Effects of microplastic properties and dissolved organic matter on phosphorus availability in soil and aqueous mediums

Plastic mulching films and phosphate fertilizers have been widely used to improve agricultural soil productivity. Microplastics (MP) and phosphorus (P) significantly accumulate in agricultural soil and water bodies. However, the effects of residual MP on P availability in soil and aqueous mediums remain unclear. In this study, available P (Olsen-P) in soils and P adsorption capacity in aqueous medium were determined to examine the influence of MP properties on P availability in laboratory. In agricultural soils, Olsen-P was significantly affected by MP types. Conventional MP (mulching film particles), such as polyethylene (PE) and polyvinyl chloride (PVC), and biodegradable MP, such as polylactic acid (PLA), substantially reduced soil Olsen-P by 9.7-38.6% and 38.4-73.6%, respectively. The size and concentration of MP strongly affected soil Olsen-P, with smaller (25 mu m) and more concentrated (5%) MP causing greater reductions in Olsen-P. In the soil contaminated with MP, increased fulvic acid content significantly increased Olsen-P levels. Microplastics exhibited strong P adsorption capacities, leading to decreased P availability in aqueous medium. Conventional MP exhibited a higher P adsorption capacity than biodegradable MP, with P distribution coefficients (K-d values) ordered as PVC (5.19 L kg(-1)) > PE (4.23 L kg(-1)) > PLA (2.48 L kg(-1)). Notably, the K-d values increased with decreasing sizes of conventional MP, whereas the opposite trend was observed for PLA. The presence of fulvic acid affected the adsorption of P by MP in aqueous medium. Increased fulvic acid content reduced P adsorption capacity of MP, thus enhancing P availability. Our findings contribute to a better understanding of P dynamics in MP-contaminated agricultural soil and aqueous medium, which could aid in formulating sustainable agricultural practices and effective environmental management strategies for plastic mulching films and P contamination.