Effect of hydrogen uptake (Hup) characteristics on survival of Bradyrhizobium japonicum (Buchasan 1980) comb. nov. in flooded soil

A pot experiment using soybean rhizobia free soil was conducted to determine the effect of hydrogen uptake characteristics on the symbiotic performance and survival of Bradyrhizobium japonicum in soybean-upland rice-soybean and soybean-lowland rice-soybean cropping sequence. The survival of Hup(+) and Hup(-) strains in non-flooded and flooded soil was also studied in unamended soil incubated at 30 deg C in laboratory. This study showed that hydrogen uptake positive [Hup(+)] strains were not superior to hydrogen uptake negative [Hup(-)] strains in terms of growth, nodulation, nitrogen fixation (acetylene reduction activity), and nitrogen uptake. During laboratory incubation Hup(-) strains did not survive flooded condition after three months of incubation. The population decreased from 10 to the eight power cells to less than 10 cells/g of soil at three months of incubation. In contrast Hup(+) strains maintained a population greater than 1 x 10 to the fourth power cells/g of soil until three months of incubation. In the pot experiment both Hup(+) and Hup(-) strains had a population of 1 x 10 to the fourth power cells/g power of soil after first soybean crop. During lowland rice culture following soybean, the population of Hup(-) Bradyrhizobium in the non rhizosphere soil decreased to less than 10 cells/g of soil. However, Hup(-) strains could survive on the surface of root and/or rhizosphere soil with a population of 1 x 10 to the third power cells/g of soil. After rice harvest the population of rhizobia was more than 100 cells/g of soil due to its survival in the rhizosphere of the rice plant. This number of rhizobia in soil brought effective nodulation of soybean following the rice crop. The population of rhizobia in the soil to 1 x 10 to the third power cells/g of soil at soybean harvest. Hup(+) rhizobia maintained a constant population of 1 x 10 to the fourth power cells/g soil throughout the rice crop under lowland as well as upland condition. There was no difference between the survival of Hup(+) and Hup(-) strains under upland condition