The influence of a cluster‐rooted species on <i>Bossiaea linophylla</i> (Fabaceae) under extremely phosphorus‐impoverished conditions: Phosphorus competition and altered plant–microbe interactions

Abstract Phosphorus (P) limitation may intensify plant competition. However, in severely P‐impoverished soils of south‐western Australia, cluster‐rooted Banksia attenuata (Proteaceae) can facilitate P acquisition of neighbouring species by mobilising tightly bound P from soil particles. It remains unclear whether such facilitation also occurs in Bossiaea linophylla (Fabaceae), a native and symbiosis‐dependent species that naturally coexists with B. attenuata , and whether these plant–plant interactions influence its association with arbuscular mycorrhizal (AM) fungi and N 2 ‐fixing symbiotic bacteria. We conducted a multifactorial glasshouse experiment to characterise plant growth, root morphological traits and symbiotic associations with AM fungi and rhizobia at the sapling stage in B. linophylla grown either in monoculture or in mixed culture with B. attenuata in pasteurised soil or in pasteurised soil inoculated with soil from their natural habitat. The total biomass of B. linophylla saplings was significantly greater in monoculture than in mixed culture with B. attenuata , indicating interspecific competition. In monoculture, we observed greater total biomass of B. linophylla in inoculated soil than in pasteurised soil, suggesting that the native soil inoculum benefited plant growth. In the mixed culture, the competitor, B. attenuata , suppressed some beneficial microbial associations with B. linophylla , specifically, rhizobia, as reflected by markedly reduced root nodule numbers. Rather than facilitation, B. attenuata exerted competitive pressure on B. linophylla at the sapling stage, primarily through competition for P. Moreover, this competitive relationship influenced the interaction between B. linophylla and rhizobia. These findings highlight that plant–plant interactions can modulate plant‐microbe relationships in extremely P‐impoverished soils. Read the free Plain Language Summary for this article on the Journal blog.