This study is designed to evaluate whether the speciation of soil arsenic (As) is altered as a result of phytoremediation and whether microbial communities in the root zone are associated with changes in speciation. It is also of interest to determine whether microbial communities are associated with the volatilization of As compounds. Plant growth experiments will be conducted in controlled environments. Poplar saplings will be grown while exposed to varying amounts of soil-based As. Changes in speciation and volatilization will be investigated through measurements of As in soil, plant and air samples collected from the test chambers. Microbial populations in the rhizosphere of the saplings will be enumerated based on their ability to change the speciation of soil As.