Pesticides: residues in soils everywhere

Crop protection products are an integral part of modern agriculture and are used in large quantities worldwide. In Switzerland, up to 2,000 tons of active ingredients are applied annually. Public interest is currently focused on active ingredient residues in natural resources such as water.

100 arable and vegetable fields tested for 46 pesticides 

Residues of plant protection products in soils have received comparatively little attention until now. For this reason, Agroscope conducted a nationwide study of 100 arable and vegetable fields that were cultivated organically, conventionally or with no-till farming. The soil samples were analyzed for 46 different pesticide residues. The extent to which these affect soil microbial life was also investigated.

The researchers showed that pesticide residues are widespread, albeit predominantly in very low concentrations in the microgram per kilogram range. There are no legally defined limits for residues in soils, such as in drinking water. Pesticide residues were found at all investigated sites, namely also in 40 organic fields. The researchers also found residues of up to 16 different active substances in soils that had already been cultivated organically for a longer period of time (>20 years). They either originate from the time when such pesticides were still applied on the respective field, or they were introduced by wind or water from neighboring fields. Surprisingly, residues of pesticides that may no longer be used in Switzerland (e.g. atrazine, linuron, chlorpyrifos) were also still detectable.

Do pesticides affect the soil ecosystem? 

In addition, the analyses showed that the concentration and number of pesticide residues in soil has a statistically negative relationship with soil microbial life. The abundance of arbuscular mycorrhizal fungi, a widespread group of beneficial plant symbionts, could be clearly linked to the number of pesticides detected in the soil: the more different residues, the fewer mycorrhizal fungi. This suggests that the application of pesticides may have long-term effects on the soil ecosystem, not least because of their partial persistence.

How to classify the results

The study shows that, regardless of the type of management, pesticide residues are a reality in agricultural soils. However, the concentrations of residues found were predominantly low, especially under organic management, and could be detected mainly thanks to the sensitive measurement method. While the present results suggest negative effects on soil microbial life, the study was not designed to test an actual causality between the measured concentrations and soil life, soil health or soil fertility. Further experiments under controlled conditions are needed for this purpose, and a corresponding Agroscope project is planned. In order to be able to evaluate the diverse measures of the Plant Protection Products Action Plan to reduce the environmental impact of plant protection products, Agroscope is also carrying out long-term and systematic soil monitoring.

Source: Agroscope

Publication of results: The paper was recently published in the journal Environmental Science & Technology, "Widespread occurrence of pesticides in organically managed agricultural soils - the ghost of a conventional agricultural past?" Riedo et al. ES&T (2021).

https://pubs.acs.org/doi/10.1021/acs.est.0c06405