Nitrogen fixers in the rhizosphere of certain desert plants

Six species of common desert range plants; Achillea sp., Aristida plumosa, Artemisia herba-alba, Haloxylon articulatum, and Heliotropium ramosissimum were colleted from Western Desert in Iraq. Counts of Azotobacter spp. and total bacteria were estimated in both rhizosphere and non-rhizosphere soils. Biomass and total nitrogen content of the plants were determined. The R/N ratio (ratio between rhizosphere to non-rhizosphere) for Azotobacter sp. ranged between 1 to 19.5 with an average of 4.96. The number of Azotobacter in the rhizosphere ranged between 6.8 X 10(3) to 47 X 10(3) cells/g, while in the non-rhizosphere it ranged from 2 X 10(3) to 21 X 10(3). Aristida plumosa harboured the highest number, i.e., 47 X 10(3) cells/g, while Artemisia herba-alba showed the least, i.e., 6.8 X 10(3) cells/g. Total counts of bacteria in the rhizosphere ranged between 17.2 X 10(6) and 97 X 10(6) cells/g, that of the non-rhizosphere between 1.5 X 10(6) and 18 X 10(6) cells/g. The R/N ratio ranged between 2.03 X 22.5 with an average of 7.28. An appreciable gain of dry weight and total nitrogen was observed. Between 13 and 152 g with an average of 78.9 g dry weight and between 347 and 6684.0 mg with an average of 2452.0 mg nitrogen per plant were found. The possible contribution of Azotobacter to the nitrogen economy of the plants is discussed and further suggested.