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 plant competition. However, in severely P-impoverished soils of southwestern Australia, cluster-rooted Banksia attenuata (Proteaceae)can facilitate P acquisition of neighbouring species by mobilising tightly bound P from soil particles. Whether such facilitation extends to native symbiosis-dependent Bossiaea linophylla (Fabaceae) and whether such plant-plant interactions influence the target plant’s association with microbial communities remains unclear. We conducted a multifactorial glasshouse experiment to characterise plant growth, root morphological traits, and symbiotic associations in B. 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 significantly greater in monoculture than in mixed culture with B. attenuata, indicating an interspecific competition. In monoculture, the greater biomass of B. linophylla was observed in the inoculated soil than in the pasteurised soil, suggesting that the microbial community benefited plant growth. In the mixed culture, the competitor, B. attenuata, suppressed some beneficial microbial associations with B. linophylla, specifically, N2-fixing symbiotic bacteria, as reflected by markedly reduced root nodule numbers. Rather than facilitation, B. attenuata exerted competitive pressure on B. linophylla, primarily through competition for P. Moreover, this competitive relationship influences the interaction between B. linophylla and some soil microbes, such as N2-fixing bacteria. These findings highlight that plant-plant interactions can modulate plant-microbe relationships in extremely P-impoverished soils.