Fate, transport, and biological impacts of microplastics in agricultural soil-wheat systems: from soil movement to food safety
Microplastics are persistent contaminants of agricultural soils and may affect soil structure, microbial activity, nutrient cycling, crop growth, and the quality of plant-derived foods. Wheat (Triticum aestivum L.) deserves specific attention because it is a major staple crop, develops an extensive fibrous root system in cultivated soil, and produces grain that is consumed directly by humans. These characteristics create a close connection among soil contamination, rhizosphere processes, crop performance, and possible food-chain exposure. Unlike broader reviews of microplastics in crops, this review follows the pathway from particle entry into wheat-growing soils to transport, transformation, rhizosphere interactions, wheat responses, and possible contamination of edible grain. The movement and effects of microplastics depend on particle size, shape, polymer type, density, surface properties, aging, soil texture, mineral composition, organic matter, water movement, root activity, and soil organisms. Available studies show that plastic particles can alter root development, nutrient acquisition, oxidative balance, photosynthesis, biomass, and yield, although the direction and magnitude of these effects differ among experimental conditions. Evidence for root internalization and vascular transport is strongest for nanoplastics and submicrometer particles under controlled conditions. Direct field evidence for the accumulation of larger microplastics in mature wheat grain remains limited. Important research gaps include the scarcity of field measurements, the use of unrealistic exposure concentrations, poor separation of microplastic and nanoplastic evidence, uncertain root-to-grain transfer, and the lack of standardized analytical methods. Future research should combine realistic field exposure, aged and mixed-polymer particles, multi-season experiments, advanced particle tracing, and contamination-controlled analysis of wheat tissues and grain.