. In cover crop systems, the substantial vegetative biomass produced in the spring is usually killed with herbicides before planting the next crop. Both NT and cover crop management practices generate plant residues. The plant residues left on the soil surface reduce soil erosion as well as nutrient and pesticide loss in runoff, conserve soil moisture, improve physical properties of soil, increase soil organic C (Blevins and Frye 1993), and enhance microbial populations and soil enzyme activities (Reddy et al. 1995b; Wagner et al. 1995).
The plant residues present on the soil surface can intercept a significant portion of herbicide applications. Herbicide intercepted by plant residues must reach the soil beneath the residues to prevent weed seedling growth and development. Herbicide intercepted and retained by residues is washed off and/or released to soil upon rainfall. Extent of washoff and/or release may vary, depending on time and amount of rainfall after the herbicide application and on the nature of any interaction between herbicide and plant residues. Many workers have reported that plant residues intercept and retain a significant portion of several soil-applied herbicides (Banks and Robinson 1982, 1986; Ghadiri et al. 1984).
Interception and retention of herbicides by plant residues could affect herbicide release to soil for target plant uptake. Plant residues left on the soil surface represent a significant mass of organic matter. Herbicide binding can be weak or strong, depending on type and degree of decomposition of plant residues. Increased herbicide retention by plant residues not only reduces the amount reaching the soil (affecting weed control) but also prolongs herbicide persistence. The latter could provide season-long weed control from herbicide desorbed from plant residues over a period of time. However, the herbicide residues persisting beyond the crop season have the potential to injure sensitive rotational crops (Anonymous 1993). Herbicide retention by plant residues, however, can also minimize the potential off-target movement of herbicides from agricultural fields. Thus, adoption of any crop management system that increases plant residue accumulation on the soil surface could have a positive impact on the environment.
Cyanazine is used extensively for annual grass and broadleaf weed control in corn, cotton, and many other crops. It is applied (sometimes incorporated) to the soil surface and plant foliage (Weed Science Society of America 1994). Cyanazine has a water solubility of 171 mg L-1 and an octanol/water partition coefficient of 127 (Weed Science Society of America 1994). Cyanazine, a heterocyclic organic base (pKa, 5.1), is readily adsorbed to soils (Weber 1970; Majka and Lavy 1977; Clay et al. 1988; Cancela et al. 1990). The triazine herbicides are generally adsorbed to soil through ionic bonds. The protonated weakly basic triazine molecules (organic cations) form ionic bonds with negatively charged functional groups of various soil colloids (Weber et al. 1969). Cyanazine adsorption in soil increases with decreasing pH (Weber 1970). Soil half-life values for cyanazine range from 5 to 31 days (Helling et al. 1988; Sirons et al. 1973; Blumhorst and Weber 1992), and the half-life decreases as soil pH increases (Blumhorst and Weber 1992). Cyanazine is more persistent in the moderately acidic soils (Blumhorst and Weber 1992). However, information on the effects of NT and cover crop management systems on cyanazine adsorption and desorption in soils and the magnitude of cyanazine adsorption and retention by cover crop residues is lacking.
The objectives of this research were (i) to characterize cyanazine adsorption and desorption in soils and plant residues from cotton tillage systems with and without a ryegrass cover crop, (ii) to study the effects of decomposition of ryegrass residue on the sorptive characteristics of cyanazine, and (iii) to determine whether the two-site or three-site equilibrium/kinetic (Amacher et al. 1988; Boesten et al. 1989) models can describe time-dependent cyanazine adsorption adequately and predict the rate of desorption.
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MATERIALS AND METHODS
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Experimental Site, Soils, and Ryegrass Residue
A continuous cotton field study under nonirrigated conditions was established in 1990 on a Dundee silt loam (fine-silty, mixed, thermic Aeric Ochraqualf) soil at Stoneville, Mississippi. The tillage systems consisted of continuous conventional tillage (CT) and NT. The CT operations were: subsoil and chisel plow in the fall, double disk, rowed up, and harrow in the spring, and three interrow cultivations in the summer. In 1993, each tillage plot was divided into two subplots. The subplots were maintained with and without ryegrass cover crop. Ryegrass had been planted in the fall of each year since 1993 and killed with paraquat (1,1'-dimethyl-4, 4'-bipyridinium ion) 3 to 4 weeks before cotton planting in the following spring. Desiccated ryegrass residue was left undisturbed on the soil surface in NT plots and incorporated into the soil with the tillage treatments in CT plots. Cotton was seeded directly into cover crop residues in NT plots. Fluometuron {N, N-dimethyl-N'-[3-(trifluoromethyl)phenyl]urea} at 0.9 kg ha-1 and norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] at 1.12 kg ha-1 were applied to the surface of soil at planting in all treatments (CT and NT with or without ryegrass). Post-emergence herbicides; cyanazine at 0.9 kg ha-1 or fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} at 0.2 kg ha-1 or MSMA (monosodium methanearsonate) at 2.24 kg ha-1 were applied during midseason, depending on the weed species, to all treatments.
On March 30, 1995, ryegrass cover crop was killed using paraquat. Cotton was planted on May 11, 1995. Frequent rains and wet field conditions caused a slight delay in planting cotton. Fluometuron and norflurazon were applied on the day of cotton planting, and cyanazine was applied as postdirected on June 15, 1995.
As the differences in soil characteristics between cover crop and no cover crop systems are more apparent in the surface layer than in lower depths, the soils from the surface 2 cm were used in the studies. Surface 2-cm soils were collected from CT and NT plots, with or without ryegrass, on May 9, 1995. Herbicide-desiccated ryegrass residues were collected from NT plots on April 17, 1995 (3 weeks before planting), May 9, 1995 (at planting), and June 15, 1995 (5 weeks after planting and before postemergence application of cyanazine). Soils (Table 1) were sieved (2-mm), and cover crop residues were chopped to about 2 cm in length and stored at 4°C. / Table 1
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Cyanazine Adsorption Kinetics
Cyanazine adsorption kinetics was measured on soils from CT and NT plots with ryegrass cover crop and on plant residues collected at planting. A 0.5-g (oven-dry equivalent) field moist soil or plant residue was weighed into a 25-mL glass centrifuge tube. Eight milliliters of technical grade cyanazine (98% purity; ChemServe, West Chester, PA) and 14C-cyanazine (14C-ring-labeled, specific activity 851.7 kBq mg-1, DuPont Agricultural Products, Wilmington, DE) in 0.01 M CaCl2 (background solution) were added to each tube, and tubes were sealed with a Teflon-lined cap. Cyanazine concentration used was 0.87 µmol L-1, with about 100 Bq mL-1. Samples were shaken on a rotary shaker for 1, 24, 48, 72, 144, and 312 h in a cold room (4°C) to minimize microbial activity. After shaking, samples were centrifuged at 7710 g for 10 min. Two 1-mL aliquots of the supernatant were counted for radioactivity using a liquid scintillation spectrometer. All volumes were determined by weight for increased accuracy. The difference between initial and final solution concentrations (µmol L-1) was attributed to adsorption (µmol kg-1). Each shaking time was replicated three times.
Cyanazine adsorption kinetics has been described using a 3-site equilibrium/kinetic model (Gaston and Locke 1994; Reddy et al. 1995a) similar to models of Amacher et al. (1988) and Boesten et al. (1989). Equations (1a) to (1c) / Equation 1A / Equation 1B / Equation 1C
Where the subscript numbers refer to adsorption sites, C (µmol L-1) is solution concentration of herbicide, Sn (µmol kg-1) are adsorbed concentrations of herbicide, ke (L kg-1) is the coefficient for instantaneous equilibrium, kf (L kg-1 h-1) is the forward rate constant, kr(h-1) is the reverse rate constant, kir(h-1) is the rate constant for apparently irreversible adsorption, and the exponent N accounts for nonlinearity. Equations (1a) to (1c) were expressed in finite difference form and optimized values for parameters obtained using a least-squares procedure. The number of parameters estimated was reduced from five to four by using the Freundlich N value obtained from the 48-h adsorption isotherm data.
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Cyanazine Adsorption
A batch equilibrium technique was employed to characterize adsorption of cyanazine. Soil or plant residues collected at planting were reacted with cyanazine solution as described in the adsorption kinetics study. Cyanazine concentrations were 0.13, 0.91, 3.99, 7.99, and 15.68 µmol L-1, with about 27 to 123 Bq mL-1. Samples were shaken for 48 h. The adsorption kinetics study showed that adsorption was nearly complete within 48 h. After shaking, the samples were centrifuged and radioactivity determined as described in the adsorption kinetics study. Each cyanazine concentration was replicated four times.
The distribution of herbicide between adsorbed and solution phases was described by the Freundlich equation, S = KfCN, where S is the amount of herbicide adsorbed (µmol kg-1) and C is the solution concentration (µmol L-1). Kf and N are empirical constants, where Kf is the Freundlich coefficient (L kg-1), and the exponent N is a dimensionless parameter commonly less than unity. Nonlinear regression techniques used to derive Kf and N minimized the weighted residual sum of squares (SAS 1991). The linearized Kd (distribution coefficient) values were also calculated.
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Cyanazine Desorption
Desorption was measured on samples equilibrated with initial cyanazine concentrations of 0.91 and 7.99 µmol L-1. After an initial 48-h adsorption and centrifugation, the desorption was induced by a successive replacement of 80% of the supernatant with herbicide-free desorbing solution. The samples were re-equilibrated on a rotary shaker for 24 h in a cold room (4°C). After shaking, samples were centrifuged at 7710 g for 10 min, and two 1-mL aliquots of supematant were counted for radioactivity as described earlier. Of the four desorption cycles, the first and second were desorbed with 0.01 M CaCl2 and the third and fourth were desorbed with methanol. After the methanol desorption, the soil samples were air-dried and plant residues samples oven-dried (45°C for a week). Duplicate subsamples of 0.2 g soil or 0.1 g of plant residues were mixed with 0.3 g cellulose and combusted using a Packard Oxidizer 306 (Packard Instruments Co., Downers Grove, IL) to determine undesorbable 14C. The 14CO2 released during combustion was trapped in Carbo-Sorb and Permafluor, and radioactivity was quantified using a liquid scintillation spectrometer. Desorbable and undesorbable data were expressed as percent of 14C-cyanazine adsorbed.
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Effect of Age of Ryegrass Residue on Cyanazine Adsorption
Ryegrass cover crop residues collected at cotton planting, 3 weeks before planting, and 5 weeks after planting cotton were used to assess the effect of residue decomposition on cyanazine adsorption. Cyanazine adsorption was characterized using a batch equilibrium technique as described in the adsorption study.
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RESULTS AND DISCUSSION
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Cyanazine Adsorption
The Freundlich parameter coefficients for the adsorption isotherms of Figure 1 are given in Table 2. The Kf values were higher for NT soils compared with CT soils regardless of ryegrass cover crop. Similarly, soils from ryegrass cover crop had higher Kf values than soils from no cover crop, regardless of tillage. The N values for cyanazine adsorption were greater than 0.90 in all soils, indicating near linearity. Accordingly, Kd values were almost similar to their respective Kf values (Table 2). / Fig. 1 / Table 2
The differences in cyanazine adsorption (Kf) among the soils from four treatments is similar to the differences in soil pH and organic C (Table 1). NT soils had slightly higher organic C and lower pH than CT soils, and the soils from ryegrass cover crop had slightly higher organic C and lower pH than soils from no cover crop. Higher cyanazine adsorption observed in soils from NT and ryegrass cover crop plots may be attributed to higher organic C content and soil pH close to the pKa (5.1). These results are similar to that observed for fluometuron under NT system by Brown et al. (1994). Fluometuron adsorption was greater in hairy vetch cover cropped soil (Kd, 2.96) than in no cover crop soil (Kd, 2.23), which was attributed to the higher organic C accumulated over a period of 11 years in hairy vetch soil (2.5%) compared with no cover crop soil (2.0%). In other studies, greater adsorption of alachlor {2-chloro-N-(2,6-dimethylphenyl)-N-(methoxymethyl)acetamide} (Locke 1992) and chlorimuron {ethyl 2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid} (Reddy et al. 1995b) in several NT soils compared with their respective CT soils was also related to higher organic C in NT soils. However, bentazon {3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide} adsorption was similar in both CT and NT soils due to a very low sorption of bentazon (Gaston et al. 1996).
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Cyanazine Adsorption Kinetics
Cyanazine adsorption in soils and ryegrass residue during a 1 to 312-h shaking indicated that adsorption increased very rapidly during the first hour, and shaking for 24 h increased adsorption slowly (Fig. 2), but shaking beyond 24 h had very little or no affect on adsorption. In an adsorption kinetics study on peat, Cancela et al. (1990) have found that cyanazine attained adsorption equilibrium within 1 h. The thermodynamic parameters of cyanazine study indicated that adsorption mechanism were hydrogen bonding and possibly adsorption of protonated species (Cancela et al. 1990). Slow but continued adsorption of cyanazine in the CT and NT soils from ryegrass plots was well described using the three-site model (1a to 1c). In contrast, the ryegrass residue samples achieved apparent equilibrium within a 48-h reaction time. Accordingly, the two-site analogue, with kir = 0, was sufficient to describe cyanazine adsorption kinetics in ryegrass residue (Fig. 2). Estimates of parameters are given in Table 3. The values for instantaneous equilibrium coefficient ke and forward rate constant kf were larger in ryegrass residue, followed by soils from NT and CT plots. Uncertainty was fairly high for estimates, particularly for kf and kir values. / Fig. 2 / Table 3
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Cyanazine Desorption
At a cyanazine concentration of 0.91 µmol L-1, the amount of CaCl2-desorbable cyanazine in two 24-h desorptions ranged from 77 to 82% of that adsorbed among the soils but was only 46% in ryegrass residue (Table 4). Overall, the amount desorbed by CaCl2 in the first step was about four times higher than the amount desorbed in the second step (Data not shown.). Methanol was used to maximize desorption following CaCl2 desorption. Two 24-h methanol desorptions removed an additional 12 to 14% of that adsorbed in the soils. However, methanol desorbed most of the remaining cyanazine from ryegrass residue (Table 4). Undesorbable (bound cyanazine) amounts were less than 8% of that adsorbed in both soils and ryegrass residue. Similarly, other workers have reported hysteresis in cyanazine desorption from soils (Clay et al. 1988). Total recovery of cyanazine was greater than 98% for soils as well as for plant residues. A similar desorption trend was observed for 7.99 µmol L-1 concentration (Table 4). The amounts of cyanazine desorbed with CaCl2 in this study were higher than the amounts reported for chlorimuron from soils (Reddy et al. 1995a). This may be attributable, in part, to differences in herbicide chemistry; chlorimuron, unlike cyanazine, is a weak acid. However, in plant residues, the patterns of cyanazine desorption by either CaCl2 or methanol are similar to that observed for chlorimuron (Reddy et al. 1995a). / Table 4