Responses of root morphology and function to elevated CO2 in terrestrial ecosystems: a meta-analysis

Background/Question/Methods

Plant root systems sensitively respond to elevated CO₂ and are important regulators of C cycling of terrestrial ecosystems. Despite considerable research efforts, our knowledge of plant root systems responses to elevated CO₂ is still fragmented. Through synthesizing data from 110 published papers of Free-Air CO₂ Enrichment (FACE) and Open-Top Chamber (OTC) experiments, we evaluated the responses of 18 variables associated with root morphology, biomass, C and N pools, and turnover by a meta-analysis. The objective of the analysis was to determine general patterns of responses of root traits to elevated CO₂, and elucidate what the implications are for these important sensors and regulators of elevated CO₂ in terrestrial ecosystems.

Results/Conclusions

CO₂ elevation increased root length (+26.0%) and diameter (+8.4%). CO₂ elevation also stimulated total root (+28.8%), fine root (+27.7%), and coarse root biomass (+25.3%), demonstrating strong responses of root morphology and biomass. CO₂ elevation increased root:shoot ratio (+8.5%) and decreased root distribution in soil surface (-8.4%), suggesting plants expand rooting systems at elevated CO₂. In addition, CO₂ elevation decreased N concentration (-7.1 %), but did not affect C concentration, thus increased C:N ratio (+7.8%). Root C (+29.3%) and N pools (+9.4%) together increased under elevated CO₂. Functional traits were also strongly affected by CO₂ elevation, which deceased turnover (-4.7%) and increased respiration (+58.9%), rhizodeposition (+37.9%), and fungal infection (+3.3%). The increases in N pool, fine root biomass, fungal infection, root system expansion and C:N ratio adjustment were observed across terrestrial ecosystems. Overall, root systems appear to act as a negative, or self-regulating, feedback to rising atmospheric CO₂ concentration.