Soil Physical Structure Controls Microplastic Accumulation and Partitioning in Pistachio Orchards

Microplastics (MPs) are increasingly recognized as pervasive contaminants in terrestrial environments. However, the influence of soil physical structure on their accumulation and internal partitioning remains insufficiently understood. This study investigated the relationship between MP abundance, size distribution, and morphology, and key hydro-structural soil properties in 42 pistachio orchard soils from a semi-arid region of Türkiye. Soil samples were analyzed for MP content (0.1-5 mm), organic matter, texture, porosity, water retention characteristics, and bulk density. Random Forest (RF) and Gradient Boosting Decision Tree (GBDT) models, combined with SHAP (SHapley Additive exPlanations) analysis, were used to identify primary predictors of MP accumulation. MP abundance ranged from 100 to 8,533 particles kg − 1 (median = 1,433 particles kg − 1 ) with the highest levels recorded in former landfill sites (median = 4,633 particles kg − 1 ). Fine MPs (0-200 µm) dominated the size distribution and showed a significant negative correlation with macroporosity (r= -0.49, p < 0.01), indicating enhanced mobility in well-connected pore systems. In contrast, larger particles (500 µm-1 mm) were positively correlated with microporosity (r = 0.49, p < 0.01) and clay content (r = 0.39, p < 0.05), suggesting size-dependent retention mechanisms. Morphology-specific analysis revealed that fragments were positively associated with aggregate stability, whereas granules showed negative relationship with available water content. SHAP analysis identified organic matter, silt content, and bulk density as the most influential predictors of MP accumulation. The RF model demonstrated superior generalization performance on the test set (R²= 0.47) compared with the GBDT model, which showed clear overfitting. These findings indicate pore-size compatibility as a key mechanism governing MP distribution and emphasize the critical role of soil structure in regulating MP dynamics in agroecosystems.