Selenium desorption in tropical soils by sulfate and phosphate, and selenium biofortification of Mombaca grass under increasing rates of phosphate fertilisation

Selenium (Se) is essential for animals and humans, and studies assessing the influence of sulfur (S) and phosphorus (P) on Se availability and biofortification in tropical soils are required. This study evaluated the adsorption of selenate (SeVI) and selenite (SeIV) in two oxidic soils with contrasting soil texture and assessed effects of phosphate and sulfate as competitive anions on Se desorption. The study also examined Se biofortification of Panicum maximum cv. Mombaca grown in Se-treated clayey soil under increasing rates of P fertilisation. In the laboratory, different Se concentrations were added to soils for adsorption studies (0, 1, 2, 4 mg L-1 for SeVI and 0, 4, 8, 16 mg L-1 for SeIV), with Se desorption performed by adding different concentrations of P (0, 2.18, 4.36, 10.90, 21.90 mg L-1) or S (0, 3.75, 7.5, 15, 30 mg L-1). Mombaca grass was grown in pots for 43 days in the clayey soil fertilised with SeVI and increasing P rates. Selenium adsorption varied depending on Se speciation, Se rate and soil texture, with SeIV showing greater adsorption than SeVI. In general, amounts of SeVI desorbed increased with increasing addition of P and (mainly) S. However, only P had a positive effect for releasing SeIV from soils. At all SeVI rates added in the clayey soil, Se contents in the shoot dry matter of Mombaca grass increased with increasing P fertilisation rate, agreeing with the desorption results. Further studies combining desorption with biofortification data are recommended for assessing the influence of P in sandy clay loam tropical soils.

Automated summary

This study evaluated the adsorption of selenate and selenite in two oxidic soils with different soil textures, as well as the competitive effects of phosphate and sulfate on selenium desorption. It was found that selenium adsorption varied depending on speciation, rate, and soil texture, with selenite showing greater adsorption than selenate. In the biofortification experiment with Panicum maximum cv. Mombaca, it was observed that increasing phosphate fertilization rates led to higher selenium content in the plant shoots.