Effects of Polypropylene and Polyethylene Terephthalate Microplastics on Nutrient Dynamics in Agricultural Soils: A Laboratory Incubation Study from Two Regions in Bangladesh

Microplastic (MP) contamination of agricultural soils is an emerging environmental concern because MPs may alter soil physicochemical properties and nutrient dynamics. However, information on polymer- and soil-dependent responses in South Asian agricultural systems remains limited. This study investigated the temporal effects of polypropylene (PP) and polyethylene terephthalate (PET) microplastics (≤20 μm) on soil pH and dissolved ion concentrations in agricultural soils from Gazipur and Mymensingh, Bangladesh. Soil–MP composite systems were prepared with a 1% (w/w) MP loading and incubated under controlled laboratory conditions for 30 days, with measurements conducted on days 1, 15, and 30. Concentrations of NH4+, Na+, SO42−, Cl−, and Br− were determined by ion chromatography. The results showed substantial temporal and treatment-dependent variation in soil pH and ion concentrations. PP-containing systems showed pronounced changes in NH4+, whereas PET-amended Mymensingh soil exhibited marked changes in SO42− and Br−. Na+ generally declined during incubation, while Cl− showed pronounced short-term variation. PCA identified distinct patterns among the measured ions, with three components explaining 91.471% of the total variance. These findings indicate that PP and PET microplastics can be associated with changes in nutrient and ion dynamics under controlled laboratory conditions, with responses varying according to polymer type, soil characteristics, and incubation time. However, the single MP concentration, short incubation period, limited polymer types, non-sterile experimental conditions, and incomplete nitrogen speciation limit direct extrapolation to field conditions. Further studies using environmentally realistic concentrations, longer incubation periods, diverse and aged polymers, complete nitrogen speciation, and soil–plant or field systems are needed to clarify the broader implications of MP contamination in agricultural soils.