A significant fraction of pesticides sprayed on crops may be returned to soils via plant residues, but its
fate has been little documented. The objective of this work was to study the fate of glyphosate associated
to plants residues. Oilseed rape was used as model plant using two lines: a glyphosate-tolerant (GT) line
and a non-GT one, considered as a crucifer weed. The effects of different fragmentation degrees and
placements in soil of plant residues were tested. A control was set up by spraying glyphosate directly on
the soil. The mineralization of glyphosate in soil was slower when incorporated into plant residues, and
the amounts of extractable and non-extractable glyphosate residues increased. Glyphosate availability
for mineralization increased when the size of plant residues decreased, and as the distribution of plant
residues in soil was more homogeneous. After 80 days of soil incubation, extractable 14C-residues mostly
involved one metabolite of glyphosate (AMPA) but up to 2.6% of initial 14C was still extracted from
undecayed leaves as glyphosate. Thus, the trapping of herbicides in plant materials provided a protection
against degradation, and crops residues returns may increase the persistence of glyphosate in soils. This
pattern appeared more pronounced for GT crops, which accumulated more non-degraded glyphosate in
their tissues.
© 2016 Elsevier Ltd. All rights reserved