Effect of different Alnus taxa on abundance and diversity of introduced and indigenous Frankia in soils and root nodules.

Abstract

The effect of host plants on the abundance and distribution of introduced and indigenous Frankia populations was assessed in soils and root nodules of four alder species, Alnus glutinosa,Alnus cordata,Alnus rubra and Alnus viridis. Plants were grown in microcosms with either a sandy soil without detectable frankiae, with or without inoculation of a mixture of Frankia isolates, or a silty clay loam soil with indigenous Frankia. The presence of frankiae in soils increased plant height and root nodule formation, with significant increases in the presence of indigenous frankiae. Abundance in soils increased significantly for both introduced and indigenous Frankia populations independent of alder species, with generally largest increases in cluster 1b frankiae. Root nodules formed by introduced frankiae did not reflect the diversity of strains inoculated, with nodules generally only formed by strain ArI3 representing cluster 1a/d. All indigenous Frankia populations detected in soil were also found in A. glutinosa nodules, while A. cordata or A. rubra nodules contained different subsets of frankiae with unique abundances dependent on plant species. These results demonstrate the intrageneric differences of host plants in the selection of specific Frankia populations in soils for root nodule formation.