Experimental test of the hypothesis that nitrogen input inhibits growth and nodulation of Alnus glutinosa growing on soils in mining areas

Alder (Alnus glutinosa) is a nitrogen-fixing species commonly used in the reclamation of mining sites. Nitrogen supply is assumed to be beneficial for soil development, soil carbon storage and overall ecosystem recovery. However, earlier studies show that in old alder plantations, growth is slow. The experiment reported here was done to test the hypothesis that a surplus of nitrogen can suppress the nitrogen fixing activity of Frankia, which is the nitrogen-fixing symbiont of alder, resulting in a reduction in the growth of alder. Potted alder seedlings were grown in a greenhouse into two types of non-sterilised spoil heap soils from the Sokolov brown coal mining district in Czechia. Two different concentrations of ammonium nitrate were tested, which mimic the addition of 25 and 50 kg N.ha−1, together with a control in which no nitrogen was added. The addition of nitrogen to the soil significantly reduced both alder growth and the formation of Frankia tubers. This indicates that once there is sufficient nitrogen in soil, alder growth will be suppressed even in the absence of competition from other plants, which has implications for using alder in reclamation.