Nitrogen Regulation Under Heavy Metal Stress in Soil–Plant Systems: Implications for Plant Growth and Phytoremediation

Nitrogen (N) is a vital macronutrient that dictates soil fertility and plant productivity. Heavy metal contamination threatens plant health by disrupting nutrient uptake and photosynthesis. Specifically, heavy metals accumulate in soils and inhibit key N-assimilatory enzymes, such as nitrate reductase. This inhibition restricts N availability, which limits protein synthesis and reduces overall plant biomass. This review synthesizes the recent literature to evaluate the mechanisms of heavy metal–N interactions, highlighting a critical scientific insight: N exerts a dual role in heavy metal stress. Externally, the chemical form of N uptake alters rhizosphere pH, which can either mobilize or precipitate toxic metal cations. Internally, optimal N assimilation fuels the synthesis of metal-chelating proteins (e.g., phytochelatins) required for cellular detoxification. A key conclusion of this review is that isolated agronomic or physiological interventions are insufficient to manage this stress in the field. Effective mitigation requires a coupled “soil-to-gene” approach. By combining macroscopic soil management (e.g., pH buffering and biochar application) to restrict external metal bioavailability with targeted molecular interventions (e.g., regulating root ion transporters and leveraging microbiome-assisted multi-omics) to enhance internal detoxification, researchers can improve phytoremediation efficiency and restore soil health in multi-contaminated environments.