Phosphorus regulates As uptake by rice via releasing As into soil porewater and sequestrating it on Fe plaque

Phosphorus (P) application rate can affect the As uptake by rice, but its mechanism lacks systematic studies. In this study, P fertilizers with different dosages (0, 75, 150, and 300 mg P₂O₅ kg^-1 soil) were used to investigate the effects of P on As release in soil porewater, As sequestration on Fe plaque and the change of abundance and communities of aioA and arsC genes in rhizosphere, and then explore its effect on As uptake by rice. Our results indicated that As content in brown rice under P0 and P75 treatments was 14.3-28.6% lower than that under P150 treatment. The total accumulation of As in brown rice under P0 treatment (1.51 μg plant^-1) was significantly lower than that under P150 treatment (2.17 μg plant^-1). Compared to P150 treatment, P0 treatment decreased the total As content in porewater but increased the proportion of As(V) to total As in porewater. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in rice roots and the Fe and As contents in Fe plaque were significantly higher under P0 treatment than under P150 treatment. Most of As (80.3-82.9%) sequestered by Fe plaque was in the form of arsenate (As(V)), and the associated As(V) on Fe plaque was 11.0% higher under P0 treatment than under P150 treatment. In addition, the abundance of aioA gene was 73.5% higher under P0 treatment than under P150 treatment, and the dominant aioA at genus level was Rhizobium and Rhodoferax. In general, P0 treatment led to higher root oxidation activity, which improved the formation of Fe plaque; and P0 treatment also improved the abundance of aioA gene in rhizosphere, thus increased the oxidation of As; so, P0 treatment indirectly enhanced As sequestration on Fe plaque, and that in turn reduced As accumulation in brown rice.