Different responses of root exudates to biochar application under elevated CO2

Abstract Biochar application and rising carbon dioxide (CO2) concentrations are likely to influence both aboveground and belowground plant processes. However, little is known about the influence of biochar and CO2 enrichment on root exudation, as well as their interactive effects. The aim of this study was to reveal whether rice paddy root exudates responded to biochar and CO2 enrichment and whether this effect varied at different plant growth stages. Rice was grown in growth chambers with different biochar addition rates (1%, 2%, 3%, 4%, and 5% by weight) under both ambient (aCO2, 400 ppm) and elevated (eCO2, 700 ppm) CO2 levels. Root exudation variables were measured at the tillering and stem elongation stages. Electrical conductivity (EC), amino acid (AA), indole acetic acid (IAA), and abscisic acid (ABA) levels were significantly increased but cytokinin (CYT) concentrations decreased in response to biochar addition rates. In addition, eCO2 increased root and shoot biomass but decreased the root-to-shoot ratio. eCO2 significantly decreased EC, potassium (K+), ABA, and CYT concentrations, while the opposite pattern was found for AA and IAA. Significant interactive effects between biochar addition and eCO2 were only found on CYT and AA concentrations, while no significant interactive effects of plant growth stage, biochar addition, and eCO2 were found on root exudation. These results indicate that the additive effects of biochar and eCO2 should be integrated into assessments of carbon storage ability in the future. Understanding the different responses of these variables within the rhizosphere is crucial for predicting changes in the carbon balance in response to biochar application and climate change.