Factors Influencing Microbial Degradation of14C-Glyphosate to14CO2in Soil

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Abstract
14C-glyphosate [N-(phosphonomethyl)glycine] degradation to 14CO2 was examined in a Spinks sandy loam, Collamer silt loam, and a Norfolk loamy sand. After 32 days, 40, 9.5, and 3% of the 14C-glyphosate was recovered as 14CO2 in the three soils, respectively. The degradation was primarily microbial. Phosphate additions stimulated 14C-glyphosate degradation to a limited extent in the Collamer silt loam but not in the Norfolk loamy sand. Additions of Fe+++ and Al+++ ions reduced degradation in the Spinks sandy loam. It is postulated that formation of colloidal Fe and Al precipitates in modified soils with concomitant adsorption of 14C-glyphosate is responsible for decreased availability of 14C-glyphosate to microorganisms. Mn++ additions were found to increase degradation. Spinks soil and carbon substrate amendments failed to substantially increase degradation rates in both soils with low degradation rates.