Microplastics deplete soil available nutrients: a global meta-analysis with machine learning reveals critical thresholds and interactive controls

Abstract

Microplastics (MPs) contamination threatens soil nutrient bioavailability, yet quantitative understanding of effect magnitudes and controlling factors remains fragmented across diverse environmental contexts. Here, we synthesized 206 studies (2853 observations) spanning 12 countries and diverse agricultural systems (soil pH 4.5–8.5; organic matter 5–80 g kg–1) to quantify MPs impacts on soil available nitrogen (AN), phosphorus (AP), and potassium (AK). We integrated random-effects meta-analysis using log response ratios (lnRR) with eXtreme Gradient Boosting (XGBoost) machine learning interpreted through SHapley Additive exPlanations (SHAP) to identify non-linear relationships and interactive controls. The dataset comprised predominantly controlled experiments (pot: 49%; incubation: 50%; field: 1%), with geographical concentration in East Asia (92%). Across all experimental setups, MPs significantly depleted AN by 5.3%, AP by 9.4%, and AK by 7.0% relative to controls. However, effects were context-dependent: pot/incubation studies showed significant depletion, while field studies reported no significant changes. XGBoost models (R2 = 0.44–0.62) indicated hierarchical control structures, with soil pH dominating AN responses (relative importance: 18.4%) and MPs concentration governing AP (15.2%) and AK (14.8%). SHAP analysis identified critical thresholds: particle size effects plateaued above 100 μm, concentration impacts saturated beyond 10 g kg–1, and pH-dependent reversals shifted from depletion in acidic soils to enrichment under alkaline conditions. Biodegradable plastics (e.g., PLA, PBAT) caused equal or greater short-term nutrient immobilization than conventional polymers, though mechanisms likely differ: biodegradable MPs may stimulate microbial biomass growth that temporarily sequesters nutrients, whereas conventional plastics primarily act through surface sorption. Partial dependence analysis indicated synergistic interactions: small particles (<50 μm) combined with high concentrations (>30 g kg–1) induced depletion 1.5–2.3-fold beyond additive expectations. These findings establish quantitative thresholds for risk assessment and demonstrate that MPs-induced nutrient limitation, observed primarily in controlled settings, could affect crop yields, necessitating interventions to prevent irreversible soil fertility degradation, though field-scale validation remains essential.