Embothrium coccineum (Proteaceae) increases the survival of the cushion plant Acaena integerrima (Rosaceae) and modifies bacterial communities in their rhizosphere

Background and Aims: Acaena integerrima and Embothrium coccineum are native plants colonizing volcanic deposits in southern South America. Both species can grow in poor nutrient availability, increasing their leaf nutrient status when growing together. Enhanced leaf P concentrations could be explained by cluster-root carboxylate exudation of E. coccineum; however higher leaf N in E. coccineum has an unknown origin and free-living N ₂ -fixing microbes could be involved. Methods : To test this possible mechanism, we conducted an experiment using volcanic material (pumice) and seedlings. We studied bacterial communities (16S rRNA and nif H) in the rhizosphere of E. coccineum and A. integerrima , both grown alone and together. To evaluate microbial communities, we used Polymerase Chain Reaction and Denaturing Gradient Gel Electrophoresis. We measured carboxylate-exudation, leaf nutrient concentrations (N, P) and leaf δ ¹⁵ N isotopic fractionation. Results : The survival and shoot growth of A. integerrima was increased when grown combined. E. coccineum showed higher N and P concentrations when grown together than alone. Carboxylate-exudation showed faster rates of citrate from both species, being malate detected only from E. coccineum. Isotopic fractionation showed values close to zero in E. coccineum , being indicative of a biologically fixed N. The rhizosphere of E. coccineum alone exhibited the most distinct presence of 16S rRNA and nifH. Conclusions : Our results show that the N ₂ -fixing bacterial communities and δ ¹⁵ N indicate reliance on diazotrophic activity in E. coccineum rhizosphere. Additionally, A. integerrima appears to benefit from this N fixation and shows the importance of carboxylate exudation as strategy on colonizing plant species.