The Fate of Glyphosate in Crop Residues

Abstract

The envitonraental fate of glyphosate [Ai-(phosphonomethyl)glycinc] was studied in six crop residue (CR) types, three from maize [Zea mays L) (Ml, M2, and M3) and thtee from soybean [Glycine max (L.) Mem] (SI, S2, and S3). Glyphosate adsorption was characterized through isotherms. The glyphosate distribution in CRs was characterized through the balance of '''C-glyphosate radioactivity among the mineralized fraction, the e.xcractabk fractions (water and NH^OH), and the nonextractable fraction. Crop residues wete 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 A^f index range for maize CR was 1.5 to 8.3 L kg"' and 2.6 to 7.4 L kg"' for soybean CR. Organic C and hemicellulose partially explained adsorption variability. The addition of mineralized and nonextractable fractions of the initial '''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 ' ''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.

Modelling the impacts of maize decomposition on glyphosate dynamics in mulch

“…This finding could be explained by a stimulation of degrading microorganisms by the labile C present in fresh residues. The glyphosate mineralization in our study was greater than that found by Rampoldi et al (), who reported that 41.7% of glyphosate had mineralized in maize residues after 56 days in a laboratory incubation at 28°C and at a moisture content of 85% w/w, but these authors incubated maize residues of a few mm size and monitored glyphosate mineralization in the absence of soil.…”

“…The increased stabilization and formation of bound residues of 2,4‐D, chlorophenols (Benoit et al , ) and atrazine in decomposing wheat straw (Benoit & Preston, ) were also reported; these changes were attributed to modifications in microbial populations and their related enzymatic activities. Rampoldi et al () found that glyphosate mineralization was affected by the origin and age of crop residues and concluded that fresh maize residues mineralized more glyphosate than decomposed residues. The rapid degradation of benazolin (Berns et al , ) after adding maize straw to soils was also attributed to greater microbial activity induced by the organic amendment.…”

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

“…Piccolo et al (1996) reported increased glyphosate adsorption with increased aliphaticity and molecular size of humic substances; however, high aromatic structural content and small molecular dimensions in humic substances showed inverse results. In previous studies, we demonstrated that glyphosate adsorbed negligible amounts on unhumified organic matter (Rampoldi et al, 2011). Collectively, these results suggested that small changes in SOM quality associated with soil management can affect extended glyphosate adsorption.…”

Glyphosate sorption to soils and sediments predicted by pedotransfer functions

“…Since the 1990s, glyphosate has been the most heavily used pesticide worldwide for weed control in agriculture, forestry and urban landscapes, with postemergent applications (Candela et al 2007;Kogan et al 2003;Ololade et al 2014;Rampoldi et al 2011;Székacs and Darvas 2012). Despite glyphosate's strong tendency to sorb to soils, its intensive use may be associated with a risk of freshwater and groundwater contamination.…”