How Everyday Microplastics Quietly Rewire Nickel Availability in Mediterranean Calcareous Soils

Microplastics are emerging as ubiquitous contaminants in agricultural soils, while nickel (Ni), although catalogued as a potentially toxic element, is an essential micronutrient and a cofactor of urease. In a pot experiment, this study examined how three microplastics (polyethylene, PE; poly(ethylene terephthalate), PET; and polystyrene, PS) affect nickel availability and the properties of two calcareous agricultural soils from central Greece. The microplastics were applied at two levels (1.5 and 3.0% by weight), and the samples were analysed at four time points (3, 6, 9, and 12 months, n = 3). Total (aqua regia) and available (DTPA) nickel, pH, electrical conductivity, organic matter, bulk density (BD), water-holding capacity (WHC), and microbial respiration (qCO2) were measured, and the data were evaluated by one-way ANOVA with Tukey HSD tests. Total nickel remained stable (about 15.1–15.4 mg/kg; non-significant), whereas the available fraction changed systematically. In Soil 1 (12 months, 1.5%), PE raised DTPA-Ni by +24.9%, PET by +19.8% and PS by −1.7%, while at 3.0% the increases reached +39.8% and +31.7%. In parallel, PE lowered bulk density by up to −16.1%, PET raised water holding capacity by up to +28.9%, and PS raised microbial respiration by up to +38.3%, producing distinct, polymer-specific responses. The changes intensified with time and dose and were milder in the more strongly buffered Soil 2. Microplastics redistribute the biologically active, rather than the total, pool of nickel, a finding that supports reappraising Ni as a nutritional micronutrient and monitoring its available fraction. Because no plants were grown, the nutritional interpretation is advanced as a hypothesis for future testing rather than as a demonstrated agronomic benefit, and the applied microplastic doses (1.5–3.0% w/w) exceed most reported field levels and were chosen to resolve mechanisms under accelerated conditions.