Plant-microbe interactions in Bossiaea linophylla (Fabaceae) are altered by competition with the cluster-rooted species Banksia attenuata (Proteaceae) in severely phosphorus-impoverished soils of southwestern Australia

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Extreme phosphorus (P) limitation is thought to intensify plantcompetition. However, in severely P-impoverished soils ofsouthwestern Australia, cluster-rooted Banksia attenuata (Proteaceae)can facilitate P acquisition of neighbouring species by mobilisingtightly bound P from soil particles. Whether such facilitation extendsto native symbiosis-dependent Bossiaea linophylla (Fabaceae) andwhether such plant-plant interactions infl uence the target plant’sassociation with microbial communities remains unclear. Weconducted a multifactorial glasshouse experiment to characteriseplant growth, root morphological traits, and symbiotic associations inB. linophylla grown either in monoculture or in mixed culture with B.attenuata in pasteurised and inoculated soil from their natural habitat.The biomass of B. linophylla was signifi cantly greater in monoculturethan in mixed culture with B. attenuata, indicating an interspecifi ccompetition. In monoculture, the greater biomass of B. linophylla wasobserved in the inoculated soil than in the pasteurised soil, suggestingthat the microbial community benefi ted plant growth. In the mixedculture, the competitor, B. attenuata, suppressed some benefi cialmicrobial associations with B. linophylla, specifi cally, N2-fi xingsymbiotic bacteria, as refl ected by markedly reduced root nodulenumbers. Rather than facilitation, B. attenuata exerted competitivepressure on B. linophylla, primarily through competition for P.Moreover, this competitive relationship infl uences the interactionbetween B. linophylla and some soil microbes, such as N2-fi xingbacteria. These fi ndings highlight that plant-plant interactions canmodulate plant-microbe relationships in extremely P-impoverishedsoils.