1. Potential Interaction of Glyphosate with the Soil Environment
L. Tribe, M.A. Fidanza, T.C. Gehris, and G.A. Khoury
Pennsylvania State University ● Division of Science ● Berks Campus ● Reading, Pennsylvania
American Society of Agronomy - Crop Science Society of America - Soil Science Society of America
International Annual Meetings
November 4 - 8, 2007
New Orleans, Louisiana
USA
ABSTRACT
Glyphosate (N-[phosphonomethyl] glycine) is one of the world’s most utilized and recognized herbicides for weed management in crop production systems. Glyphosate is actually a phosphonic analogue of the natural amino acid glycine. Glyphosate is considered a systemic, non-selective, postemergence herbicide effective against both broadleaf and grass weeds, and is especially useful for the control of perennial weeds. The potential persistence of glyphosate in the soil is explored in this current research. A theoretical model was developed to explore the interaction of glyphosate with montmorillonite clay. The model was constructed using Spartan Pro (Wavefunction, Inc, Irvine, CA, USA), a commonly used chemical modeling computer program. The model revealed that glyphosate was attracted to clay layers based on molecular mechanics calculations. Correlations between the equilibrium interatomic distances suggest changes in energy associated with the stabilization of several configurations of this model.
INTRODUCTION
Glyphosate is a systemic, non-selective, foliar-applied, postemergence herbicide effective on annual and perennial broadleaf and grassy weeds in agricultural cropping systems. Glyphosate is rapidly and tightly adsorbed to soil, and the organic matter, clay, silt, or sand contents, or soil pH, has been reported to have minimal effect on adsorption. However, high concentrations of metallic cations in clay soils generally increase the amount of glyphosate adsorbed. Strong adsorption to soils is generally supported by observations of low plant phytotoxicity with soil applications, since many crops can be seeded or transplanted immediately into glyphosate-treated sites. Glyphosate is considered to have a “moderate” persistence in soil, with a typical field half-life of 47 days, or laboratory half-life of about 25 days. Montmorillonite, a 2:1 smectite clay, is a common component of many agricultural soils in the USA and throughout the world. Although the interaction between glyphosate and the soil environment has been investigated, the possibility for the glyphosate compound or its moieties to accumulate within the soil warrants further study. Therefore, the objective of this theoretical research was to explore the potential persistence of glyphosate in the clay fraction of the soil environment.
MATERIALS and METHODS
A theoretical mathematical model was constructed using Spartan Pro (Wavefunction Incorporated, Irvine, CA, USA). To model the interaction of the fully protonated glyphosate molecule with montmorillonite clay, molecular modeling calculations were performed with the glyphosate molecule placed within the montmorillonite interlayer. The glyphosate molecule was allowed to move freely and without constaints, which helps to avoid producing biased results while simulating the interactions of large molecules.
Figure 1. Glyphosate (C3H8NO5P),
N-(phosphonemethyl) glycine.
The zwitterion pictured here is the common form found in an aqueous solution.
REFERENCES
Amrhein, N. et al Plant Physiol. 66:830.
Boerboom and Wyse Weed Sci. 36:291.
Coupland, D Pestic. Sci. 15:226.
Dewey and Appleby Weed Sci. 31:308.
Jachetta, J.J. et al Plant Physiol. 82:100.
Marshall, G. et al Pestic. Sci. 18:55.
Martin and Edgington Pestic. Biochem. Physiol. 16:87.
Sandberg, C.L. Et al Weed Res. 20:195.
Wauchope, R.D. et al Rev. Environ. Contam. Toxicol. 123:1.
For further questions or comments, please contact:
Lorena Tribe, Ph.D
Assistant Professor of Chemistry
Pennsylvania State University – Berks Campus
RESULTS
Figure 2. Montmorillonite is a
2:1 smectite clay, which has an
octahedral aluminum layer sandwiched between two tetrahedral siloxane layers,
which may be penetrated by water, ions, and other solutes.
Figure 3. Glyphosate without (a) and with (b) the addition of water molecules to the interlayer of montmorillonite clay. a b
Figure 4. A linear trend was observed between the addition of water molecules and its effect on the interlayer differences (a, b, c). c
PMG3-
PMG2-
PMG-
Eads’
Ecryst
Egly+Ewater
Etot
Species
Bidentate
Monodentate
Monodentate PMG-Montmorillonite.
Bidentate PMG-Montmorillonite.
Figure 6. Similar trends were observed for the interactions between PMG and montmorillonite in the presence and absence of water, which helps to confirm the validity of developing a model for the interactions without water.
Figure 5. The initial seven configurations of the potassium-montmorillonite and glyphosate system.
4.9179
2.3703
3.5034
3.7406 7
4.0065
2.9495
3.4854
4.4708
5.2736
4.7970
3.3724
4.4193 6
3.9482
2.0336
4.0208
4.3764
3.9753
8.1715
8.7161
9.4065 5
2.6152
6.4015
6.0904
6.5193
2.6471
4.5351
3.9294
4.6724 4
4.1317
6.6739
9.7783
2.6304
5.3585
6.4503
6.3416 3
2.6811
2.6573
5.4055
6.0360
3.7951
2.9544
3.5271
4.5862 2
4.2505
5.2592
3.2986
4.2772
2.7672
2.2676
3.6996
4.3373 1
3.7571
2.3382
3.9200
4.4851
Closest O
4.7500
1.9994
3.2576
4.2712
4.3945
1.8969
3.5822
4.3822
4.2521
2.0806
3.4571
4.2763
4.5098
1.8759
3.6358
4.4582
2.7052
1.7388
3.2537
4.4191
4.1315
1.9316
5.0750
5.6846
5.0404
7.0407
9.4299
9.9598
6.2802
2.0124
4.5902
4.9865
3.1268
3.4367
5.9777
6.6549
5.4025
3.2118
5.0195
5.7762
4.1306
5.4469
3.3715
4.1802
2.8766
6.5060
3.9030
4.3810
4.2147
2.6435
3.8169
4.2049
4.9886
2.3860
4.6744
5.7744
PMG
ncN
H(COO)-O
H(N)-O
P-O
P-Si
Model
Table 1. The effect of pH on the adsorption of glyphosate was modeled by considering the same seven initial configurations
for PMG-, PMG2-, and PMG3- without water molecules.
CONCLUSION
Molecular mechanics was useful in calculating and visualizing the final conformations of glyphosate with montmorillonite. All models utilized the complete montmorillonite structure to allow glyphosate the maximum freedom when approaching molecular surfaces. The swelling of montmorillonite as a function of humidity showed the d001 interlayer to swell to 16.8A, which was a change of 6.7A. In the glyphosate-montmorillonite calculations for PMG, the charged amino moiety was attracted to the negative surface of the interlayer for 6 of 7 conformations chosen. In the exception, the initial position of glyphosate may have led the glyphosate to be trapped in a local minimum. When adding water, the same results were observed. The effect of pH on adsorption revealed that as alkalinity increased, the number of times the amino group was closest decreased. The adsorption energy glyphosate with montmorillonite for neutral PMG revealed that the system was more stable when the NH2+ moiety was closest to the surface based on the general trend towards more negative adsorption energies. Based on adsorption energies for monodentate and bidentate systems, it can be concluded that either complex can form between glyphosate and montmorillonite clay, since all bonding energies were negative for all acidities of PMG.