Soil phosphorus drives subcontinental patterns of carbon isotope discrimination across Australia.

Several transects have been established to study the sensitivity of carbon isotope discrimination (Δ13C) in woody plants to mean annual precipitation (MAP) across Australia. These have shown a surprising divergence in Δ13C-MAP sensitivity among subcontinental regions. We analysed previously reported data alongside new measurements from a transect in northeastern Queensland to explore potential drivers of regional-scale Δ13C-MAP sensitivity. Multiple lines of evidence indicated this sensitivity is related to soil phosphorus. In phosphorus-poor regions, Δ13C decreased less with decreasing MAP than in phosphorus-rich regions. Along two contrasting transects in northern Australia, Δ13C correlated with leaf phosphorus in the phosphorus-poor Northern Territory, but not in phosphorus-rich northeastern Queensland, where it instead correlated with leaf nitrogen. Common garden experiments for species from phosphorus-poor vs phosphorus-rich regions showed contrasting relationships between Δ13C and species range MAP. Finally, using an Australia-wide leaf gas exchange dataset, we showed that soil phosphorus influenced the ratio of intercellular to ambient CO2 concentrations (ci : ca), which in turn controls Δ13C; the influence was through stomatal conductance, not photosynthetic capacity. Higher stomatal conductance in phosphorus-poor regions appeared to moderate the decrease in Δ13C with decreasing precipitation. We suggest that high transpiration rates in these regions help to facilitate phosphorus foraging in phosphorus-impoverished, ancient soils.