Synergistic enhancement of selenium accumulation in rice: elucidating the role of biochar and selenobacteria in reshaping root microbiome assembly

Aims: Selenium (Se) bioavailability often limits the production of Se-rich crops in naturally Se-enriched soils. While biochar and selenobacteria individually show promise in enhancing Se mobility, their synergistic effects and underlying mechanisms remain poorly understood. Methods: This work investigated the synergistic impact of rice straw-derived biochar and selenobacterium Bacillus megaterium on Se accumulation in rice and elucidated the associated shifts in root microbiome. Results: The findings demonstrated a synergistic interaction between the treatments, inducing an increase in shoot Se content surpassing that observed with any single treatment. This effect was accompanied by an obvious decrease in total soil Se, particularly evident with co-application, highlighting their synergistic ability of mobilizing the native soil Se reservoir effectively. Biochar was the dominant driver of soil fertility improvement, causing increased availability of phosphorus, potassium and organic carbon. It also played key roles in reshaping root-associated bacterial communities, reducing rhizosphere bacterial diversity while simultaneously enriching specific beneficial taxa like Paenibacillaceae and Bdellovibrionaceae under co-application treatment. Co-occurrence network analysis further identified compartmentalized associations between specific microbial taxa and shoot Se content. Specifically, key taxa such as Planctomycetes and Mortierellaceae in the rhizosphere, Thermoleophilia, Solirubrobacterales, and Sordariomycetes in the rhizoplane, alongside Rubrivivax and Sphingomonadaceae in the endosphere, were revealed as being significantly correlated. Peroxidase activity showed strong correlative associations with Se accumulation across compartments. Conclusions: The combined use of biochar and Bacillus megaterium enhances Se bioavailability and translocation by beneficially restructuring the root microbiome, offering a sustainable strategy for the biofortification of rice in naturally Se-enriched soils.