Dynamic modeling of plant uptake and leaching of pesticides applied at an apple orchard

Apple production is a major agricultural activity in many regions around the globe. Over the past decades, a range of different pesticides have been used for preventing fungal and insect infestations. Evaluating the fate of pesticides in plants and soil is important for determining human health risks arising from chemical residues in fruits as well as ecological risks, among others resulting from pesticide fate in soils and eventually leaching to groundwater.

This study aimed at improving the understanding of complex fate and transport processes interacting in the soil-plant system of an apple orchard. Field experiments were done with different insecticides and fungicides that are frequently applied in agricultural practice. Pesticide concentrations in soil and different plant parts were observed at different times under a multiple pesticide applications scenario.  A coupled soil-plant model was set up for numerically simulating pesticide fate and comparison with observed pesticide concentrations. This model considers a tipping buckets approach for soil water and solute transport, linked with a dynamic numerical model for plant uptake and translocation of chemicals within plants, implementing a fruit growth model for explaining the fruit growth dilution. Moreover, risks posed for food safety were estimated.

This model approach was successful for describing observed pesticide residues. Up to 25% of the applied chemicals were deposited on leaves and up to 0.6% on fruits, and up to 61% entered the topsoil directly after application. A decrease in fruit concentration was observed, which could be explained by biodegradation and growth dilution as the main contributions, as well by wash-off. First estimates of dietary risks indicated that the ingestion of the treated apples may not lead to relevant acute or chronic human health risks. The contribution of the different pathways leading to pesticide residues and their dynamics in plant material was highly influenced by precipitation patterns, fruit growth dilution and pesticide characteristics. Our model approach can contribute to an improvement of process understanding concerning the fate of pesticides in apple orchards and pesticide utilization. It has a high potential for supporting decision making with respect to food safety and minimizing risks associated to pesticide use.