The effect of fluctuations of salinity in three different seasons on diazotrophic populations and N(2) fixation in six mono cropped rice field soils of the coastal region of the Gangetic delta of West Bengal, India, was studied. The average pH, ECe, organic carbon and total nitrogen of the soils ranged from 4.99-7.08, 2.02-19.58 dSm(-1), 4.68-12.03 g kg(-1) and 0.44-1.70 g kg (-1), respectively. The average log colony forming units of the bacterial populations and N(2)-fixation in the soils varied from 4.61 to 5.86 and 2.74 to 4.52 mg N(2) fixed 50 ml (-1) culture media respectively, with the lowest value recorded in summer. Recovery of microorganisms and N(2)- fixation gradually decreased with extraneous addition of NaCl in the culture media. All the eight isolates were Gram positive, spore and capsule formers. They could utilize glucose, sucrose, mannitol, starch, citrate and nitrate, and were catalase and gelatinase positive, but indole, methyl red and Vogues Proskauer reaction negative. The organisms produced alkaline reaction on TSI agar slant. The acetylene reduction assay of the isolates at 0 and 1% NaCl in the culture media were 4.51-164.52 and 1.72-100.6 nmole C(2)H(4) ml(-1) culture media in 72 h, respectively. The isolates could fix 2.42-4.45 and 2.04-4.08 mg N(2) fixed 50 ml(-1) culture media at 0 and 1% NaCl in the culture media respectively. 16S rDNA sequences of the isolates were similar to the species: Bacillus sp. isolate 28A, Bacillus sp. MOLA 87, Bacillus sp. By113 (B)Ydz-dh, Bacillus sp. PN13, Bacillus licheniformis strain RH101, Bacterium Antarctica 14, Bacillus sp. PN13 and Bacillus megaterium.
Keywords: Coastal arable soils; DNA sequencing; Diazotrophs; Dinitrogen fixation; Salinity.