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

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 PO₄^3-, 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. [ABSTRACT FROM AUTHOR]