Nickel in agri-food systems: a review

Nickel (Ni) is a widely distributed element in environmental and agri-food systems, deriving both from geogenic sources, such as ultramafic rocks and serpentine soils, and from anthropogenic inputs related to industrial activities, fossil fuel combustion and phosphate fertilization. Its ubiquitous presence favors its transfer into terrestrial ecosystems and the food chain, where plants represent the main interface between environmental compartments and the human diet. In the plant system, Ni plays a biphasic role: it is an essential micronutrient, acting as a cofactor of urease and involved in nitrogen metabolism, but it becomes phytotoxic at relatively high concentrations, inducing oxidative stress, photosynthetic alterations and nutritional imbalances. Absorption, transport, and accumulation in tissues depend on species, genotype, soil and environmental conditions, and agronomic practices, with direct implications for content in consumer products. In humans, exposure occurs predominantly via the dietary route; in sensitized individuals, even low concentrations can cause systemic manifestations, including systemic dermatitis and systemic Ni allergy syndrome (SNAS). The adoption of Regulation (EU) 2024/1987 introduced binding maximum limits for Ni in several food categories, strengthening the protection of the general population. In parallel, the increasing diffusion of "Ni-free" and "Ni-tested" products highlights an emerging demand, in the absence, however, of harmonized criteria for the use of such claims. In this context, the term "Ni-free" is generally used to refer to products with Ni concentrations below the detection or quantification limits of the analytical method used. Despite the extensive literature available, there remains a significant gap in the integration of environmental, agronomic, and regulatory perspectives to quantitatively identify the main factors governing Ni transfer along the soil-plant-food continuum. This review analyzes Ni transfer pathways from environmental sources to plant systems, the food chain, and human exposure, integrating physiological, agronomic, toxicological, and regulatory aspects. Particular attention is paid to closed-loop soilless systems, in which the Ni content depends on technical inputs and not on soil geochemistry. The results of the mass balance analysis indicate that mineral fertilizers can account for more than 80% of the total Ni input in the nutrient solution (NS), while pesticides and growing media play a secondary role. Overall, this review provides an integrated framework for understanding Ni transfer across agri-food systems, highlighting the central role of agronomic inputs in determining Ni accumulation in crops. These findings have direct implications for agricultural management and food security and support the development of reliable and standardized approaches for "Ni-free" production systems.