Glyphosate Transport in Two Louisiana Agricultural Soils: Miscible Displacement Studies and Numerical Modeling

Joshua Padilla,H Selim

doi:10.3390/soilsystems2030053

Abstract

Glyphosate (N-(phosphonomethyl) glycine) (GPS) is currently the most commonly used herbicide worldwide, and is generally considered as immobile in soils. However, numerous reports of the environmental occurrence of the herbicide coupled with recent evidence of human toxicity necessitate further investigation as to the behavior of GPS in the soil environment. Batch sorption studies along with miscible displacement experiments were carried out in order to assess the mobility of GPS in two Louisiana agricultural soils; Commerce silt loam and Sharkey clay. Batch results indicated a high affinity of both soils for solvated GPS, with greater affinity observed by the Sharkey soil. GPS sorption in the Commerce soil was most likely facilitated by the presence of amorphous Fe and Al oxides, whereas the high cation exchange capacity of the Sharkey soil likely allows for GPS complexation with surface exchangeable poly-valent cations. Miscible displacement studies indicate that GPS mobility is highly limited in both soils, with 3% and 2% of the applied herbicide mass recovered in the effluent solution from the Commerce and Sharkey soils, respectively. A two-site multi-reaction transport model (MRTM) adequately described GPS breakthrough from both soils and outperformed linear modeling efforts using CXTFIT. Analysis of extracted herbicide residues suggests that the primary metabolite of GPS, aminomethylphosphonic acid (AMPA), is more mobile in both soils, although both compounds are strongly retained.

Highlights

Glyphosate (N-(phosphonomethyl) glycine) (GPS) is the active ingredient in several herbicidal formulations and is the most widely used herbicide in the world
Generally regarded as relatively immobile in soils, several investigations indicate that GPS transport from application sites to the surrounding environment is significant [1,2,3,4]. These results coupled with recent evidence of human toxicity [5,6,7,8] require further investigation into the mobility of GPS in soils
Kjær et al [9] conducted an eight-month field trial to assess GPS mobility and detected the herbicide in tile-drained leachates at concentrations that exceeded European Union maximum admissible levels for up to seven days after rainfall events, which the authors attributed to macropore transport

Summary

Introduction

Glyphosate (N-(phosphonomethyl) glycine) (GPS) is the active ingredient in several herbicidal formulations and is the most widely used herbicide in the world Such widespread use can be attributed to the compound’s high weed killing efficiency coupled with its incorporation in agricultural systems utilizing Roundup ready cultivars. Generally regarded as relatively immobile in soils, several investigations indicate that GPS transport from application sites to the surrounding environment is significant [1,2,3,4]. These results coupled with recent evidence of human toxicity [5,6,7,8] require further investigation into the mobility of GPS in soils.

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Results

Conclusion