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Sue Everett: Ecologist and former British Association of Nature Conservationists Chair, Sue is co-founder of two national wild-plant charities, was a Director of the consultancy Naturebureau for 12 years and helped establish the Chartered Institute of Ecology & Environmental Management. She currently practices as a land management adviser and writes the Conservation News column in British Wildlife magazine.

Pesticide residues in soils: Recent research findings

A new study[1]  has found pesticide residues in the soils of 11 European countries in six different cropping systems. Some 83% of 317 surface soil samples from 11 European countries were found to contain residues of 76 different types of compounds; of the soils testing positive for residues, over half (58%) contained more than one type of pesticide with 25% containing one compound.

Glyphosate and its metabolite aminomethylphosphonic acid (AMPA), DDT and its metabolites, and broad-spectrum fungicides  were the compounds most frequently found and at the highest concentrations. These compounds occasionally exceeded their predicted environmental concentrations in soil but were below the respective toxic endpoints for standard in-soil organisms.

The 76 prioritized pesticides’ residues analysed correspond to less than 20% of the active substances available on the EU market, indicating that the total amount of pesticide residues in EU soils may be higher than presented in this study and the actual residue mixtures even more extensive and complex, also with regard to possible effects on soil life. The authors also flag up concerns regarding EFSA pesticide-testing, including the small number of indicator soil-organisms used in standard toxicity tests and the lack of testing for pesticide combinations. They also point out that pesticides tend to accumulate in topsoil which may be eroded by wind and water, leading to wider environmental exposure to plant-protection products.

A further publication[2] focuses on glyphosate residues in European soils. Glyphosate and/or AMPA were present in 45% of the topsoils collected, originating from eleven countries and six crop systems, with a maximum concentration of 2 mg kg− 1. Several glyphosate and AMPA hotspots were identified across the EU. The authors call for residue threshold values in soils, which they say are urgently needed in order to define potential risks for soil health and off-site effects related to export by wind and water erosion.

Another study[3] looked into pesticide residues in Australian soils. The soil survey of 40 different paddocks from around Australia detected residues of 11 chemicals out of the 15 analysed. Glyphosate and its primary metabolite, AMPA, were the most commonly detected residues, with AMPA residues present in every topsoil sample taken. Trifluralin residues were also detected in over 75% of the paddocks surveyed, both in topsoil and in the 10-30 cm soil layer, indicating some vertical movement despite the strong tendency of trifluralin to remain close to the site of application.

Diflufenican and diuron residues were frequently detected in samples from WA paddocks, but less so in NSW-Qld and SA. Notably, in a number of paddocks (especially in WA but also in other states), the authors found glyphosate in quantities greater than expected from a single spray, considered to demonstrate a degree of accumulation of glyphosate and its metabolite AMPA over time. Although the half-life of glyphosate is relatively rapid (10-40 days), a significant portion of the glyphosate (and AMPA) is bound to soil and is much less accessible for continued degradation. This, combined with the high frequency of glyphosate use, can lead to a build-up of glyphosate and AMPA in soil. Accumulation of trifluralin was also apparent in a number of paddocks in WA.

Conclusions of the research:

●      Glyphosate, trifluralin and diflufenican are routinely applied in grain cropping systems and their residues, plus the glyphosate metabolite AMPA, are frequently detected at agronomically significant levels at the commencement of the winter cropping season

●      The risk to soil biological processes is generally minor when herbicides are used at label rates and given sufficient time to dissipate before re-application

●      However, given the frequency of glyphosate application, and the persistence of trifluralin and diflufenican, further research is needed to define critical thresholds for these chemicals to avoid potential negative impacts to soil function.

Compiled by Sue Everett, January 2019

[1] Vera Silva, Hans G.J. Mol, Paul Zomer, Marc Tienstra, Coen J. Ritsema, Violette Geissen. Pesticide residues in European agricultural soils – A hidden reality unfolded. Science of The Total Environment, 2019; 653: 1532 DOI: 10.1016/j.scitotenv.2018.10.441

[2] Vera Silva, Luca Montanarella, ArwynJones, OihaneFernández-Ugalde, Hans G.J.MolcCoen,J.Ritsema, VioletteGeissen. Distribution of glyphosate and aminomethylphosphonic acid (AMPA) in agricultural topsoils of the European Union. Science of The Total Environment Volume 621, 15 April 2018, Pages 1352-1359 

[3] Vera Silva, Luca Montanarella, ArwynJones, OihaneFernández-Ugalde, Hans G.J.MolcCoen,J.Ritsema, VioletteGeissen. Distribution of glyphosate and aminomethylphosphonic acid (AMPA) in agricultural topsoils of the European Union. Science of The Total Environment Volume 621, 15 April 2018, Pages 1352-1359