Interactions between arsenic and nitrogen regulate nitrogen availability and arsenic mobility in flooded paddy soils

In paddy soils, arsenic (As) stress influences nitrogen (N) transformation while application of N fertilizers during rice cropping affects As transformation. However, specific interactive effects between As and N in flooded paddy soils on As mobility and N availability were unclear. Here, we examined N and As dynamics in flooded paddy soils treated with four As levels (0, 30, 80 and 150 mg kg^-1) and three urea additions (0, 4 and 8 mmol N kg^-1). Arsenic contamination inhibited diazotrophs (nifH) and fungi but promoted AOA and denitrification genes (narG, nirK, nirS), decreasing dissolved organic N, NH₄⁺-N and NO₃^--N. Besides, urea application stimulated As- and Fe-reducing bacteria (arrA and Geo) coupled with anammox. On Day 28, the addition of 8 mmol N kg^-1 increased total As concentrations in solutions of soils treated with 30 and 80 mg As kg^-1 by 2.4 and 1.8 times compared with the nil-N control. In contrast, at 150 mg As kg^-1, it decreased the total As concentration in soil solution by 63 % through facilitating As(III) oxidation coupled with NO₃^--N reduction. These results indicate that As contamination decreases N availability, but urea application affects As mobility, depending on As contamination level.