A great number of studies have been conducted on the role of Azotobacter - a free living N₂-fixer - in agricultural soils, but our information is lacking on the importance of Azotobacter in pasture soils. This research was conducted to study the relationship between ecological and edaphical factors with the population of the bacteria in soil. Soil samples were taken from two sites in Azarbaidjan (Mishou-dagh and Khadjeh), having different ecology, soils and vegetation types. We investigated also the effect of inoculation of rhizosphere with soil-isolated A. chroococcum on the growth of wheat (Triticum aestivum L. cv. Omid) during 56 days of growth in a pot experiment. The treatments included: 1) inoculation with A. chroococcum (+A), 2) N fertilizer at a rate of 35 mg/Kg as (NH₄)₂SO₄ (+N), 3) +A+N and 4) –A–N as control with four replications. Results showed that the population of Azotobacter chroococcum correlated positively with organic carbon and negatively with soil pH. In soils covered with vegetation the population of bacteria was higher than that in uncovered soils and the highest population of bacteria was found in soil samples collected from the rhizosphere of grass species and the lowest in that of legume species. In inoculation experiment, shoot and, particularly root growth, increased in the order of +N, +A and +A+N in comparison with control (–A–N) plants. These treatments also stimulate uptake and transport of K into shoot. Interestingly, the effect of +A treatment was significantly higher than that of +N and +A+N treatments. The same results were obtained for nitrogen. A higher uptake and particularly transport of K into shoot was attributed to low molecular weight organic molecules produced in inoculated roots and transport K into shoot. Results showed that inoculation with Azotobacter not only enhanced growth and chlorophyll content, but also stimulated uptake and translocation of ions into shoot. The stimulatory impact of inoculation is most likely a general effect that could also be important in micronutrients translocation into shoot.

Keywords: Population of Azotobacter, Inoculation with Azotobacter, Nutrient uptake and transport, Pasture soils.

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