Agricultural soils can contain high arsenic (As) concentrations due to specific geological contexts or pollution. Agricultural practices, in particular fertilizer amendments, could influence As speciation and mobility, thus increasing its transfer to crops and its toxicity. In the present study, field-relevant amounts of fertilizers were applied to soils from a cultivated field that was a former ammunition-burning site. Potassium phosphate, ammonium sulfate and organic matter were applied to these soils in laboratory experiments to assess both their impact on As leaching and on As bioavailability to Lactuca sativa. Results indicate that none of the fertilizers significantly influences the speciation and quantity of mobile As, although trends showed an increase of mobility with KP and a decrease of mobility with ammonium sulfate. The most probable number of AsIII-oxidizing microbes and AsIII-oxidizing activity were strongly linked to As levels in water and soils. In the reference un-polluted soil, ammonium sulfate negatively affected AsIII-oxidizing activity. KP amendment induced a small increase of As content in plants, and the polluted soil amended with ammonium sulfate was significantly less phytotoxic than the un-amended soil. Results suggest that ammonium sulfate, the most frequently applied fertilizer on site, influences the interactions between As, microorganisms and plants.Read less <