Altered root traits due to elevated CO₂: a meta-analysis

Aim: Plant root traits regulate belowground C inputs, soil nutrient and water uptake, and play critical roles in determining sustainable plant production and consequences for ecosystem C storage. However, the effects of elevated CO₂ on root morphology and function have not been well quantified. We reveal general patterns of root trait responses to elevated CO₂ from field manipulative experiments. Location: North America, Europe, Oceania, Asia. Methods: The meta-analysis approach was used to examine the effects of CO₂ elevation on 17 variables associated with root morphology, biomass size and distribution, C and N concentrations and pools, turnover and fungal colonization from 110 published studies. Results: Elevated CO₂ increased root length (+26.0%) and diameter (+8.4%). Elevated CO₂ also stimulated total root (+28.8%), fine root (+27.7%) and coarse root biomass (+25.3%), demonstrating strong responses of root morphology and biomass. Elevated CO₂ increased the root:shoot ratio (+8.5%) and decreased the proportion of roots in the topsoil (-8.4%), suggesting that plants expand rooting systems. In addition, elevated CO₂ decreased N concentration (-7.1%), but did not affect C concentration, and thus increased the C:N ratio (+7.8%). Root C (+29.3%) increased disproportionately relative to root N pools (+9.4%) under elevated CO₂. Functional traits were also strongly affected by elevated CO₂, which increased respiration (+58.9%), rhizodeposition (+37.9%) and fungal colonization (+3.3%). Main conclusions: These results suggest that elevated CO₂ promoted root morphological development, root system expansion and C input to soils, implying that the sensitive responses of root morphology and function to elevated CO₂ would increase long-term belowground C sequestration.