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Spatial and Temporal Variability of Carbofuran Degradation in Soil

USDA-ARS-Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames IA, 50011,

D.R. Shelton

USDA-ARS-PDL, Building 050, BARC-WEST, Beltsville, MD 20705.

^* Corresponding author.

ABSTRACT

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 of carbofuran (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.

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