Contrasting release of redox-sensitive arsenic and uranium induced by heterotrophic denitrification in alluvial sediments

) and without nitrate were incubated in triplicates. In the control mesocosms, we observed a progressive decline in Eh, accompanied by the release of Mn and Fe due to reductive dissolution of (oxyhydr)oxides, which led to a release of As. In contrast, in the nitrate-amended mesocosms, heterotrophic denitrification caused (sub)oxic conditions, which inhibited the reductive dissolution of (oxyhydr)oxides, but led to the oxidative release of U. Sequential and single extractions showed that the geochemical fractions of U did not change and that about 93 % occurred as reduced U(IV) before and after the experiment. Our findings suggest that agricultural inputs of nitrate can cause the release of geogenic U, while simultaneously reducing the reductive release of As.