Comparing effects of ammonium and nitrate nitrogen on arsenic accumulation in brown rice and its dynamics in soil-plant system

Nitrogen is widely used in agricultural production, including in arsenic-contaminated soil. However, there is a lack of systematic studies on the arsenic (As) dynamics in soil porewater–root surface–rice system after applying different nitrogen fertilizers to As-contaminated soil. Three treatments (CK, Nitrate-N, and Ammonium-N) were applied before rice planting to investigate the effect of nitrogen forms on arsenic accumulation in brown rice and its dynamics in soil-plant system. Compared to Nitrate-N and CK treatments, Ammonium-N treatment increased rice biomass and yield. Compared with CK treatment (0.12 and 4.48 mg/kg), Ammonium-N treatment (0.08 and 2.71 mg/kg) reduced the As concentrations and Nitrate-N treatment (0.18 and 6.20 mg/kg) increased the As concentrations in brown rice and shoots. Nitrogen application reduced Fe plaques concentration on roots, but Ammonium-N treatment increased the total As accumulation in Fe plaques. In soil porewater, Nitrate-N treatment reduced the concentration of As (III) in the first month (5.2–47.0 μg/L) but increased its concentration in the last month (137.6–183.1 μg/L). However, the concentration of As (III) in Ammonium-N treatment (58.5–168.6 μg/L) was similar to that in CK treatment (72.8–138.8 μg/L) in porewater. The application of Nitrate-N caused the decrease of As in porewater during the seedling stage; however, the As concentration in rice grains was increased due to the increase of As (III) in porewater during the ripening stage. The application of Ammonium-N reduced the As concentration in rice tissues by increasing biomass and accumulating more As in Fe plaques.