Elevated CO2 and biochar differentially affect plant C:N:P stoichiometry and soil microbiota in the rhizosphere of white lupin (Lupinus albus L.)

Biochar application is a potent climate change mitigation strategy in agroecosystems. However, little is known about the interactive effects of elevated CO2 (eCO2) and biochar on plant nutrient uptake and soil microbial processes. A pot experiment was conducted to investigate the effects of eCO2 and biochar addition on plant C:N:P stoichiometry and rhizobacterial community for better management of nutrient balance and use efficiency in a future climate scenario. White lupin (Lupinus albus L.) was grown for 30 days in topsoil and subsoil with or without 2% corn-stubble biochar under ambient CO2 (aCO2: 390 ppm) or eCO2 (550 ppm). Elevated CO2 increased, but biochar decreased, plant biomass and shoot N and P uptake, with no interactions in either soil layer. Elevated CO2 decreased shoot N concentration by 16% and biochar decreased shoot P concentration by 11%. As a result, eCO2 increased shoot C:N ratio by 20% and decreased the N:P ratio by 11%. Biochar decreased shoot C:N ratio by 8% in the subsoil under eCO2. However, biochar increased shoot C:P ratio by an average of 13% and N:P ratio by 23% in the subsoil. Moreover, plants grown in the subsoil showed lower shoot N (35%) and P (70%) uptake compared to the topsoil. The results indicate that N and P are the more limiting factors that regulate plant growth under eCO2 and biochar application, respectively. Elevated CO2 and biochar oppositely affected dominant rhizobacterial community composition, with the eCO2 effect being greater. The microbiota in the subsoil held a greater diversity of contrasting species than the topsoil, which were associated with nutrient cycling, hydrocarbon degradation and plant productivity. These results enrich our understanding of potential soil nutrient cycling and plant nutrient balance in future agroecosystems.