11 Jul 2022
11 Jul 2022
Status: a revised version of this preprint is currently under review for the journal BG.

The influence of elevated CO2 and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland

Johanna Pihlblad1,2, Louise C. Andresen3, Cartriona A. Macdonald1, David S. Ellsworth1, and Yolima Carrillo1 Johanna Pihlblad et al.
  • 1Hawkesbury Institute of environment, Western Sydney University, Penrith Australia
  • 2Birmingham Institute for Forest Research, University of Birmingham, Birmingham United Kingdom
  • 3Department of Earth Sciences, University of Gothenburg, Gothenburg Sweden

Abstract. Elevated carbon dioxide (eCO2) in the atmosphere increases forest biomass productivity, but only where soil nutrients, particularly nitrogen (N) and phosphorus (P) are not limiting growth. eCO2, in turn, can impact rhizosphere nutrient availability. Our current understanding of nutrient cycling under eCO2 is mainly derived from surface soil, leaving mechanisms of the impact of eCO2 on rhizosphere nutrient availability at deeper depths unexplored. To investigate the influence of eCO2 on nutrient availability in soil at depth, we studied various C, N and P pools (extractable, microbial biomass, total soil C and N, and mineral associated P) and nutrient cycling processes (enzyme activity and gross N mineralization) associated with C, N, and P cycling in both bulk and rhizosphere soil at different depths at the Free Air CO2 enrichment facility in a native Australian mature Eucalyptus woodland (EucFACE) on a nutrient-poor soil. We found that the depth-induced decrease in nutrient availability, gross N mineralization was counteracted by the root influence and by eCO2. Increases in available PO43-, adsorbed P and the C : N and C : P ratio of enzyme activity with depth were observed. We conclude that the influences of roots and of eCO2 can affect available-nutrient pools and processes well beyond the surface soil of a mature forest ecosystem. Our findings indicate a faster recycling of nutrients in the rhizosphere, rather than additional nutrients becoming available through SOM decomposition. If the plant growth response to eCO2 is reduced by the constraints of nutrient limitations, then the current results would call to question the potential for mature tree ecosystems to fix more C as biomass in response to eCO2. Future studies should address how accessible the available nutrients at depth are to deeply rooted plants, and if fast recycling of nutrients is a meaningful contribution to biomass production and the accumulation of soil C in response to eCO2.

Johanna Pihlblad et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-145', Anonymous Referee #1, 29 Jul 2022
    • AC1: 'Reply on RC1', Johanna Pihlblad, 10 Oct 2022
  • RC2: 'Comment on bg-2022-145', Anonymous Referee #2, 03 Aug 2022
    • AC2: 'Reply on RC2', Johanna Pihlblad, 10 Oct 2022

Johanna Pihlblad et al.

Johanna Pihlblad et al.


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Short summary
Elevated CO2 in the atmosphere increases forest biomass productivity, but only where soil nutrients, mainly nitrogen and phosphorus, are not limiting growth. This study explores mature trees capability to increase nutrient availability at depth by studying bulk and rhizosphere at different depths under eCO2. We found that phosphate was increased at depth by faster recycling of nutrients in the rhizosphere, not through SOM decomposition, questioning if mature trees can fix more carbon under eCO2.