Associative N2-fixation in pearl millet and sorghum: levels and response to inoculation. K KLeeauthorS PWaniauthorTYoneyamaauthorNTrimurtuluauthorRHarikrishnanauthorN2-fixation was compared in pearl millet (Pennisetum glaucum), sorghum, pigeonpeas, chickpeas, and groundnuts and the responses of the two cereal crops to inoculation with N2-fixing bacteria was examined. The overall mean nitrogenase activity (C2H2 reduction) in pearl millet throughout the growing period was less than 0.1% of that in pigeonpeas and the activity in sorghum was only 1.3% of that in chickpeas. Whether assessed by the determination of the N balance in pots, using pigeonpeas as reference, or by the measurement of the natural abundance of 15N (δ15N) using groundnuts 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 pots showed a significant response to inoculation. It is concluded 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. MilletsSorghum1994Japanese Society of Soil Science and Plant Nutrition Article