Effects of Nitrogen Form and Application Rate on Cadmium and Mineral Element Uptake and Translocation in Rice

The simultaneous challenges of cadmium (Cd) contamination and mineral nutrient imbalance in paddy systems necessitate the development of effective agronomic strategies. This study systematically investigated the coordinated effects of different nitrogen fertilizer forms on the accumulation and translocation of Cd and mineral elements in rice. A hydroponic experiment was conducted using four N sources, including urea (U), nitrate-N (N), ammonium-N (AN), and a mixed ammonium-nitrate source (NN), which were applied at two concentrations (2.9 and 5.8 mM L−1). We evaluated Cd accumulation, mineral element uptake, and translocation in rice seedlings under Cd stress. The results indicated that both the form and level of nitrogen markedly affected Cd accumulation. The AN treatment exhibited a strong Cd-reduction effect, especially at the higher nitrogen level, where it significantly reduced Cd concentration in roots and shoots by 68.75% and 26.81%, respectively. In contrast, the N treatment increased Cd accumulation in roots. Nitrogen fertilization also differentially influenced the accumulation of mineral elements, resulting in notable alterations in root Ca, Mg, Cu, and Zn concentrations, while shoot mineral concentrations remained relatively stable. Correlation and random forest analyses revealed a highly significant positive correlation between root Cd concentration and Mg and Cu concentrations, a significant negative correlation with Zn concentration, and a synergistic relationship between the translocation of Cd and that of Ca, Mg, and Cu. Analysis of ion channel tolerance rates further indicated that the AN treatment suppressed Cd uptake by reducing the permeability of root trace element channels to Cd. These findings demonstrate that nitrogen forms modulate Cd accumulation and partitioning by regulating competitive ion uptake and coordinated translocation. In particular, the AN treatment shows promising potential for reducing Cd accumulation while maintaining mineral nutrient balance, thereby providing a theoretical foundation for precise nitrogen management in Cd-contaminated paddy fields.