Tripartite symbioses regulate plant–soil feedback in alder

Plant–soil feedbacks regulate plant productivity and diversity, but potential mechanisms underpinning such feedbacks, as the allocation of recent assimilate, remain largely untested especially for plants forming tripartite symbioses. We tested how soils from under alder Alnus glutinosa beneath other species same different families affected growth nutrition, colonization roots by nitrogen-fixing Frankia bacteria ectomycorrhizal fungi. also measured soil environment carbon capture pulse labelling seedlings with 13CO2. then linkages between foliar nutrient stoichiometry origin using statistical modelling approaches. Performance nitrogen nutrition were best on home birch Betula pendula (both Betulaceae), whereas performance Douglas fir Pseudotsuga menziesii (Pinaceae) was poor. Plants growing in P. virtually devoid ectomycorrhizas, natural abundance 15N signatures leaves more enriched indicating distinct acquisition pathways. Seedlings these had smaller 13C fixation root rates, leading to respiration rates microbes. Statistical models showed that predictors N concentration fine net CO2 exchange mesocosms. The phosphorus mesocosms origin; tended have greater compared while no treatments. Foliar not correlated available or total any soils. Home resulted communities roots, which could be responsible concentration. Our findings show association relieved limitation seedling triggering a feedback loop. propose relief likely increases demand, may promote formation ectomycorrhizas nutrient-deficient symbioses therefore generates positive plant–soil enables acquire mineral nutrients otherwise inaccessible trade carbon. A free Plain Language Summary can found within Supporting Information this article.