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.

1. 澳大利亚与南非的数百种山龙眼科(Proteaceae)植物通常生长在磷(P)贫瘠的土壤中，均采用释放羧酸盐的磷动员策略。在澳大利亚西南生物多样性热点区，有两种柱果木属(Xylomelum，山龙眼科)植物分布范围较广，但分布局限于该范围内。
2. 我们以1 μM磷浓度在水培条件下培养了西方柱果木(Xylomelum occidentale)，并在其自然生境中采集了叶片、种子、根际鞘与非根际土壤样品。
3. 西方柱果木不会产生具有功能的簇根，其所栖息的土壤磷贫瘠程度较该区域典型山龙眼科生境的土壤更低。基于叶片锰浓度（作为根际羧酸盐浓度的替代指标）以及非根际与根际土壤的磷组分测定结果，我们推断西方柱果木可在不释放羧酸盐的情况下获取有机磷。溶液态31P核磁共振光谱(Solution 31P-NMR)明确了该植物所利用的有机磷形态。
4. 西方柱果木所采用的策略与山龙眼科的典型策略存在本质差异，可在不释放羧酸盐的情况下获取土壤有机磷。我们推测，这一新颖策略可能同样存在于同域分布的、缺乏羧酸盐分泌策略的非山龙眼科植物，以及类似生境中的植物类群中。由于土壤有效磷浓度过低，这些同域分布物种难以从菌根共生关系中获益。
5. 综合与结论。本研究的发现首次提供了野外实证证据，证明西方柱果木可在不分泌羧酸盐的情况下有效利用土壤有机磷，并解释了其在古老磷贫瘠景观中分布相对局限的原因，有助于深化对多样磷获取策略如何在超多样生态系统中共存这一科学问题的理解。