Tripartite symbioses regulate plant-soil feedback in alder

• Plant-soil feedbacks regulate plant productivity and diversity, but potential mechanisms underpinning such feedbacks, such as the allocation of recent plant assimilate, remain largely untested especially for plants forming tripartite symbioses.    • We tested how soils from under alder (Alnus glutinosa) and beneath other species of the same and different families affected alder growth and nutrition, and colonisation of roots by nitrogen-fixing Frankia bacteria and ectomycorrhizal fungi. We also measured how the soil environment affected carbon capture and allocation by pulse-labelling seedlings with 13CO2. We then tested for linkages between foliar nutrient stoichiometry and carbon capture and allocation and soil origin using statistical modelling approaches.    • Performance of alder and nitrogen nutrition were best on home and birch (Betula pendula) soils (both Betulaceae), while performance on Douglas fir (Pseudotsuga menziesii) (Pinaceae) soil was poor. Plants growing in P...