(1992) Effect of Water Potential, Temperature, and Soil Microbial Activity on Release of Starch-Encapsulated Atrazine and Alachlor

Starch encapsulation has been proposed as a method for controlling the rate at which pesticides are released into the soil. Relatively ittle is known about what environmental f ctors influence controlled release. A series of laboratory studies were initiated to improve our understanding of how water potential, temperature, and soil microbial activity influence rate of release of starch-encapsulated razine (2chloro-4-ethylamino-6-isopropylamino-s-triazine) and alachlor (2-chloro2’,6’-diethyI-N-(methoxymethyl)-acetanilide). Wat r potential, posed using polyethylene glycol, significantly influenced swelling of the starch matrix and rate of release of both herbicides. At a water potential of 0 MPa, complete release required 21 d for atrazine and 7 d for alachlor. As water potential declined, so did rate of release. At a water potential of 1.5 MPa, less than 50~ of the encapsulated atrazine and less than 80% of the encapsulated alachlor had diffused out of the starch matrix after 28 d. Temperature also influenced rate of release of both herbicides. At 35 °C nearly three times more atrazine and two times more alachlor was released from starch granules than at 15 °C at all sampling times. Soil microbes increase the rate of release. After 21 d there was a twofold increase in the percentage of atrazine released from starch granules applied to nonsterile soil compared with granules applied to sterile soil. Effect of soil microbes on rate of alachlor elease was apparent only at early times. After 5 d there was a 20% increase in the percentage of alachlor eleased from starch granules when microbes were present compared with release from starch granules applied to sterile soil. After 14 d essentially all of the alachlor had been released from starch granules applied to either sterile or nonsterile soil. The different influences that water potential, temperature, and soil microbes have on rate of release between atrazine and alachlor are likely due to differences in water solubility of atrazine (30 mg -I) and alachlor ( 240 mg L-~). Release of a starch-encapsulated compound is governed mainly by a diffusion process. When starch granules are applied to the soil they imbibe water, swell, and the encapsulated compound diffuses out of the starch matrix. As a consequence, environmental factors influencing diffusion and uptake of water by the starch granules should influence rate of release. Temperature is known to influence diffusion, and water potential may be one measure of the availability of water to the starch granule. Amylase (EC 3.2.1.1) is an enzyme that breaks down starch. Treatment of starch-encapsulated pesticides with amylase has been shown to enhances diffusion of the encapsulated compound from the starch matrix (Trimnell et al., 1985); however, the role that amylase produced by soil microbes may play in rate of release has not been evaluated. The objective of this research was to evaluate the effect of water potential, temperature, and soil microbial activity on rate of release of starch-encapsulated atrazine and alachlor. Atrazine and alachlor are widely used herbicides that exhibit differences in water solubility (30 mg L-~ for atrazine and 240 mg L-1 for alachlor) and should, therefore, exhibit differences in rate of release from the starch matrix (Baur, 1980). Field investigations are currently being conducted in several states to evaluate the effect of starch encapsulation of atrazine and alachlor on groundwater issues (M. M. Schreiber, 1990, personal communication). Results of the present study should have immediate application in helping interpret results of field studies. INCREASED AWARENESS concerning the potential for agricultural chemicals to serve as sources for groundwater contamination has increased the level of research directed at understanding pesticide behavior (Burkart et al.,. 1990). Modification of pesticide behavior by encapsulating the chemicals in a starch matrix is one experimental approach receiving increased attention (Wing et al., 1987). Starch encapsulation has been shown to reduce volatilization (Schreiber et al., 1978) and leaching (Gish et al., 1991) losses of some herbicides. Starch encapsulation modifies pesticide behavior by controlling the rate of release of the encapsulated compound (Baur, 1980; Schreiber and White, 1980; Wing et al., 1987). Our knowledge of potential factors influencing rate of release is limited to the chemical nature of the starch matrix (Coffman et al., 1984; White and Schreiber, 1984; Wing et al., 1987, 1988) and the nature of the chemical being encapsulated (Baur, 1980; Schreiber and White, 1980) with little knowledge of what effect environmental variables have on rate of release. Both authors, USDA-ARS, Hydrology Lab., Bldg. 265, BARCE, Beltsville, MD 20705. Received 1 Oct. 1991. *Corresponding author. Published in J. Environ. Qual. 21:382-386 (1992) MATERIALS AND METHODS