Changes in the physiological and agricultural characteristics of pest-based Bradyrhizobium japonicum inoculants after long-term storage

Commercial soybean inoculants processed with sterilised peat and stored at 20 degrees C for 1-8 years were used as experimental materials to assess th e changes in the physiological activity of Bradyrhizobium japonicum after s torage. Viable counts decreased and physiological characteristics of the ba cterium changed during storage, with an increase in the time taken for colo ny appearance on a medium without yeast extract, an increase in the lag tim e for nodule appearance on soybean grown in glass tubes and a decrease in s urvival on seeds. All the inoculants produced a significant increase in gra in yield in a field experiment. The percentage of efficient cells in the fi eld (relative to the plate counts) decreased as the length of storage incre ased. These results suggest that the physiological activity of B. japonicum cells changes after storage. Practical implications for inoculant quality control are discussed.