Methodological Approach for Detecting Polypropylene Microplastics in Agricultural Soil in Southern Portugal

Plastics have been widely used in agriculture, particularly as mulching materials, due to their ability to improve soil conditions and enhance productivity. However, their degradation into microplastics (MPs) raises significant environmental and agronomic concerns, as these particles may change soil properties, affect microbial communities, and pose risks to surrounding ecosystems. While methodologies for MP detection in aquatic environments are well established, the analysis of MPs in soils remains challenging due to the complexity and heterogeneity of soil matrices. Currently, there is no standardized protocol for the determination of MPs in soils. This study critically evaluated and compared three different pre-treatment methods for removing organic matter from soil prior to MP analysis in an agricultural soil, and proposes a comprehensive methodology comprising two main phases: (i) organic matter removal, a crucial step of MP particles, and (ii) density separation of MP particles. Three distinct removal chemical methods were tested using samples from an agricultural soil in Southern Portugal. The most effective method was then applied to assess MP particles in an experimental field, using soil samples collected before mulching and 14 months later beneath a polyethylene-based soil cover. This was one of the first studies contributing to the establishment of a routine methodology for monitoring MPs in soils, particularly the agricultural soils, ensuring compliance with the future “Directive for Soil Monitoring”.