The effect of exogenous phosphate (P, 200 mg·kg^-1 soil) on the lability and phyto-availability of arsenic (As) was studied using the diffusive gradients in thin films (DGT) technique. Lettuce were grown on the As-amended soils following the stabilization of soil labile As after 90 day’s incubation. Phosphate (P) application generally facilitated plant growth except one grown on P-sufficient soil. Soil labile As concentration increased in all the soils after P application due to a competition effect. Plant As concentration increased in red soils collected from Hunan Province, while decreases were observed in the other soils. Even though, an overall trend of decrease was obtained in As phytoavailability along with the increase of DGT-measured soil labile P/As molar ratio. The functional equation between P/As and As phytoavailability provided a critical value of 1.7, which could be used as a guidance for rational P fertilization, thus avoiding overfertilization.

外源磷酸盐（P，200 mg·kg ^-1使用薄膜中的扩散梯度（DGT）技术研究了砷（As）的不稳定性和植物有效性。在90天的温育后，土壤不稳定的As稳定下来后，将莴苣种植在As改良土壤上。施用磷肥（P）通常可促进植物生长，但磷肥可在磷充足的土壤上生长。土壤的不稳定由于磷的竞争作用，在所有土壤中，砷的浓度都会增加。从湖南省采集到的红壤中，随着植物浓度的增加，其他土壤中的植物浓度降低。即使随着DGT测得的土壤不稳定P / As摩尔比的增加，总的砷利用率也有所下降。P / As和As的植物利用率之间的函数方程提供的临界值为1.7，