LAUSR

We conducted pot experiments to assess the bioavailability of cadmium (Cd) in contaminated rhizosphere soil and accumulation in rice organs in response to nitrogen (N) supply ((NH 4 ) 2 SO 4 , NH 4 NO 3 , NH 4 Cl). The results showed that the concentration of bioavailable Cd in rice rhizosphere soil was (NH 4 ) 2 SO 4 treatment > NH 4 Cl treatment > NH 4 NO 3 treatment at the same level of N application and growth period; the Cd concentration in rice roots was (NH 4 ) 2 SO 4 treatment > NH 4 NO 3 treatment > NH 4 Cl treatment; and the Cd concentration in rice straw was NH 4 NO 3 treatment > NH 4 Cl. The Cd concentration in rice roots, straws, and seeds at the maturity stage was (NH 4 ) 2 SO 4 treatment > NH 4 Cl treatment. With the same N fertilizer, excessive N promoted Cd accumulation in rice at later growth stages. This suggested that sulfate (SO 4 2− ) influenced Cd concentration in rice. NH 4 Cl application maintained a low Cd level in different rice organs with the same N level. This confirmed that NH 4 Cl is a safe N source for rice planting in polymetallic sulfide mining areas. The study concludes that appropriate NH 4 Cl levels for Cd-contaminated paddy soil with high-S-content could obtain rice grains with Cd concentrations below the food safety standards (0.2 or 0.4 mg·kg −1 ).