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Soil salinization is a major problem in irrigated agriculture. A field study was conducted in the Sariosiyo district in the Surkhandarya region of southeast Uzbekistan to evaluate soil nitrogenase activity and nitrogen-fixing bacteria populations in Saline serozem soils under wheat, maize, and alfalfa, as well as from adjacent fallow land. Composite soil samples were randomly collected from depths of 0-10, 10-20, and 20-30 cm in autumn, winter, spring, and summer, which were then 2-mm sieved and subjected to selected microbial population analysis and enzymatic studies. The results showed that alfalfa contributed both to high nitrogenase activity and to a large nitrogen-fixing bacteria population. The fallow soil had a lower nitrogenase activity and bacterial population. The nitrogenase activity was higher in the soil from a depth of 10-20 cm in spring. Results suggest that cropping, especially suitable crop rotation, is essential to support greater microbial biomass population and nitrogenase activity for improving the biological fertility of saline and nitrogen-poor calcareous and soils.