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 (c_i : c_a), 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.