Cyanobacterial collections (eleven strains) have been used to screen their growth and nitrogen fixing abilities in both sterile and non-sterile soils under different incubation periods. The effects of cyanobacterial exudates on germination of rice grains were also investigated. The maximum growth of cyanobacteria represented by their biomass was recorded for Nostoc rivulare and Nostoc microscopicum after 60 day old cultures in sterile soil. However, in non-sterile soil, the greatest biomass was detected for Calothrix brevissima, Nostoc microscopicum; and Nostoc carneum at the same incubation period. The most potent strains for nitrogen fixation in sterile soil were Anabaena variabilis, Nostoc spongiaeforme andNostoc linckia after 90 days of incubation. Meanwhile, Nostoc muscorum and Calothrix brevissima recorded the highest values of nitrogen fixation in non-sterile soil at the same incubation period. Pretreatment of rice grain with cyanobacterial exudates of Nostoc linckia, Nostoc muscorum and Anabaena flos-aquae stimulated the germination by 25, 14 and 10%, respectively. The results revealed that nitrogen-fixing abilities and rice grain germination appeared to be strain-specific. Moreover, the persistence of Nostoc muscorum and Calothrix brevissima in non-sterile soil for 90 days with a maximum activities indicate that both of them can be used as non-indigenous strains.
cyanobacterial biomass, cyanobacteria, germination, nitrogen fixation, rice grain