Rhizosphere-driven increase in nitrogen and phosphorus availability under elevated atmospheric CO2 in a mature Eucalyptus woodland

Background and aims: Rhizosphere processes are integral to carbon sequestration by terrestrial ecosystems in response to rising concentrations of atmospheric CO ₂. Yet, the nature and magnitude of rhizosphere responses to elevated CO ₂, particularly in nutrient and water-limited forest ecosystems, remain poorly understood. Methods: We investigated rhizosphere responses (enzyme activities and nutrient availability) to atmospheric CO ₂ enrichment (ambient +150 μmol CO ₂ mol ⁻¹) in a phosphorus-limited mature eucalypt woodland in south-eastern Australia (the EucFACE experiment). Results: Following 17 months of treatment, the activity of rhizosphere soil exoenzymes related to starch and cellulose degradation decreased between 0 and 10 cm and increased from 10 to 30 cm depth under elevated CO _2. This response was concurrent with increases in nitrogen and phosphorus availability and smaller C:P nutrient ratios in rhizosphere soil under elevated CO ₂. Conclusions: This nutrient-poor eucalypt woodland exhibited rhizosphere responses to atmospheric CO ₂ enrichment that increased nutrient availability in rhizosphere soil and suggest accelerated rates of soil organic matter decomposition, both of which may, in turn, promote plant growth under elevated CO ₂ concentrations.