Comparative fate of glyphosate and triclopyr herbicides in the forest floor and mineral soil of an Acadian forest regeneration site

Following applications of three different salt formulations of glyphosate (Vision®, ouchdown®, and Mon14420) and an ester formulation of triclopyr (Release®) to an Acadian forest regeneration site in New Brunswick, Canada, the fate and persistence of herbicide residues in the forest floor and underlying mineral soil were investigated. Within 14 days of treatment, maximal residue levels (average 8.3 µg·g dry mass-1) were observed in the forest floor matrix following application of the glyphosate formulations, with higher values (45.7 µg·g dry mass-1) observed for triclopyr. Residue maxima in the underlying mineral soil were, on average, 5.7-fold lower than those in the forest floor. In both matrices, glyphosate residues declined exponentially with time, irrespective of the formulation applied. Among the glyphosate treatments no significant differences (p = 0.16, p = 0.97, for forest floor and mineral soil respectively) were observed in the estimated times to 50% dissipation (DT50). Overall, average DT50 values for glyphosate were estimated as 12 ± 2 and 10 ± 3 days for the forest floor matrix and mineral soil, respectively. Triclopyr residues, particularly in the forest floor, were characterized by a series of transient increases, possibly reflecting temporally varying inputs from dew, rainwash, or litter fall from surrounding treated vegetation. Triclopyr residues also dissipated with time, with approximate DT50 values ranging from 39 to 69 days in the forest floor and mineral soil, respectively.