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

<p>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 <i>Xylomelum </i> (Proteaceae) species are widely distributed, but restricted within that distribution.</p> <p>2. We grew <i>X. occidentale</i> in hydroponics at 1 μM P. Leaves, seeds, rhizosheath and bulk soil were collected in natural habitats.</p> <p>3. <i>Xylomelum occidentale</i> 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 <i>X. occidentale</i> accesses organic P, without releasing carboxylates. Solution ³¹P-NMR revealed which organic P forms <i>X. occidentale</i> accessed.</p> <p>4. <i>Xylomelum occidentale</i> 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, and plants in similar habitats. These co-occurring species are unlikely to benefit from mycorrhizal associations, because plant-available soil P concentrations are too low.</p> <p>5. <i>Synthesis.</i> Our findings show the first field evidence of effectively utilising soil organic P by <i>X. occidentale</i> 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.</p>