Understanding the environmental fate of pesticides in South African planted forests: Part 1 – concentrations of pesticides in soil and risk posed to non-target soil organisms

Pesticides are used within forest plantations to manage the negative impacts caused by pests (including weeds) and pathogens. However, these chemicals have the potential to negatively affect the environment, including non-target soil organisms such as earthworms and microorganisms. It is therefore imperative that relevant pesticide environmental fate data is available to guide responsible pesticide use and/or the application of risk mitigation measures (where necessary). To this end, a 24-month field study, covering the period from pre-plant to canopy closure, was conducted to investigate the soil fate of commonly used pesticides in South African forest plantations and assess the risk they pose to non-target soil organisms. The trial was established in a Eucalyptus stand managed for pulpwood production in the KwaZulu-Natal Midlands, South Africa. Pesticides were applied at different stages of stand development according to standard operational practices. Pesticides (active ingredients) applied included glyphosate, triclopyr, metazachlor, cypermethrin, azoxystrobin, and tebuconazole. Following each application, soil samples were collected at pre-determined intervals (based on the DT₅₀ value of each pesticide) from two depths (0–10 cm and 10–50 cm) to evaluate persistence and leaching potential. The results were largely positive. Glyphosate, azoxystrobin, and foliar-applied cypermethrin degraded rapidly and posed a low risk to non-target soil organisms. While triclopyr, tebuconazole, metazachlor, and soil-applied cypermethrin persisted for more than 90 days, their concentrations either remained below risk thresholds or require further investigation to fully determine their ecological impact.