The black alder - Alnus glutinosa - lives in symbiosis with bacteria from the actinomycetes group. These are capable of withdrawing nitrogen from the atmosphere. Through this partnership, the alder, despite its relatively high nutritional requirements, can colonize nitrogen-poor locations.
The bacteria penetrate the root through the root hairs. The plant reacts with changes in the tissue to compartmentalize the microorganisms, root nodules are growing. The individual nodules are formed from short, thickened and forked branched roots and grow, like coral, to the size of an apple. The bacteria withdraw carbohydrates from their host and produce herewith, together with nitrogen, amino acids and proteins, which are then available to the alder. The degree of nitrogen fixation depends on the availability of nitrogen in the soil. In nitrogen-poor places, significantly more root nodules are formed and more nitrogen is bound than in eutrophic soils. The nitrogen gained by symbiosis benefits not only the symbiotic partners. Black alder leaves are rich in nitrogen (up to 3% of dry matter) The trees permit themselves the luxury to let green leaves fall in autumn, with a hardly lower nitrogen content. Through the easily degradable layer of fallen leaves - the C / N ratio of the alder lies at about 15 - a lot of nitrogen gets into the soil, in intact alder forests more than 70 kg per hectare per year.
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