Xylomelum occidentale (Proteaceae) accesses relatively mobile soil organic phosphorus without releasing carboxylates

1. Hundreds of Proteaceae species in Australia and South Africa typically grow on phosphorus (P)‐impoverished soils, exhibiting a carboxylate‐releasing P‐mobilising strategy. In the Southwest Australian Biodiversity Hotspot, two Xylomelum (Proteaceae) species are widely distributed, but restricted within that distribution.
2. We grew X. occidentale in hydroponics at 1 μM P. Leaves, seeds, rhizosheath and bulk soil were collected in natural habitats.
3. Xylomelum occidentale did not produce functional cluster roots and occupied soils that are somewhat less P‐impoverished than those in typical Proteaceae habitats in the region. Based on measurements of foliar manganese concentrations (a proxy for rhizosphere carboxylate concentrations) and P fractions in bulk and rhizosheath soil, we conclude that X. occidentale accesses organic P, without releasing carboxylates. Solution 31P‐NMR spectroscopy revealed which organic P forms X. occidentale accessed.
4. Xylomelum occidentale uses a strategy that differs fundamentally from that typical in Proteaceae, accessing soil organic P without carboxylates. We surmise that this novel strategy is likely expressed also in co‐occurring non‐Proteaceae that lack a carboxylate‐exuding strategy. These co‐occurring species are unlikely to benefit from mycorrhizal associations, because plant‐available soil P concentrations are too low.
5. Synthesis. Our findings show the first field evidence of effectively utilising soil organic P by X. occidentale without carboxylate exudation and explain their relatively restricted distribution in an old P‐impoverished landscape, contributing to a better understanding of how diverse P‐acquisition strategies coexist in a megadiverse ecosystem.