Microplastic-Induced Decoupling of Soil Nickel Bioavailability and Phytoaccumulation in Two Industrial Crops

Microplastics are an increasingly pervasive contaminant of agricultural soils, where they frequently co-occur with heavy metals such as nickel (Ni), an element still catalogued mainly as a pollutant yet now recognised as an essential plant micronutrient and the cofactor of urease. A pot experiment assessed how three of the most common microplastics—polyethylene (PE), poly(ethylene terephthalate) (PET) and polystyrene (PS), added at 2% v/v—affect nickel behaviour in two calcareous agricultural soils differing chiefly in texture (Soil 1, a clay loam; Soil 2, a silty clay), cropped with industrial hemp (Cannabis sativa L., cv. Fedora 17) and tobacco (Nicotiana tabacum L., cv. Burley). Forty-eight pots (2 soils × 4 treatments × 2 crops × 3 replicates) were analysed for pseudo-total (aqua regia), bioavailable (DTPA-extractable) and plant-tissue Ni. Pseudo-total Ni was unchanged across all treatments (15.1–15.4 mg kg−1), confirming that microplastics are modifiers and not sources of the metal. DTPA-extractable Ni rose significantly under PE and PS in the coarser-textured Soil 1 (1.87 and 1.80 versus 1.69 mg kg−1) but not in the finer-textured Soil 2. Tissue Ni nevertheless declined under every polymer, in the order control > PE > PS > PET, and was consistently lower on Soil 2. The Bioavailability Factor and the soil-referenced transfer indices (BAF, TC) followed the same pattern, whereas the internal root-to-shoot Transfer Factor remained close to unity, indicating that microplastics restrict the acquisition of nickel rather than its internal distribution. Neither crop showed visible symptoms of phytotoxicity and both completed their full growth cycle; because biomass was not quantified, no conclusion regarding yield is drawn. Chemically measured availability therefore ceases to predict plant uptake under microplastic amendment, with implications both for the expected efficiency of Ni phytoextraction and for the supply of an essential micronutrient on calcareous soils.