Selenium regulated the responses of soil bacterial communities to short-term elevated atmospheric CO₂ stress

Recent studies highlight essential metallic micronutrients (e.g., zinc, nickel, copper, iron, manganese) in influencing soil microbial structure and functions. However, the roles of metalloid micronutrients like selenium (Se) are still underexplored. To fill this gap, this study conducted an experiment in which soils were incubated at ambient (435 ppm) and elevated (800 ppm) atmospheric carbon dioxide (CO₂) concentrations within the short term of 6 weeks. The results exhibited that elevated CO₂ significantly impacted the α-diversity of the bacterial community (p < 0.05). Yet, the Se addition (1 and 10 ppm) noticeably accelerated the effects of elevated CO₂ on the diversity by 3 and 2 weeks, respectively. In addition, the 10 ppm Se addition significantly decreased the bacterial α-diversity (p < 0.05) compared to the other two Se treatments under the ambient CO₂. In comparison, the highest relative abundance of nitrate reduction genes under elevated CO₂ occurred in weeks 4 and 5 following the Se addition. Network analysis presented that the elevated CO₂ formed a strongly interconnected bacterial network (node: 275, edge: 3824) with 1 ppm Se addition, while 10 ppm Se addition decreased the connection of the network (edge: 1460). Structural equation modeling showed that elevated CO₂ levels and Se addition significantly affected soil physicochemical properties (p < 0.05), which in turn influenced bacterial α-diversity and functional genes related to the nitrogen cycle; bacterial α-diversity also directly impacted carbon and sulfur cycles (p < 0.05 and p < 0.05, respectively). Altogether, the current findings provide a basis for a new strategy to mediate the soil microbial responses to future climatic changes through the application of micronutrients; however, careful consideration of the appropriate dosage is essential.