The Fate of Glyphosate in Crop Residues
Résumé
The environmental fate of glyphosate [N-(phosphonomethyl)glycine] was studied in six crop residue (CR) types, three from maize (Zea mays L.) (M1, M2, and M3) and three from soybean [Glycine max (L.) Mem] (S1, S2, and 53). Glyphosate adsorption was characterized through isotherms. The glyphosate distribution in CRs was characterized through the balance of (14)C-glyphosate radioactivity among the mineralized fraction, the extractable fractions (water and NH(4)OH), and the nonextractable fraction. Crop residues were characterized by elemental composition, organic C, total N, and biochemical parameters (soluble fraction, cellulose, hemicellulose, and lignin). Total microbial activity (TMA) was also assessed. Limited and reversible glyphosate adsorption on soybean and maize CRs was determined. The sorption coefficient K(f) index range for maize CR was 1.5 to 8.3 L kg(-1) and 2.6 to 7.4 L kg(-1) for soybean CR. Organic C and hemicellulose partially explained adsorption variability. The addition of mineralized and nonextractable fractions of the initial (14)C-glyphosate applied on the CRs averaged 56%; however, differences were detected between soybean and maize CRs. Mineralization and nonextractable residues were 30.7 +/- 11 and 32.5 +/- 6% (soybean CR) and 44.3 +/- 12 and 17 +/- 7% (maize CR), respectively. We hypothesized that glyphosate molecules could be used initially by microorganisms as a labile C source. High variability in (14)C-glyphosate mineralization was observed in all crop residues, suggesting that the magnitude of the glyphosate mineralization process would be regulated by accessibility and the lability of other carbonate sources.