Distinct arsenic uptake feature in rice reveals the importance of N fertilization strategies

The environmental behavior of arsenic (As) is commonly affected by the biogeochemical processes of iron (Fe) and nitrogen (N). In this study, field experiments were conducted to explore As uptake in rice and As translation and distribution in As-contaminated iron-rich paddy soils after applying different forms of N fertilizers, including urea (CO(NH2)2), ammonium bicarbonate (NH4HCO3), nitrate of potash (KNO3), and ammonium bicarbonate + nitrate of potash (NH4HCO3 + KNO3). The results indicated that applying nitrate N fertilizer inhibited the reduction and dissolution of As-bearing iron minerals and promoted microbial-mediated As(III) oxidation in flooded soil, thus reducing the soil As bioavailability. The concentrations of total As and inorganic As ratio (iAs/TAs) in rice grain decreased by 32.4 % and 15.4 %, respectively. However, the application of ammonium nitrogen promoted the reductive dissolution of As-bearing iron minerals and stimulated microbial As(V) reduction in flooded soil, leading to the release of As from soil to porewater. The total As concentration and inorganic As uptake ratio in rice grain increased by 20.1 % and 6.2 %, respectively, when urea was applied, and by 29.6 % and 10.5 %, respectively, when ammonium bicarbonate was applied. However, the simultaneous application of NH4+ and NO3- had no significant effect on As concentration in rice grain and its transformation in paddy soils. Ammonium nitrogen enhanced the organic As concentration in rice grain because the increased As(III) promoted As methylation in soil. In contrast, nitrate decreased the organic As uptake by rice grain because the decreased As(III) diminished As methylation in soil. The results provide reasonable N fertilization strategies for regulating the As biogeochemical process and reducing the risk of As contamination in rice.