The effects of the M. malabathricum rhizosphere
In this study, we investigated the diversity of diazotrophic
bacteria in the habitat around the root of M. malabathricum
in three soil types. Several plant species are known to release
mucilage composed of detached cells and root exudates, and
containing carbohydrates, organic acids, amino acids, vitamins
and other compounds, into rhizosphere soil. Up to 20%
of photosynthetically fixed carbon was released into the soil
during the vegetative period of spring wheat (15). The
amount and/or composition of root exudates released into the
rhizosphere varies with plant species, growth stage and other
environmental factors (15, 16). In hydroponic culture, M.
malabathricum is one plant species that can exude large
amounts of mucilage from its roots (37). It is also known that
the activity and biomass of microorganisms are greater in
rhizosphere soil than in bulk soil. Bacterial densities, metabolic
signatures and genetic structures are also modified
by the addition of artificial root exudates to bulk soil (4).
Diazotrophic bacteria, which utilize root exudates for their
growth, might accumulate in the rhizosphere of plants grown
in low-nutrient conditions (11). However, in this study, the
pattern of distribution of the nifH gene did not differ significantly
between the rhizosphere and bulk soils in peat and
sandy clay. There was a large difference in diazotrophic bacterial
diversity with soil type. In peat and sandy clay soils, H
indicated that the diversity of nifH was higher in rhizosphere
than in bulk soil (Table 3). It seems that root exudates did not
affect the accumulation of nifH diversity in the rhizosphere.
In contrast, in acid sulfate soil, H was lower in the
rhizosphere than bulk soil (Table 3). These results indicated
that particular bacterial species might accumulate in the
rhizosphere of M. malabathricum in acid sulfate soil.