Associative N2-fixation in pearl millet and sorghum: Levels and response to inoculation

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Abstract
A study was conducted to assess the agronomic importance of associative N2-fixation in pearl millet (Pennisetum glaucum (L.) R. Br.) and sorghum (Sorghum bicolor (L.) Moench). To achieve this objective, the rate of N2-fixing activity was measured by comparing the magnitude of N2-fixation between pearl millet or sorghum, and pigeonpea (Cajanus cajan (L.) Millsp), chickpea (Cicer arietinum L.), or groundnut (Arachis hypogaea L.) and by examining the response of the crops to inoculation with N2-fixing bacteria. The overall mean nitrogenase activity (C2H2 reduction) in pearl millet throughout the growing period was less than 0.1% of that in pigeonpea and the activity in sorghum was only 1.3% of that in chickpea. Whether assessed by the determination of the nitrogen (N) balance in pots, using pigeonpea as reference, or by the measurement of the natural abundance of 15N (δ15N) using groundnut as reference, N gain by these cereals, even when they were inoculated with N2-fixing bacteria was not observed. Inoculation trials were conducted in pots and in the field with Azospirillum lipoferum and Azotobacter chroococcum. Only one trial with sterilized soil in pot showed a significant response to inoculation. The results obtained indicate that associative N2-fixing systems in pearl millet and sorghum do not exert an agronomically significant impact through N input on the production of these crops at present.