Dinitrogen fixation (acetylene reduction) associated with forage grasses in Texas : enumeration, characterization and inoculation studies of bacterial isolates

Abstract

The most generally limiting nutrient to growth of forage grasses is fixed nitrogen in the form of No3- or NH4+. Dinitrogen fixation by bacteria associated with forage grass roots in a system which would provide plants with fixed nitrogen would be of great economic importance because the cost of nitrogen fertilizers has escalated over the past 7 years. The following experiments were conducted to evaluate some of the ecological aspects of the grass-bacteria association and to re-establish the association between forage grass roots and diazotrophic bacteria cultured from roots of grasses growing in Texas soils. The effect of fixed nitrogen on nitrogenase activity of a grass-bacteria association was studied. Panicum coloratum L. seedlings were inoculated with the diazotrophic bacterium Azotobacter and fed with nutrient solutions containing various levels of fixed nitrogen. Acetylene reduction (AR) activity on root systems was used to determine nitrogenase activity by associated diazotrophic bacteria. A high level of fixed nitrogen suppressed AR activity, but after a change from a high to an intermediate level, AR activity increased significantly. Ecological aspects of the natural association of diazotrophic bacteria with grass roots of forage grasses, classifying the bacteria, and correlating population counts with AR rates of soil-root cores. Diazotrophic Enterobacter cloacae and Klebsiella pneumoniae were consistently found associated with a wide variety of forage grass roots collected from moist sites in unfertilized pastures in southeast Texas. Three other bacterial species were cultured one time each from three different plant root systems. There was a lack of linear correlation between AR activity and number of diazotrophic bacteria/g dry root. Re-establishment of the diazotrophic bacteria-grass root system for supplying grasses with fixed nitrogen was studied under greenhouse conditions. Cynodon dactylon (L.) Pers., Pancium coloratum L., and Paspalum dilatatum Poir. Seedlings were inoculated in quadruplicate with 18 diazotrophic bacteria which were either E. cloacae or K. pneumoniae...