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

Abstract

Background and aims: Rhizosphere processes are integral to carbon sequestration by terrestrial ecosystems in response to rising concentrations of atmospheric CO2. Yet, the nature and magnitude of rhizosphere responses to elevated CO2, particularly in nutrient and water-limited forest ecosystems, remain poorly understood. Methods: We investigated rhizosphere responses (enzyme activities and nutrient availability) to atmospheric CO2 enrichment (ambient +150 μmol CO2 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 CO2. This response was concurrent with increases in nitrogen and phosphorus availability and smaller C:P nutrient ratios in rhizosphere soil under elevated CO2. Conclusions: This nutrient-poor eucalypt woodland exhibited rhizosphere responses to atmospheric CO2 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 CO2 concentrations.