A survey was carried out for two years in some selected farms at Al-Kharj, Saudi Arabia to assess the status of the nodules under field conditions by Sinorhizobium meliloti on alfalfa (Medicago sativa L.) plants and monitoring the introduced S. meliloti strains activity under Saudi soils conditions. The samples were collected at regular seasonal intervals from the selected areas. The microbiological examination included the determination of total nodules number as well as the effectiveness of N2-fixation. Results revealed that, soils in the selected areas in Saudi Arabia have sufficient bacteria of the proper kind to nodulate the alfalfa plants. However, these nodules are high in number, small in size, white in color and proven to be ineffective in nitrogen fixation in the most farms. Inoculation of alfalfa seeds with imported S. meliloti strains in Saudi Arabia failed to improve the plant’s ability to fix its need from the atmospheric nitrogen. The imported strains were always over competing with the native strains in Saudi soils and did not survive the long hot and dry summer season. Seasonal variation of nodulation was observed and it was found that summer season severely affected the nodulation to nearly nil. This reducing effect on nodules number exert a very slow recovery in the second year. Nitrogenase activity was always very low in all the collected samples in both inoculated and non-inoculated farms, which indicates that the ability of fixing nitrogen by S. meliloti strains under Saudi soils conditions is very low.
Alfalfa, Sinorhizobium meliloti, nodules count, N2-ase activity, seasonal variation
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