Abstract:
Soil microbes play a key role in ecosystem functioning by being part of numerous biogeochemical
cycles and organic matter decomposition. For this reason, microbial inoculants could contribute
immensely to the sustainable agriculture as natural nutrient mobilizers. In this study, we isolated
and characterized nitrogen-fixing bacteria extracted from soil to investigate their potential to be
used as bioinoculants to improve crop growth and development. Fifteen isolates of bacteria were
isolated from the Kekanadura Forest Reserve in Matara, Sri Lanka (5°98'51.79"N-80°59'12.90"E).
Their N-fixing potential was confirmed by the widely used peptone water qualitative assay.
Agriculturally important traits such as phosphate solubilization, synthesis of indole-3-aceti acid
(IAA) and antagonisms against plant pathogen, Sclerotium rolfsii were also tested. Phosphate
solubilization was tested by growing the bacteria in Pikovskayas agar medium, which contains
calcium phosphate as the insoluble phosphate source. Strain DzpC, DzpE, DzpF, DzpQ and Azp1
showed clear zone surrounding bacterial growth indicating solubilization of phosphate in the
medium. The IAA production was tested spectrophotometrically in broth cultures supplemented
with tryptophan as the precursor molecule. Five strains, DzpC, DzpE, DzpF, DzpI and DzpQ
produced IAA in the range of 9.9-64.1 mg/mL. The antagonistic potential of strains was tested in
dual culture plate assay. Thirteen strains showed growth inhibition of S. rolfsii at varying degrees.
Strain DzpC, DzpP and Azp1 showed the strongest antagonistic activity. When overall results were
considered, DzpC, DzpE, DzpF, DzpI and DzpQ were the best performing strains. Bacteria were
identified by their colony morphology and sequencing of 16S rRNA gene. Seven out of fifteen
strains were morphologically similar to actinomycetes. Four strains were identified as Bacillus
toyonensis, Lysinibacillus macroides, Rhizobium tropici and Rhizobium sp. by DNA sequencing.
Since actinomycetes and endospore-forming bacteria such as Bacillus and Lysinibacllus are
generally resistant to adverse environmental conditions, potential application of majority of the
strains isolated in this study for commercial formulation in crop cultivation is high. Furthermore,
since bacterial strains were isolated from the native soil, they are expected to have better
adaptability to Sri Lankan soil compared to exotic strains.
