Carbon-14-Glyphosate Behavior in Relationship to Pedoclimatic Conditions and Crop Sequence

The recognition of glyphosate [(N-phosphonomethyl) glycine] behavioral patterns can be readily examined using a pedoclimatic gradient. In the present study, glyphosate adsorption-desorption and degradation were examined under different scenarios in relationship to soil properties and soil use applications. Three sites with varied pedoclimatic conditions and two crop sequences were selected. Adsorption-desorption and glyphosate distribution in mineralized, extractable, and nonextractable fractions were assessed under laboratory conditions. Glyphosate sorption was characterized by isotherms and glyphosate degradation using the distribution of C-14-glyphosate radioactivity among mineralized fractions, two extractable fractions (in water, ER1; in NH4OH, ER2), and nonextractable fractions. Results showed sorption indices (distribution coefficient K-d and Freundlich sorption coefficient K-d 13.4 +/- 0.3-64.1 +/- 0.9 L kg(-1) and 16.2-60.6, respectively), and hysteresis increased among soil sites associated with decreasing soil particle size <2 mu m, soil organic matter, and other soil properties associated with soil granulometry. A multiple stepwise regression analysis was applied to estimate the relationship between K-d values and soil properties. Cation exchange capacity, water field capacity, and Bray-1 P were the soil properties retained in the equation. Soils under continuous soybean [Glycine max (L.) Merr.] (monoculture) treatment exhibited reduced glyphosate adsorption and decreased hysteresis desorption relative to soils under rotation. To our knowledge, these results are the first to demonstrate that soils with identical properties exhibited different glyphosate retention capacities based on crop sequence. We propose possible explanations for this observation. Our results suggested that characterization of the variability in soil property gradients can serve to determine glyphosate behavioral patterns, which can establish a criterion for use in reducing potential environmental risks.