Influence of various soil factors on nitrogen fixation by Azospirillum spp

The influence of soil pH, redox potential and added organic matter on N₂-fixation by Azospirillum was studied. Application of rice straw to alluvial, laterite and acid-sulphate Pokkali soils under submerged conditions enhanced the population of N₂-fixing Azospirillum spp. An acid-sulphate saline soil of extremely low pH (3.2) harboured Azospirillum spp with appreciable N₂-fixing activity. Enrichment cultures originating from soils with low pH (<4.0) possessed lower N₂-fixing activity compared to cultures from soils with higher pH values (upto 6.6). Azospirillum cultures from soils that had undergone prolonged waterlogging showed lower N₂-fixing activity than cultures isolated from soils submerged for a few days. A relationship was shown between the in vitro N₂-fixing activity of Azospirillum cultures and the redox status of the soil samples; activity was high when the soil redox potential was between ?50 to ?150mV. The results show that the N₂-fixing activity of Azospirillum cultures is governed by fluctuations in soil redox potential, pH and organic matter.