Divergent Phosphorus-Mining Strategies in Simple and Compound Cluster Roots in Extremely Phosphorus-Impoverished Soils in Southwest Australia

Most Proteaceae and some Fabaceae species produce specialised cluster roots (CRs), and are abundant in severely phosphorus (P)-impoverished soils in southwest Australia. Two types of CRs, compound and simple, have been identified. However, the difference in their P-mining strategies remains unclear. Therefore, we conducted glasshouse and field experiments to compare the P-acquisition strategies among 18 CR-producing species in Proteaceae and Fabaceae. Proteaceae produced a significantly larger mass of CRs than Fabaceae. Particularly, Banksia species produced the largest mass of compound CRs and exhibited the greatest net plant-absorbed P in pots and consistently higher mature leaf manganese concentration in the field. In contrast, Hakea and Grevillea species produced less mass of simple CRs but three times as much soil adhered to their CRs per CR dry weight, resulting in greater absorbed P per CR weight. All plants depleted similar P compounds from soil and accessed c. 52% of P that was not extracted by a NaOH-EDTA solution, suggesting that both CRs shared a common physiological function for mining scarcely available P. This study highlights two divergent P-acquisition strategies: greater biomass investment in compound CRs versus greater P-acquisition efficiency in simple CRs.