Migration and transformation of cadmium in rice - soil under different nitrogen sources in polymetallic sulfide mining areas.

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)SO, NHNO, NHCl). The results showed that the concentration of bioavailable Cd in rice rhizosphere soil was (NH)SO treatment > NHCl treatment > NHNO treatment at the same level of N application and growth period; the Cd concentration in rice roots was (NH)SO treatment > NHNO treatment > NHCl treatment; and the Cd concentration in rice straw was NHNO treatment > NHCl. The Cd concentration in rice roots, straws, and seeds at the maturity stage was (NH)SO treatment > NHCl treatment. With the same N fertilizer, excessive N promoted Cd accumulation in rice at later growth stages. This suggested that sulfate (SO) influenced Cd concentration in rice. NHCl application maintained a low Cd level in different rice organs with the same N level. This confirmed that NHCl is a safe N source for rice planting in polymetallic sulfide mining areas. The study concludes that appropriate NHCl 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).