Nitrogen fertilizer management affects remobilization of the immobilized cadmium in soil and its accumulation in crop tissues

Immobilization of soil cadmium (Cd) has been the strategy mostly used in remediation of Cd-contaminated arable soil. However, Cd might be remobilized after the immobilization process through the acid-soluble and complexation effects. Development of agronomic management technologies to prevent soil Cd remobilization after the immobilization process was an important pathway to control the food safety of agricultural products in soils with the immobilized Cd. In this study, the ammonia (NH4+-N) and nitrate (NO3−-N) forms with concentrations of 60, 90, and 150 mg-N kg−1 soil were performed for evaluating their effects on Cd remobilization with planted or unplanted treatments and Cd accumulation in tissues of edible amaranth (Liuye). With an initial soil palygorskite-bound fraction Cd concentration of 0.6 mg kg−1, bioavailable Cd in rhizosphere soils and Cd in crop shoots respectively increased from 11.4 to 20.6 μg kg−1 (dry soil weight) and 6.92 to 14.92 mg kg−1 (dry plant weight) in planted NH4+-N treatments, while significantly lower concentrations of bioavailable Cd in rhizosphere soils and Cd in crop tissues were observed with planted NO3−-N treatments. Compared with that of planted NO3−-N treatments, decreasing pH value (i.e., 7.64 to 7.18) induced by root proton efflux during the absorption of NH4+-N, enhancive organic/amino acid (oxalic acid, lactic acid, l-proline, and so on) secretion from roots, and increasing abundance of bacteria distributed in phyla Proteobacteria, Cyanobacteria, and Bacteroidetes with Cd mobilization ability in rhizosphere soils were the main reasons found in this study for the higher Cd remobilization in soils and Cd accumulation in crop under NH4+-N treatments. Moreover, the direct effect of NH4+-N on remobilization of immobilized Cd by upregulating the expression abundances of genes associated with pyruvate metabolism and amino acids metabolism was more significant than that of NO3−-N. In summary, the use of NO3−-N as preferred N fertilizer was more efficient to ensure the food safety of agricultural products than that of NH4+-N in Cd-contaminated arable soil after immobilization process.