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

Pesticides are used within forest plantations to manage the impacts of pests (including weeds) and pathogens. However, these chemicals can adversely affect the environment, including non-target soil organisms such as earthworms and microorganisms. It is therefore imperative that pesticide environmental fate data is available to guide responsible pesticide use and risk mitigation measures. To this end, a 24-month field study, covering the period from pre-plant to canopy closure, investigated the soil fate of commonly used pesticides in South African forest plantations and the risks posed to non-target soil organisms. The trial was established in a Eucalyptus stand managed for pulpwood in the KwaZulu-Natal Midlands, South Africa. Pesticides were applied at different stand-development stages according to standard operational practices. Pesticides applied included the herbicides glyphosate, triclopyr and metazachlor; the insecticide cypermethrin; and the fungicides azoxystrobin and tebuconazole. Following each application, soil samples were collected at pre-determined intervals (based on pesticide DT₅₀ value) from two depths (0–10 cm and 10–50 cm) to evaluate pesticide persistence and vertical mobility. Pesticide concentrations were compared with standard ecotoxicity thresholds to assess potential risk. Results indicated that glyphosate and foliar-applied cypermethrin dissipated rapidly, reaching below detection limits (BDL) (< 0.01 mg kg− 1) within 12–13 days; and azoxystrobin was BDL by 81 days. Triclopyr, tebuconazole, metazachlor, and soil-applied cypermethrin persisted for > 90 days. Risks to non-target soil organisms were generally low, but triclopyr, metazachlor, and soil-applied cypermethrin may require further study to assess chronic effects. The findings should be interpreted with caution due to study limitations.