**5. Conclusions**

Both the adsorption and desorption processes of freshly added Se(IV) in acid and alkaline soils revealed distinct differences between the two groups of soils. Acid soils adsorbed significantly higher amounts of added Se(IV) than alkaline soils, and alkaline soils desorbed more Se. Fe, Al and Mn oxides, and particularly amorphous Fe oxides content, were the key parameters controlling the sorption/desorption of Se(IV) in the studied soils. Indeed, increased Feo concentration led to higher Se(IV) sorption and to lower Se desorption from the studied soils. Soil pH and the equilibrium solutions' pH strongly influenced both sorption and desorption patterns, providing more positively charged sites on oxides surfaces, leading to higher Se(IV) sorption. Furthermore, metal oxide chemistry at low pH values favored the formation of stronger surface complexes, thus suppressing the Se desorption from acid soils by a weak salt. Overall, the results of this study showed that metal oxides content and pH determine Se geochemistry in soils. Considering that biofortification through plant uptake is also crop/plant-dependent, Se(IV) application in agricultural soils should be site-specific, since a high Se leaching hazard in alkaline soils with low metal oxides concentration may emerge, and low Se availability in acid soils with high metal oxides contents can appear.

**Author Contributions:** I.Z.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing—original draft. D.G.: Methodology, Validation, Resources, Data curation, Writing—review and editing. I.M.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing—original draft, Writing—review and editing, Visualization, Supervision, Project administration. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
