Conclusion
This study revealed the diversity of the nifH gene associated
with M. malabathricum roots in three soil types. Our
results suggested that the diversity and phylogenetic composition
of nifH were dependent on soil properties, and not
remarkably different between rhizosphere and bulk soils in
peat and sandy clay. However, in acid sulfate soil the diversity
of the nifH gene shifted between rhizosphere and bulk
soils. The nifH gene sequences were amplified not from
RNA but from DNA extracted directly from soil, so the
functions and expression of nifH in soil could not be determined.
Furthermore, this information does not clarify the
role of these diazotrophic bacteria in the supply of N to M.
malabathricum. Diazotrophic bacteria may accumulate,
depending on their environment, and their role in the acquisition
of N for M. malabathricum may also differ among soil
types. Furthermore, the co-culture of diazotrophic bacteria and
non-diazotrophic bacteria was reported to enhance or induce
nitrogen-fixing activity (21, 29). So we need to pay attention
to microbial interactions between diazotrophic bacteria
and other indigenous bacteria in the rhizosphere of M.
malabathricum. Further study of the role of diazotrophic
bacteria in the rhizosphere of M. malabathricum is progressing.
It is also necessary to clarify the relationship between
nifH gene expression and root exudates, and to compare the
diversity of the nifH gene in the rhizosphere of different plant
species.