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.
Potential activity of seven enzymes associated with C, N and P mineralisation were determined for bulk and rhizosphere soil respectively. For this we used fluorometrically labelled substrates, following the method of Bell et al., 2013. Two g frozen soil was mixed to a slurry (1:33 w:v) with MilliQ water in a laboratory blender for one minute. The slurry was pipetted into 96 well plates with three technical replicates and given fluorescent substrates (4-methylumbelliferone; MUB and 7-amino-4-methylcoumarin: MUC) in accordance with the Bell et al. protocol (2013). The samples were then incubated at 25 °C for three hours and analysed for fluorescence with a CLARIOstar plate reader (BMG LABTECH GmbH, Germany). Four enzymes (α-D-glucopyranoside (AG), β-Dglucopyranoside (BG), β-D-cellobioside (CB), and β-D-xylopyranoside (XYL)) targeted C-rich compounds (sugar, cellulose, hemicellulose), two enzymes (L-Leucine-7-aminopeptidase (LAP) and N-acetyl-β-Dglucosamine (NAG)) targeted N-rich compounds (proteins and chitin), and acid phosphatase (PHOS) targeted organic compounds with P. These enzymes are considered representative of the total enzyme pool active in the soil.
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