| Increasing evidence has been accumulating supporting significant grass-diazotrophic associations. However, characterization of these associations in hot desert grasses is lacking.; Nitrogenase activity was measured in excised roots amended with glucose. Grass roots had up to 18-fold greater nitrogenase activity than soil cores. This confirms association between N{dollar}sb2{dollar}-fixing bacteria and the plant species. The C{dollar}sb4{dollar} perennial grasses tested had greater nitrogenase activity than the C{dollar}sb3{dollar} annual herb, Baileya multiradiata, desert marigold. Nitrogenase activity was highly variable among plant species and within a species. Higher rates of nitrogenase activity were associated with higher soil water contents. The phenophase of the plant species affected nitrogenase activity. Nitrogenase activity was greatest during vegetative and flowering phases of the grass phenology. Nitrogenase activity was higher during the hot wet season than during the cold season or warm dry season suggesting that climatic conditions or plant growth affected nitrogen fixation activity.; Nitrogen fixation associated with intact plant-soil systems was studied in the greenhouse. There were no significant differences in acetylene reduction among grasses. Plant phenology had a significant effect on nitrogenase activity. The highest nitrogenase activity was observed during vegetative growth. The study confirmed that N{dollar}sb2{dollar} is fixed in soil and in the rhizosphere of grasses tested.; Inoculation studies were conducted on the C{dollar}sb4{dollar} perennial grass, Sporobolus cryptandrus, under greenhouse conditions. In the first experiment, inoculated plants had greater height, more tillers, longer flag leaf length, greater root length, more biomass, greater nitrogenase activities, and higher N content than control plants. The nitrogenase activity was greater during the vegetative phase than reproductive phases of the grass. In the second experiment, clipping after inoculation had similar significant effects on plant parameters and N content. However, nitrogenase activity and N concentration in plants and soil were lower than in the first experiment. The mixed culture treatment had the major positive effect on the grass in both experiments.; The results indicated that associative N{dollar}sb2{dollar} fixation may be ecologically important in N{dollar}sb2{dollar} fixation and cycling in the Chihuahuan Desert. |
