Effects of elevated CO₂ on hydraulic performance and carbon assimilation of Schefflera arboricola

Purpose: The growth and carbon assimilation of plants can be promoted through increasing atmospheric CO₂ concentration, which facilitates plant carbon sequestration. However, plant growth is normally limited by soil–water content and the lack of soil nutrients under elevated CO₂ condition. This study explores the effects of elevated CO₂ concentration on plant behaviour, with an emphasis on hydrological performance and carbon assimilation in biochar-amended soil. Methods: A tree species, Schefflera arboricola, was grown in soil with and without biochar amendment at two CO₂ concentrations, 400 ppm (aCO₂) and 1000 ppm (eCO₂). Plant characteristics including root architecture, sap flow, and stomatal conductance were measured to reflect plant transpiration and carbon assimilation. Soil–water content and suction were measured to explore soil–plant hydraulic interactions. Results: Plant transpiration induced higher soil suction at eCO₂. However, the benefits towards plant transpiration provided by eCO₂ were diminished when soil suction was higher than 550 kPa. When compared with that in the same soil conditions at aCO₂, plant transpiration rate and carbon assimilation at eCO₂ conditions were increased by about 660% and 200%, respectively. Linear relationships between stomatal conductance and sap flow rate under different CO₂ concentrations were proposed to estimate plant carbon assimilation. Conclusion: The benefits of eCO₂ on plant transpiration were limited by soil–water content, which can be solved by biochar application. This study revealed the importance of examining the soil–plant hydraulic interactions when estimating plant carbon assimilation.