Controlling factors for glyphosate sorption: and the prediction of sorption coefficients for cultivated soils of Southern Greenland and elsewhere

Abstract

Den mest benyttede generelle plantegift i verden er glyfosat, den aktive ingrediens i populære produkter som Monsantos Roundup. Det globale forbrug af glyfosat stiger fortsat i 2017. Der er bekymringer for miljøpåvirkningen fra både glyfosat selv, nedbrydningsproduktet AMPA, den kation det distribueres med, overfladeaktive -og andre hjælpestoffer. Sædvanligvis forventes glyfosat at nedbrydes inden nedsivning til grundvand kan forekomme, men under det danske varslingssystem for udvaskning af pesticider er glyfosat fundet over grænseværdien i grundvand. Glyfosat er vandopløseligt, og binder sig til jordpartiklernes overflader efter indviklede og endnu ikke fuldt afdækkede principper. Dette studie beskæftiger sig med styrken af bindingerne mellem glyfosat og jord, og de jordparametre der kontrollerer styrken. Modeller for bestemmelse af den lineære sorptionskoefficient for glyfosat ved koncentrationen 0.23 mg\L diskuteres og forbedres. En samling af jordprøver er nøje udvalgt. Danske marker med udtalte gradienter i lerindhold, pH og organisk materiale er medtaget. Sammen med marker fra Grønland opnås en gradient i jordenes alder, lige fra de helt nyopdyrkede arealer, arealer der har været under plov i et par generationer, og til de århundrede gamle landbrugsjorde fra Danmark. Jorde med forskellig oprindelse er medtaget, fra Danmarks glaciale aflejringer til det grønlandske grundfjeld. Problemer med at fremskaffe data for jorde af vulkansk oprindelse gør at disse i hovedparten ikke er medtaget. Jordparametre er indhentet fra videncentret på Århus Universitet, Foulum, eller målt i laboratoriet. Sorptionskoefficienter for glyfosat er målt ved masseeksperimenter på C 14 -mærket glyfosat og liquid scintillation counting. Lineære regressionsanalyser er udført i Matlab. Jordparametrene pH, indhold af ler, indhold af finsilt, indhold af oxalat ekstraherbar fosfor og i aller højeste grad indhold af oxalat ekstraherbar jern viser sig at være vigtige for forudsigelsen af sorptionskoefficienter for glyfosat. Parametrene elektrisk ledningsevne, indhold af organisk materiale og oxalat ekstraherbar aluminium viser sig kun at have lille eller slet ingen indflydelse. Jordens oprindelse og landbrugshistorie findes at påvirke forholdet mellem jordparametre og sorptionskoefficienter for glyfosat. Modeller for bestemmelse af sorptionskoefficienter for glyfosat ud fra oxalat ekstraherbar jern og enten pH eller oxalat ekstraherbar fosfor findes at have R 2 over 0.6 på regionalt eller højere niveau.

The most used non-selective herbicide in the world is glyphosate, the active ingredient of popular formulations such as Monsanto's Roundup. The global use of glyphosate is still in 2017 on the rise. There are concerns for the environmental safety of both glyphosate itself, its degradation product AMPA, the cation it is distributed by, surfactants and other adjuvants. Glyphosate is generally expected to degrade before leaching to groundwater resources can occur, but under the Danish Pesticide Leaching Assessment Programme glyphosate has been found in groundwater above the limit value. Glyphosate is water soluble, and binds to soil surfaces in complex ways still not fully understood. This study focuses on the strength of the bonds made between glyphosate and soil, and the soil parameters controlling the strength. Models for predicting the linear glyphosate sorption coefficient at the glyphosate concentration 0.23 mg\L are discussed and improved. A sample set is carefully selected. Danish fields with pronounced gradients in clay content, pH and content of organic carbon are included. In junction with Greenlandic fields a gradient in soil age is obtained, ranging from areas recently introduced to farming, areas that have been farmed for only a few generations and the centuries old farmlands of Denmark. Soils of different origin are included, ranging from the glacial deposits of Denmark to the crystalline rocks of Greenland. Inability to obtain data rendered an attempt at including soils of volcanic origin unsuccessful for the most part. Soil parameters are obtained from the knowledge base of Aarhus University, Foulum, or measured in the laboratory. Glyphosate sorption coefficients are measured by batch experiments on C 14 -marked glyphosate and liquid scintillation counting. Linear regression analyses are performed in Matlab. The soil parameters of pH, content of clay, content of fine silt, content of oxalate extractable phosphorus and most of all content of oxalate extractable iron prove important in predicting glyphosate sorption coefficients. Parameters of electrical conductivity, content of organic carbon and oxalate extractable aluminium are found to have only a weak or no influence. The historical management and origin of the soils are found to influence the relationships between soil parameters and glyphosate sorption coefficient. Pedotransfer functions for predicting a glyphosate sorption coefficient based on oxalate extractable iron and either pH or oxalate extractable phosphorus are found to have coefficients of determination above 0.6 at regional level or higher.

A master thesis from Aalborg University

Controlling factors for glyphosate sorption: and the prediction of sorption coefficients for cultivated soils of Southern Greenland and elsewhere