Nitrate induces increases in cadmium accumulation by reducing manganese competitive absorption of wheat plant

Manganese (Mn^2 +) fertilization can competitively inhibit cadmium (Cd) uptake by crops, while the newly formed Mn oxides adsorb bioavailable Cd in the soil, thereby achieving effective remediation of Cd-contaminated soils. However, the potential influence of co-existing nitrate (NO₃^-) in soils has often been ignored in previous studies. Here, the effects of Mn²⁺ on wheat Cd accumulation, soil metal fractionation, and microbial community upon NO₃^- addition were investigated through pot and hydroponic experiments. Counterintuitively, Mn²⁺ and NO₃^- addition enhanced Cd accumulation in wheat grains (0.1 %-39.7 %) in a NO₃^- dosage-dependent manner. NO₃^- addition weakened the competitive absorption of Mn²⁺ by lowering soil pH, increasing Cd bioavailability, and promoting the transport of Cd into wheat plants. The competitive advantage of Mn over Cd was further diminished by soil microorganisms, which accelerated Mn²⁺ oxidation while inhibiting NO₃^- reduction. Furthermore, higher NO₃^- levels facilitated co-accumulation of nitrate and cytoplasmic Cd in wheat leaves, which aggravated oxidative stress and further undermined the protective effect of Mn. This study demonstrates that NO₃^--containing fertilizers weaken the remediation efficiency of Mn-based amendments in Cd-contaminated soils and highlights the need of considering interactions among remediation agents, microbes, and coexisting ions for food safety.