ASSOCIATIVE N2-FIXATION IN PEARL-MILLET AND SORGHUM - LEVELS AND RESPONSE TO INOCULATION

A study was conducted to assess the agronomic importance of associativ e N2-fixation in pearl millet (Pennisetum glaucum (L.) R. Br.) and sor ghum (Sorghum bicolor (L.) Moench). To achieve this objective, the rat e 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 hy pogaea L.) and by examining the response of the crops to inoculation w ith N2-fixing bacteria. The overall mean nitrogenase activity (C2H2 re duction) 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 nitroge n (N) balance in pots, using pigeonpea as reference, or by the measure ment of the natural abundance of N-15 (delta N-15) using groundnut as reference, N gain by these cereals, even when they were inoculated wit h N2-fixing bacteria was not observed. Inoculation trials were conduct ed in pots and in the field with Azospirillum lipoferum and Azotobacte r chroococcum. Only one trial with sterilized soil in pot showed a sig nificant response to inoculation. The results obtained indicate that a ssociative 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.