2,4-D mineralization in unsaturated and near-saturated surface soils of an undulating, cultivated Canadian prairie landscape

Abstract

The herbicide 2,4-D [2,4-(dichlorophenoxy) acetic acid] is one of the most widely used pesticides in the Canadian prairies and is frequently detected as a ground and surface water contaminant. The objective of this paper was to determine the magnitude and extent of variation of 2,4-D mineralization in a cultivated undulating prairie landscape. Microcosm incubation experiments, using a 4 × 3 × 2 factorial experimental design (soil moisture, 4 levels: 60, 85, 110, 135% of field capacity; slope position, 3 levels: upper-, mid- and lower-slopes; soil depth, 2 levels: 0–5 and 5–15 cm), were used to assess 2,4-D mineralization. The first-order mineralization rate constant (k₁) varied from 0.03 to 0.22 day^{− 1}, while total 2,4-D mineralization varied from 31 to 52%. At near-saturated conditions (110 and 135% of field capacity), the onset of 2,4-D degradation was delayed in soil obtained from the upper- and mid-slopes but not in soils obtained from the lower-slope position. The k₁ and total 2,4-D mineralizationwas significantly influenced by all three factors and their interactions. The Freundlich sorption coefficient of 2,4-D ranged from 0.83 to 2.46 ug ^1–1/ng^{− 1} mL^1/n and was significantly influenced by variations in soil organic carbon content across slope positions. The infield variability of 2,4-D sorption and mineralization observed across slope positions in this undulating field was comparable in magnitude and extent to the regional variability of 2,4-D sorption and mineralization observed in surface soils across Manitoba. The large variability of 2,4-D mineralization and sorption at different slope positions in this cultivated undulating field suggests that landform segmentation models, which are used to delineate slope positions, are important considerations in pesticide fate studies.

Keywords:

• 2,4-D
• herbicide
• mineralization
• sorption
• slope position
• soil depth
• soil moisture content

Acknowledgments

Research funding for this project was provided by the Natural Sciences and Engineering Council of Canada (NSERC) and through a University of Manitoba Graduate Fellowship (UMGF) awarded to Janna Shymko.

Notes

^a Hydrometer method; SCL = sandy clay loam.

^bField Capacity (gravimetric) determined using laboratory leaching columns.

^cDetermined using a 2:1, 0.01 M CaCl₂:soil.

^dDetermined using dichromate oxidation method (Nelson and Sommers^{[ 41 ]}).

^eFreundlich sorption coefficient.

*Means within columns with the same letter are not significantly different (Student-Newman Keuls Test, α = 0.05).

^a Significant p-values are given in boldface (α = 0.05).

*Means of four replicates.

** Means within columns with the same letter are not significantly different (Student-Newman-Keuls Test, α = 0.05).

* Means of four replicates.

*Means of four replicates.