Soil microbes and nutrient inputs influence root nodulation and tree performance in Alnus glutinosa

The symbiotic relationship between the nitrogen-fixing bacteria Frankia alni and the pioneer tree species Alnus glutinosa plays an important role for tree performance, helping trees thrive in nitrogen-poor environments. However, F. alni nodulation can vary greatly between different soils and this could be due to biotic and abiotic characteristics of the soil. Here we examine how microbial communities from young (∼10 years) and mature (≥100 years) forests and gradients of available nitrogen (N) and phosphorus (P) influence F. alni nodulation and tree performance. In mesocosm experiments, A. glutinosa seedlings were inoculated with bacteria, fungi, or both, cultured from young and mature forest soils, alongside F. alni. The impacts of N and P availability were examined through controlled nutrient manipulations. Results demonstrated that fungal communities from mature forests suppressed the growth-promoting effects of F. alni, although nodule biomass itself was not directly influenced. Further, we isolated and identified bacteria and fungi that were found to contribute to the observed inhibitory effects on F. alni-mediated growth promotion. Increased N availability significantly reduced nodule biomass, and we established a threshold at which reliance on symbiosis diminished. Conversely, P-addition stimulated nodulation and tree growth. These findings highlight the pivotal influence of N and P availability in determining the A. glutinosa-F. alni symbiosis, while also providing evidence that specific microbes in the soil influence these dynamics.