Enumeration and identification of nitrogen-fixing bacteria from forage grass roots

Root-soil cores were collected from forage grasses growing in a subtropical region of Texas and tested for acetylene reduction activity. The population density of nitrogen-fixing bacteria was measured, using various media and incubation conditions. Bacteria were confirmed as nitrogen fixing, using the acetylene reduction assay, and were classified according to standard biochemical and cultural methods. The majority of the nitrogen-fixing bacteria isolated from roots were Enterobacter cloacae or Klebsiella pneumoniae. Root-associated, nitrogen-fixing bacteria were isolated from 21 of 24 root-soil cores. The population densities of nitrogen-fixing bacteria ranged from approximately 10 to 3 x 10 per g of root. Population density on roots was significantly correlated with the rate of acetylene reduction but the relationship was not linear.