“…Hydro(bio)geochemical dynamics in the vadose zone form the basis of Se mobility and thus bioavailability through the food chain. , Depending upon the pH condition, inorganic Se(VI) (mainly SeO 4 2– ) forming outer-sphere complexes on Fe/Mn-oxide minerals (e.g., goethite, ferrihydrite, birnessite, and hydrous Fe/Mn oxides) adsorbs less strongly than Se(IV) (mainly HSeO 3 – and SeO 3 2– ) forming inner-sphere complexes. − Likewise, adsorption of SeO 4 2– on Al (hydr)oxides (e.g., gibbsite and allophane) and clay minerals (e.g., montmorillonite and kaolinite) is generally weaker than that of SeO 3 2– . , This engenders SeO 4 2– the most mobile and bioavailable species under most soil conditions . In comparison, reduced Se species (e.g., Se 0 and selenides), owing to their poor solubility, are much less mobile and hardly bioavailable. , In organic-rich soils (e.g., Mollisols), Se binding to organic matter, in particular under acidic conditions (pH 4.9–6.9), can reduce Se mobilization, making such soils a significant reservoir of Se. ,, Partial degradation of the organic-bound Se yields organic Se species including selenocystine and selenomethionine, whereas complete oxidation coupled to the reductive dissolution of Fe oxides releases inorganic Se into the pore water. − Therefore, oxidation of the reduced/organic-bound Se species to Se(IV)/Se(VI) may strengthen Se mobility and bioavailability in the vadose zone.…”