Pesticide mixtures in soil: A global outlook

In modern agriculture, pesticides are used in combinations to protect crops. The co-existence of multiple pesticides in soil can threaten the soil biodiversity that maintains ecosystem services, hence further posing a long-term risk on food security. Here, we introduce an assessment of global soil contamination by the residue of pesticide mixtures in nine cropping systems using a fully mechanistic, spatially explicit, and time-resolved model at 0.5° × 0.5° spatial resolution (approximately 55 × 55 km at the equator) fed with georeferenced agricultural quantities, soil properties, and hydroclimatic variables. We found that 8.3 million km2 of treated land have more than one detectable pesticide, with pendimethalin, glyphosate, paraquat, chlorpyrifos, and chlorothalonil being the five most frequently detected. The highest pesticide mixture content was found in the 'orchards and grapes' cropping system (95th percentile at 7.3 mg kg soil-1). Globally, the pesticide mixture in the topsoil of approximately 1.88 million km2 exceeded 1 mg kg soil-1 for more than 180 d in a year. We estimate that 0.2 million tonnes of pesticides leach below the root zone each year globally, with glyphosate contributing the greatest fraction. The major hotspots of soil pesticide contamination are located in South America and Asia, mainly in Brazil, Argentina, Chile, China, Malaysia, and Japan. Our study shows that the accumulation of pesticide mixtures in soil is a global environmental issue that has to be explicitly accounted for in the sustainability assessment of agricultural production. We propose the use of mechanistic modelling as a tool to aid in designing pesticide management strategies and minimise residue contamination.