Diversity and Salinity Tolerance of Indigenous Bradyrhizobium japonicum Isolates Recovered from Saline Areas

Soil salinity can restrict soybean production and the activity of beneficial nitrogen-fixing rhizobia. This study investigated the recovery, phenotypic characteristics and in vitro salinity tolerance of indigenous Bradyrhizobium japonicum isolates from salt-affected soybean-growing areas of Sangli and Kolhapur districts, Maharashtra, India. Twenty-five soybean root nodule and rhizosphere soil samples were processed, yielding 15 isolates, equivalent to a recovery frequency of 60%. Four selected isolates (Iso-7, Iso-9, Iso-11 and Iso-21) were screened on yeast extract mannitol agar containing 2–10% NaCl. The highest NaCl concentrations supporting growth were 8% for Iso-7, 6% for Iso-21, 4% for Iso-9 and 2% for Iso-11. None grew at 10% NaCl. Only Iso-7 exhibited detectable phosphate solubilisation, producing a 4.5 mm halo, while potassium solubilisation was not detected in any of the four isolates. All four produced hydrogen cyanide; ammonia production was strong in Iso-7, weak in Iso-21 and undetectable in Iso-9 and Iso-11. All four isolates tested positive for catalase, oxidase and starch hydrolysis and negative for indole production, gelatin liquefaction and methyl red reactions. Nitrate reduction varied among isolates. Their cells were Gram-negative and rod-shaped. These findings identify Iso-7 as the most salt-tolerant of the selected isolates and a candidate for subsequent confirmation of identity, symbiotic performance and evaluation under saline growing conditions.