Spatial and Temporal Variability of Carbofuran Degradation in Soil

Loss of pesticide efficacy resulting from enhanced rates of microbial degradation has been observed with several pesticides including the insecticide carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Soils in which this phenomenon occurs are often referred to as "problem soils." Several previous studies have documented the temporal aspects of the conversion of a nonproblem soil to a problem soil, and have compared carbofuran degradation rates in problem vs. nonproblem soils. There have been few studies of the spatial or temporal variability of pesticide degradation, and no studies of the variability of carbofuran degradation, however. Our study was designed to evaluate the spatial variability of carbofuran degradation activity in a conventional-till and a no-till corn (Zea mays L.) field, and to assess temporal variations of carbofuran degradation activity. Soil samples were collected at two positional locations in each field (in-row and between-row) at three times during the growing season. Within the planting furrow, maximum rates of carbofuran degradation were higher and resulting half-lives ofcarbofuran (DT-50~) were lower than in samples collected between corn rows. Interactive effects of both microbial biomass and soil water content appeared to contribute to the observed differences in carbofuran degradation kinetics as well as to the positional differences observed. Temporal variations in carbofuran degradation appeared to be dominated by soil water content. At this time it remains unknown whether the observed increase in carbofuran degradation activity, in the row, occurred in response to the banded application of carbofuran, or to increased C availability in the rhizosphere. N/~ICROBIAL DEGRADATION is an important mech/.V.I. anism controlling the fate of pesticides in soil. In the absence of microbial growth, rates of pesticide degradation often adhere to first-order kinetics and are a function of the population densities and metabolic competence of pesticide-degrading microorganisms, pesticide bioavailability and soil factors affecting microbial activity. Rao and Davidson (1980) summarized first-order rate constants for a variety of pesticides from numerous studies, giving some indication of the variability in degradation rates with different soils and under different experimental conditions. . In contrast, the ability of soil microorganisms to utilize a pesticide as a growth substrate results in sigmoidal kinetics, at least under conditions conducive to microbial growth and where concentrations of substrate are greater than K~ and/or exceed initial biomass (Simkins and Alexander, 1985; Focht and Shelton, 1987). Where substrate concentrations are less than Ks and/or initial biomass, or conditions are not conducive to growth, apparent first-order or linear kinetics may be observed. In problem soils, losses of pesticide efficacy appear to be associated with accelerated rates of microbial degradation, indicative of microbial growth. Although direct evidence of increases in population densities has been difficult to T.B. Parkin, USDA-ARS-Nati. Soil Tilth Lab., 2150 Pammel Dr., Ames IA, 50011, and D. R. Sheiton, USDA-ARS-PDL, Building 050, BARC-WEST, Beitsville, MD 20705. Received 12 Aug. 1991. *Corresponding author. Published in J. Environ. Qual. 21:672-678 (1992). document (Scow et al., 1990; Moorman, 1988), pure cultures of microorganisms capable of utilizing pesticides as sole C and/or N sources have been isolated from "problem soils" (Chaudhry and Ali, 1988; Kams et al., 1986). Pesticide degradation rates in soil are dependent upon a variety of environmental and management factors that can affect both population densities and activity of pesticide-degrading microorganisms. Many of these parameters can vary dramatically as a function of time and location. Few studies, however, have been conducted to systematically examine the spatial and temporal variability associated with field rates of pesticide biodegradation. Walker and Brown (1983) examined spatial variability associated with simazine (6-ChloroN-N’-diethyl-l,3,5-triazine-2,4-diamine) and metribuzin [4-Amino-6-(1,1-dimethylethyl)-3-(methyl~hio)-l,2,4-triazin-5(4H)-one] degradation. Coefficients of variation associated with the first-order half lives of simazine and metribuzin degradation were 23 and 24.8%, respectively, for soil samples collected from the same plot. Plot to plot variability of degradation, determined in composite samples from 10 distinct plots were 6.9% for simazine and 21% for metribuzin. These results indicate that the small scale, within plot variability is a major component of the total variability. These previous studies involved pesticides that were not utilized as microbial growth substrates. In instances where a pesticide can be used as a growth substrate repeated applications may enrich for pesticide-metabolizing microorganisms resulting in significantly enhanced rates of biodegradation. This phenomenon has been observed with the soil insecticide, carbofuran. Laboratory studies have documented both the sigmoidal kinetics of carbofuran metabolism (Camper et al., 1987; Parkin et al., 1991) and that repeated application of carbofuran to soils results in enhanced rates of microbial degradation (Kaufman et al., 1985; Scow et al. 1990; Harris et al., 1984; Chapman et al., 1986). Also, in field comparisons, soils having a history of carbofuran application exhibit higher degradation rates than soils not previously exposed to carbofuran (Felsot et al., 1985, 1982; Turco and Konopka, 1990). No studies have examined the spatial variability associated with carbofuran degradation activity in the field, however. In contrast to herbicides, which are uniformly applied to the field, carbofuran is typically applied in a granular formulation at planting time, and is placed with the seed directly in the planting furrow. This necessarily results in extreme spatial heterogeneity of carbofuran concentrations between and within the corn row that, in turn, may give rise to spatial heterogeneity in rates of carbofuran degradation. In addition, the practice, of superimposing corn rows and carbofuran applications year-after-year in no-till cropping systems may result in sustained, localized populations Abbreviations: NT-IN, no-till in-row; NT-BTW, no-till betweenrow; CT-IN, conventional-till in-row; CT-BTW, conventional-till between-row.