Phosphorus addition shifts elevated CO2 effects on soil respiration from rhizosphere to root pathways in a mature eucalypt woodland.

Phosphorus (P) limitation may constrain the capacity of forests to remove CO2 from the atmosphere under elevated CO2 (eCa). One key mechanism is that limited P availability requires plants to invest additional fixed carbon belowground, which then returns to the atmosphere via soil respiration. However, the extent and pathways of this response remain unclear. We quantified total soil respiration and inferred its component fluxes before and after alleviating P limitation via fertilization in an established eCa experiment in mature Eucalyptus woodland. eCa increased soil respiration by 22.5% regardless of P availability. Dependence of the stimulation on soil moisture indicated it was driven primarily by root-derived respiration rather than soil organic matter-derived heterotrophic respiration. Fine-root production was stimulated by eCa only after P addition, suggesting that P fertilization triggered a shift from rhizomicrobial respiration, associated with microbial use of root exudates and residues, to autotrophic respiration from root growth and metabolism. These findings indicate that within 9 months of P addition, EucFACE plants may have shifted P-acquisition strategies under eCa. As the first ecosystem-scale study on the interactive effect of eCa and P limitation, this research improves mechanistic understanding of short-term belowground responses and provides insight to motivate follow-up studies.