Mechanisms of Biochar in Modulating Soil Organic Selenium Transformation and Enhancing Soil Selenium Availability

Selenium deficiency poses a significant threat to human health. The low bioavailability of selenium in soil largely limits the improvement of selenium content in crops. Selenium in soil mainly exists in an organically bound form. Biochar has the ability to regulate the organic matter content of soil; however, the impact of biochar on the transformation of organically bound selenium in soil remains poorly understood. Therefore, this study investigates the effect of biochar on organically bound selenium in typical medium–to–high selenium soils from Yimen County, Yuxi City, Yunnan Province. Reed straw (RS), apple wood (AW), and walnut shells (WS) were used as biomass materials for biochar preparation. The study utilized organically bound selenium transformation incubation and pot experiments to explore the role of biochar in transforming organically bound selenium in soil. The results showed that organically bound selenium was the dominant selenium form in the soil, accounting for 66.31% of the total selenium content. Both pot experiments and incubation trials indicated that the addition of biochar significantly increased the levels of water–soluble and exchangeable selenium in the soil. The addition of biochar mainly promotes the conversion of fulvic acid–bound selenium into water–soluble and exchangeable selenium. In the absence of carbon sources, humic acid–bound selenium can also be converted to water–soluble and exchangeable selenium. Correlation analysis revealed that soil water–soluble selenium was significantly negatively correlated with soil total selenium (r = −0.792 **, p < 0.01), soil phosphatase activity (r = −0.645 *, p < 0.05), abundance taxa of Chloroflexi (r = −0.751 *, p < 0.05), Chytridiomycota (r = −0.674 *, p < 0.05), and Basidiomycota (r = 0.722 **, p < 0.05), while it was significantly positively correlated with soil urease activity (r = 0.809 **, p < 0.01), and significantly negatively correlated with abundance taxa of Myxococcota (r = −0.800 **, p < 0.01). Compared with the initial soil, the WS treatment (initial soil water–soluble selenium 0.31 μg·kg−1, exchangeable selenium 0.11 μg·kg−1) significantly increased the soil water–soluble selenium by 34.9 times and exchangeable selenium by 100.2 times. Additionally, the selenium content in garlic increased by 170% compared to the control group.