ABSTRACT
High-N2-fixing activities of Frankiapopulations in root nodules on Alnus glutinosa improve growth performance of the host plant. Therefore, the establishment of active, nodule-forming populations of Frankia in soil is desirable. In this study, we inoculated Frankia strains of Alnus host infection groups I, IIIa, and IV into soil already harboring indigenous populations of infection groups (IIIa, IIIb, and IV). Then we amended parts of the inoculated soil with leaf litter of A. glutinosa and kept these parts of soil without host plants for several weeks until they were spiked with [15N]NO3 and planted with seedlings ofA. glutinosa. After 4 months of growth, we analyzed plants for growth performance, nodule formation, specificFrankia populations in root nodules, and N2fixation rates. The results revealed that introducedFrankia strains incubated in soil for several weeks in the absence of plants remained infective and competitive for nodulation with the indigenous Frankia populations of the soil. Inoculation into and incubation in soil without host plants generally supported subsequent plant growth performance and increased the percentage of nitrogen acquired by the host plants through N2 fixation from 33% on noninoculated, nonamended soils to 78% on inoculated, amended soils. Introduced Frankiastrains representing Alnus host infection groups IIIa and IV competed with indigenous Frankia populations, whereas frankiae of group I were not found in any nodules. When grown in noninoculated, nonamended soil, A. glutinosa plants harbored Frankia populations of only group IIIa in root nodules. This group was reduced to 32% ± 23% (standard deviation) of the Frankia nodule populations when plants were grown in inoculated, nonamended soil. Under these conditions, the introducedFrankia strain of group IV was established in 51% ± 20% of the nodules. Leaf litter amendment during the initial incubation in soil without plants promoted nodulation by frankiae of group IV in both inoculated and noninoculated treatments. Grown in inoculated, amended soils, plants had significantly lower numbers of nodules infected by group IIIa (8% ± 6%) than by group IV (81% ± 11%). On plants grown in noninoculated, amended soil, the originalFrankia root nodule population represented by group IIIa of the noninoculated, nonamended soil was entirely exchanged by aFrankia population belonging to group IV. The quantification of N2 fixation rates by 15N dilution revealed that both the indigenous and the inoculatedFrankia populations of group IV had a higher specific N2-fixing capacity than populations belonging to group IIIa under the conditions applied. These results show that through inoculation or leaf litter amendment, Frankiapopulations with high specific N2-fixing capacities can be established in soils. These populations remain infective on their host plants, successfully compete for nodule formation with other indigenous or inoculated Frankia populations, and thereby increase plant growth performance.
Growth performance of sandalwood during nursery stage as affected by different host plants
