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 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, 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.
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.