Modelling the impacts of maize decomposition on glyphosate dynamics in mulch

Abstract

The retention of crop residues as mulch on the soil surface in conservation agriculture systems greatly influences the fate of pesticides, as most of the applied pesticide is intercepted by mulch before moving to the soil. This work was conducted in order to model the effect of maize decomposition on glyphosate degradation in mulch and soil. Labelled 14 C-glyphosate degradation was monitored for 49 days in three treatments with the same soils but with maize residues at different stages of decomposition (0, 20 and 49 days). Fresh residues of maize (0 days) exhibited an evolution of their biochemical fractions to a greater extent than decomposed residues. Glyphosate mineralization was faster in the 0-day treatment in mulch residues and in the soil layer below the mulch. However, a greater formation of non-extractable residues (NERs) was observed in mulch residues and soils in the 20-and 49-day treatments than in the 0-day treatment. Modelling maize mulch decomposition with the COP-soil model indicated that microbial activity was different in the three treatments and depended on the initial composition of maize residues. Glyphosate mineralization in mulch and soil can be simulated with an assumption of co-metabolism by coupling the modules of pesticide degradation and mulch carbon decomposition. Glyphosate and its metabolites, including soluble and adsorbed fractions, were simulated with the same adsorption coefficients for all treatments. The simulation of NER formation, however, suggested that more than one microbial population may be involved in the degradation process and could be added in the future development of the model.

Effect of rainfall regimes and mulch decomposition on the dissipation and leaching of S -metolachlor and glyphosate: a soil column experiment

“…Using 14 C labelled glyphosate, the authors have recently quantified strongly sorbed (extracted by NH 4 OH) and non‐extractable residues in similar experimental systems but without rainfall conditions. At day 0, both fractions represented more than 40% of initial applied glyphosate, with the majority being found in soil below the mulch . This could explain the much lower mass recovery of glyphosate (54%) compared with S ‐metolachlor (89%) at the beginning of the incubation.…”

“…The interpretation of the glyphosate data is more difficult owing to the difficulties encountered in establishing satisfactory mass recoveries. As previously mentioned, the extraction procedure (water and sonication) did not allow for the recovery of all glyphosate in the soil because of the strong adsorption of glyphosate to soil . In addition, the quantification and detection limits of glyphosate were much greater than those of S ‐metolachlor.…”

“…At day 0, both fractions represented more than 40% of initial applied glyphosate, with the majority being found in soil below the mulch. 52 This could explain the much lower mass recovery of glyphosate (54%) compared with S-metolachlor (89%) at the beginning of the incubation. Dissipation half-life estimation took into account the differences in pesticide recovery by normalising to 100% the amount recovered at 0 days.…”

“…As previously mentioned, the extraction procedure (water and sonication) did not allow for the recovery of all glyphosate in the soil because of the strong adsorption of glyphosate to soil. 52,58 In addition, the quantification and detection limits of glyphosate were much greater than those of S-metolachlor.…”

Mulch of plant residues at the soil surface impact the leaching and persistence of pesticides: A modelling study from soil columns

“…Aslam et al (2013) suggested that the adsorption of pesticides was associated with the chemical composition of crop residues. The characteristics of the origin and decomposition degree of crop residues have also been reported to influence pesticide degradation (Rampoldi et al, 2008;Aslam et al, 2014).…”