The number of Bradyrhizobium SP. ( Lupinus) applied to seed and its effect on rhizosphere colonization, nodulation and yield of lupin

Abstract

In field experiments we investigated the roles of inoculum potential of Bradyrhizobium sp. ( Lupinus) and quantity of peat used to inoculate seed on the nodulation and yield of lupins. Within the range 0.125–3 times the Australian recommended rate of peat application (2.5 g peat kg −1 lupin seed), the amount of peat had no effect on nodulation or grain yield. In the first experiment, seven inoculum potentials were applied within the range log 10 0.32–6.28 bradyrhizobia per seed in 7, 10-fold increments which spanned the recommended rate of login 5.55 per seed. Inoculum potentials of log 10 6.27 and 5.27 improved the colonization of lupin rhizospheres and increased early nodulation, nodule number and nodule mass. Nodule mass was increased from 65 to 393 mg plant −1 at 43 days by increasing the inoculum from log 10 4.27 to 6.27 bradyrhizobia seed −1. Grain yield and % N in the grain were not significantly different ( P > 0.05) between potentials of logio 4.27 and 6.27. In the second experiment, higher potentials of 6.80 and 7.28 further improved rhizosphere colonization and increased nodule mass. Studies of the survival of the inoculum, during inoculation, sowing and in the soil, identified a large mortality factor; 95% of bacteria died between inoculation and sowing and of those surviving, 83% died after 22.5 h in the soil. These observations have important implications for setting new standards for commercial inoculants and for emphasizing the care needed in handling inoculated seed to reduce the death of bradyrhizobia in the period between inoculation and sowing.