J. Agric. Food Chem. 1990, 38, 1985-1988
1985
Paraquat Behavior in Costa Rican Soils and Residues in Coffee Manuel A. Constenla,$ Dave Riley,*?$Steve H. Kennedy,$ Carlos E. Rojas,' Luis E. Mora,$ and John E. B. Stevens5 CONICIT and School of Chemistry, University of Costa Rica, San JosB, Costa Rica, and Jealott's Hill Research Station, IC1 Agrochemicals, Bracknell, Berkshire RG12 6EY, U.K.
The paraquat deactivation capacities of soils, from 20 coffee plantations, generally ranged from 100 to 500 mg of paraquat/ kg of soil. Even when paraquat had been applied up to twice per year (1.2 kg ha-' year) for 20 years, residues were less than 10% of the soils' deactivation capacities and in the majority of cases were less than 1?6. Residues in coffee berries and beans were at or below the limit of determination of 0.02 mg/kg. This confirmed that strongly adsorbed residues in soil are not absorbed by the crop and there is no translocation of residues into the crop following accidental contamination of bushes during spraying.
INTRODUCTION The adsorption, deactivation, and degradation of paraquat in soil have been intensively studied over the past 30 years, particularly in temperate regions. However, no data were available for soils in Central America. Therefore, considering its importance to agriculture in the region and its widespread use, the adsorption, deactivation, and persistence of paraquat in soil and residue levels in coffee, grown on soils where paraquat has been frequently used, have been investigated. This paper describes a series of studies carried out on soil and crops from Costa Rica. Paraquat is rapidly and strongly adsorbed by soil; consequently, it has no residual biological activity and does not leach (Calderbank, 1968;Riley et al., 1976). The strong binding is primarily due to the formation of a chargetransfer complex between paraquat, a divalent cation, and negatively charged clay and organoclay surfaces (Haque and Lilley, 1972; Khan, 1973). The amount and strength of binding depend on the amount and type of clay present. Soil organic matter can also adsorb large amounts of paraquat, but not as strongly as clay minerals. Adsorbed paraquat has been arbitrarily classified into two types: loosely bound, which can be desorbed with saturated ammonium chloride, and tightly bound, which cannot be desorbed with saturated ammonium chloride (Tucker et al., 1967). The latter can only be released by destroying the clay by refluxing with 9 M sulfuric acid. Tightly bound residues do not have any activity on plants or other soil organisms and are not absorbed by plants or earthworms (Riley et al., 1976). A bioassay technique has also been developed to measure the strong adsorption capacity of soils. Paraquat is equilibrated with soil in a dilute slurry and the equilibrium solution bioassayed with wheat. The strong adsorption capacity (SAC-WB) is defined as the concentration of soil-adsorbed paraquat when the concentration of paraquat in the equilibrium solution is sufficient to inhibit the elongation of 14-day-oldwheat roots by 50% relative to an untreated control. Under field conditions soil residues up to the SAC-WB value, and sometimes higher, have no biological activity (Riley et al., 1976). Paraquat residues on plants, and possibly soil surfaces, are photochemically degraded (Slade, 1965; Calderbank, 1968); thus, not all the paraquat applied reaches the soil. t
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University of Costa Rica. IC1 Agrochemicals. 002 1-856 1f 90f 1438-1985$02.50/0
Soil contains microorganismsthat can rapidly degrade unadsorbed paraquat in culture solutions (Funderburk and Bozarth, 1967; Calderbank, 1968; Riley et al., 1976; Carr et al., 1985). However, the availability of strongly adsorbed residues to microorganisms is greatly reduced, and like many other bound residues in soil their rate of degradation is slow. Long-term field studies in the United States and the United Kingdom have shown strongly adsorbed residues have a half-life of the order of 10 years. For example, in one trial paraquat was applied at 4.5 kg ha-' year-' for 16 years, and soil residues were monitored. The half-life was approximately 7 years, and soil residue levels tended toward a plateau level where the rate of degradation equaled the rate of application (Hance et al., 1985). The rate of degradation is sufficient to ensure that the deactivation capacity of almost all soils will not be exceeded as a result of indefinite use of paraquat. EXPERIMENTAL METHODS Soil Samples. Soil samples were collected as described in the adsorption and degradation sections below. All samples were passed through a 2-mm sieve prior to being analyzed or used in the laboratory studies. The following physicochemical properties were measured: percent sand, percent silt, percent clay, percent OM (organic matter), pH (in water), and CEC (cation-exchangecapacity). Soil Tightly Bound and Strong Adsorption Capacities. Ten soils from coffee-growingregions were collected at 0-5-cm depth during the period August 1982-April 1983 (Table I). Samples (20 g) were shaken for 24 h with 200 mL of a solution containing 30 mg/L paraquat. The supernatant liquid was then analyzed for paraquat and the amount adsorbed calculated by difference from the amount applied. Adsorption coefficients, Kd, were calculated by using the equation K= [adsorbed paraquat] (mg/kg)/[paraquatin solution] (mg/L)
Samples of the supernatant solution were also analyzed after 2, 4,8, and 16 h of equilibration. The samples were then treated as follows to remove unadsorbed and loosely bound paraquat: filtered, washed with water, shaken for 24 h with 200 mL of water, filtered, shaken for 24 h with 200 mL saturated ammonium chloride, filtered, and washed with water. The remaining tightly bound paraquat was extracted by refluxing the soil with 9 M HzS04 for 5 h. A further 20 soil samples were collected (about 12 cores approximately 15 cm deep bulked together) from coffee-growing areas in July and August 1985 (Table 11). The strong adsorption capacities (SAC-WB)of the soils were determined by using a wheat bioassay (Riley et al., 1976). Briefly, samples (10 g) 0 1990 American Chemical Society
1986 J. Agric. Food Chem., Vol. 38. No. 10, 1990
Constenla et al.
Table I. Paraauat AdsorDtion Characteristics of Costa Rican Soils D 80
location Sta. Maria de D o h Dulce Nobre Tres Rios Frailes TarrazG Brazil Sacta Ana San Roque Grecia La Isabel Turrialba Atirro Turrialba San Marcos Tarrazli Sarchi Valverde Vega Barba Heredia
texture clay loam sandy loam sandy clay loam sandy clay loam sandy loam clay loam clay clay loam sandy loam sandy clay loam
sand 34 68 53 49 54 41 39 44 57 46
c
silt 33 29 24 25 29 27 17 24 29 26
I-
r
rr
clay 33 3 23 26 17 32 44 32 14 28
OM 7.6 14.1 14.3 7.7 8.6 5.4 2.1
1.7 9.1 3.7
CEC, mequiv/ 100 g
20 36 36 37 36 29 21 22
33 24
pH 5.0 7.0 4.9 6.4 6.1 5.3 5.0 5.6 5.6 5.8
tightly bound paraquat, mg/kg 168 81 100 17 147 273 188 80
paraquat adsorption equilibrium0 solution, adsorbed, mg/L mg/k Kd 2.0 280 140 293 0.7 419 1.1 289 263 1.3 287 221 291 0.9 323 1.3 287 221 0.1 299 2990 1.1 289 263 0.7 293 419 1.3 287 221
100
142
20 g of soil equilibrated with 200 mL of solution containing 30 mg/L paraquat. Table 11. Paraauat Strong AdsorDtion CaDacitv (SAC-WB) and Residues of Soil from Costa Rican Coffee Growing Areas C c CEC, paraquat paraquat coarse fine r; 'r T mequiv/ SAC-WB, residues, residues, % location texture sand sand silt clay OM 100g pH mg/kg mglkg of SAC-WB 2 27 31 San Pedro silty clay loam 40 8.1 29 5.1 95 3.3 3.5 1 27 34 38 7.9 370 1.1 silty clay loam 29 5.7 Centro de Palmares
