Approach to Selenium Application in Different Soil Concentrations for Encouraged Yield, Distribution, and Biofortification of Common Buckwheat Seeds (Fagopyrum esculentum Moench)

The soil application of essential trace elements, such as selenium and its various agrochemical species, presents a real challenge for modern agriculture. However, unknown exceeding threshold concentrations could target potential toxicity within the soil–plant–organism. When applied at optimal levels and combined with the common buckwheat—a crop of the future known for its high nutritional value—this poses a novel academic approach. Therefore, the aim of this research is to examine the effect of three concentrations (150, 300, and 600 g/ha) of selenium species (sodium selenite and sodium selenate) on mobility and distribution within the common buckwheat plant, including its impact on the biofortification. The research was carried out during the 2022 and 2023 seasons through pot experiments in semi-regulated conditions located in the Central European agronomic region. Following manual harvesting, chemical analysis was conducted using methods such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), along with yield determination. The results confirmed the positive effect of Se6+ 150 g/ha and Se4+ 150 g/ha and 300 g/ha on seed yield. Oppositely, Se6+ 600 g/ha caused a decrease in seed yield of 23.87%. For biofortification of common buckwheat is most suitable Se6+ in a dose of 150 g/ha, where the Se content in seeds, 3.30 ± 0.46 mg/kg, was achieved. The soil fertility index, based on PCA, indicated that Se6+ at 150 g/ha exhibited the highest biofortification efficiency without compromising yield.