Pasteurisation of soil from an extremely phosphorus-impoverished environment increases phosphorus availability and enhances the facilitative effect of cluster-rooted species on Agonis flexuosa (Myrtaceae)

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Soil pasteurisation is commonly used to remove biotic factors that influence plant growth, but it can unavoidably affect some soil abiotic properties. These abiotic changes are often overlooked in extremely phosphorus (P)-impoverished soils, where even slight increases in P availability can influence both plant growth and interspecific interactions. Therefore, we aimed to determine whether pasteurisation of extremely P-impoverished soil affects the growth of Agonis flexuosa and its interaction with Banksia attenuata. In a controlled glasshouse experiment, we evaluated plant growth, root morphological and physiological traits of target species A. flexuosa grown either in monoculture or mixed culture with B. attenuata, in a pasteurised soil treatment (PST) and an inoculated soil treatment (IST), where one-third of the pasteurised soil was replaced with unpasteurised field soil. Soil resin P concentration was 20% higher in PST than in IST. In PST compared with IST, A. flexuosa had two-fold higher leaf P concentration, greater total biomass, developed thicker roots with lower tissue density, and showed higher rhizosheath carboxylate concentrations. Under PST, the total biomass of A. flexuosa was greater when grown with B. attenuata, accompanied by lower rhizosheath acid phosphatase activity than in the absence of B. attenuata. Under IST, the biomass of A. flexuosa was indistinguishable in the presence or absence of B. attenuata. Moreover, in the presence of Banksia, its biomass under PST was only 49% of that under IST. Pasteurisation of soil from extremely P-impoverished soils increased P availability, leading A. flexuosa root traits more towards nutrient absorption and enhancing its facilitation by B. attenuata. These effects were likely due to the release of P from microbial biomass and lower competition for P from microbes. These results demonstrate that attention should be paid to changes in soil nutrient availability due to pasteurisation to avoid misattributing plant responses to other factors.