Soil selenium transformation across different parent materials in Pothwar uplands of Pakistan

Selenium (Se) is an essential trace element for humans but non-essential for plants. Selenium bioavailability depends on selenium forms in soil. Selenium contents may vary with the source of parent material, and pedogenesis controls its vertical distribution. The objectives of this study were to determine the variation in Se fractions with parent material and pedogenesis in Pothwar uplands of Pakistan. Triplicate soil profiles at different development stages from selected parent materials (sandstone, alluvium, loess, and shale) were sampled at different genetic horizons level. The soils were characterized for pH, dissolved organic carbon, total organic carbon, dithionite extractable amorphous iron, and cation exchange capacity. Soil selenium was fractioned into ion-exchangeable, metal oxides, organic, humic, sulfide, and residual selenium. Selenium in the soil extract was measured using HVG-AAS after centrifuge and passing through a 0.45-μm cellulose free filter. The distribution of total selenium content and selenium fractions varied with the type of parent material and soil genesis. Total selenium contents differed significantly across the different parent material. The highest total selenium was in shale followed by loess, alluvium, and sandstone, respectively. Exchangeable ion, metal oxides, and organic matter selenium were more in loess or the same level as of shale. Evaluation of the distribution of selenium fractions enabled us to estimate the occurrence of potential selenium deficiency and toxicity to humans and animals based on the parent materials and pedogenesis.