Coupled effects of elevated CO₂ and biochar on microbial communities of vegetated soil

Soil microbial communities are important for plant growth and establishing healthy ecosystems. Although biochar is widely adopted as a sustainable fertilizer, its influence on soil ecological functions is still unclear, especially under climate change such as elevated carbon dioxide concentration (eCO₂). This study explores the coupled effects between eCO₂ and biochar on microbial communities in soil planted with tree seedlings of Schefflera heptaphylla. Root characteristics and soil microbial communities were examined and interpreted with statistical analysis. Results show that biochar application at ambient carbon dioxide concentration (aCO₂) always improves plant growth, which is further promoted under eCO₂. Similarly, β-glucosidase, urease and phosphatase activities are enhanced by biochar at aCO₂ (p < 0.05). In contrast, only urease activity increases with biochar added at eCO₂ (p < 0.05). The beneficial effects of biochar on soil enzyme activities become less significant at eCO₂. Depending on biochar type, biochar can increase bacterial diversity and fungal richness at aCO₂. However, at eCO₂, biochar does not significantly affect microbial richness (p > 0.05) while microbial diversity is reduced by peanut shell biochar (p < 0.05). Owing to better plant growth under biochar application and eCO₂, plants are likely to become more dominant in specializing the microbial communities that are favourable to them. In such community, the abundance of Proteobacteria is the greatest and increases after biochar addition at eCO₂. The most abundant fungus also shifts from Rozellomycota to Ascomycota and Basidiomycota. These microbes can improve soil fertility. Even though the microbial diversity is reduced, using biochar at eCO₂ can further promote plant growth, which in turn enhances carbon sequestration. Thus, biochar application can be an effective strategy to facilitate ecological restoration under climate change and relieve the problem of eCO_2.