Effect of Inoculation and Leaf Litter Amendment on Establishment of Nodule-Forming
Frankia
Populations in Soil
Authors: Nickel Anja , Pelz Oliver , Hahn Dittmar , Saurer Matthias , Siegwolf Rolf , Zeyer Josef 
Published: July 2002

High-N
2
-fixing activities of
Frankia
populations 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 [
15
N]NO
3
and planted with seedlings of
A. glutinosa
. After 4 months of growth, we analyzed plants for growth performance, nodule formation, specific
Frankia
populations in root nodules, and N
2
fixation rates. The results revealed that introduced
Frankia
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 N
2
fixation from 33% on noninoculated, nonamended soils to 78% on inoculated, amended soils. Introduced
Frankia
strains 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 introduced
Frankia
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 original
Frankia
root nodule population represented by group IIIa of the noninoculated, nonamended soil was entirely exchanged by a
Frankia
population belonging to group IV. The quantification of N
2
fixation rates by
15
N dilution revealed that both the indigenous and the inoculated
Frankia
populations of group IV had a higher specific N
2
-fixing capacity than populations belonging to group IIIa under the conditions applied. These results show that through inoculation or leaf litter amendment,
Frankia
populations with high specific N
2
-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.