Microplastics alter soil phosphatase activity and phosphorus mobilization : a global meta-analysis integrated with machine learning

Abstract

Agricultural soils are a major reservoir and conduit within the anthropogenic water cycle, receiving microplastics (MPs) through sewage-sludge/biosolid land application, reclaimed-water irrigation, plastic-mulch degradation and atmospheric deposition, and exporting dissolved nutrients to surface and ground waters via runoff and leaching. Phosphatase enzymes govern organic-phosphorus mineralization and therefore the size of the dissolved phosphorus (P) pool that drives aquatic eutrophication, yet how MPs alter soil acid (ACP) and alkaline (ALP) phosphatase activity, and which conditions create the greatest soil-to-water transfer risk, remains unresolved. We synthesized 171 global studies (866 ACP and 688 ALP observations) in a meta-analysis integrated with an interpretable machine-learning ensemble that stacked XGBoost, LightGBM and Random Forest, with SHAP and accumulated local effects (ALE) interpretation. MPs significantly increased ACP activity by 9.89%, whereas the overall ALP effect was non-significant (+1.40%) but strongly context-dependent, ranging from significant stimulation (e.g., biodegradable MPs: +15.15%; aged MPs: +25.60%) to significant decrease under certain conditions (e.g., with nitrogen addition: −9.45%). Soil pH was the dominant control on ACP responses (mean |SHAP| = 0.088) and clay content on ALP responses (0.062), each ranking first across all five best-performing XGBoost configurations, indicating divergent driver hierarchies for the two enzymes. In the machine-learning analysis, ALP suppression was concentrated in clay-rich soils, whereas low-clay soils showed neutral-to-positive responses. By identifying where MP contamination most strongly perturbs phosphatase-mediated P mobilization, our findings connect a terrestrial contaminant to diffuse nutrient pollution of receiving waters and provide a transferable framework for contaminant–water-quality synthesis.