ISOLATION AND NITROGENASE ACTIVITY OF BUR KHOLDERIA-VIETNAMIENSIS, A NITROGEN-FIXING BACTERIUM ASSOCIATED WITH RICE (ORYZA-SATIVA L) ON A SULFATE ACID SOIL OF VIETNAM

To study the dominant diazotrophs associated with rice in Vietnam, you ng rice plants were grown on a Vietnamese sulphate acid soil in a phyt otron; under these conditions, a maximum nitrogenase activity (estimat ed with acetylene reduction activity technique) of ca 360 nmol C2H4 h( -1) plant(-1), was measured on day 8. At this stage, N-2-fixing bacter ia were counted and isolated, using the 'spermosphere model' technique , in which exudates of an aseptically grown rice plantlet are the only C source for bacteria. The contribution of N-2-fixing bacteria to the microflora (total bacteria, 6.5 x 10(8) colony forming units g(-1) so il) was shown to be close to 40% in the initial rhizosphere soil. Twel ve representative isolates were further studied and proved to be Burkh olderia vietnamiensis. They exhibited a low pH-adapted nitrogenase act ivity. Four of them were selected for comparison with six other strain s of Azospirillum lipoferum, A brasilense and Enterobacter cloacae for their nitrogenase activities in the presence of axenically grown rice plantlets. Strain TVV75 of B vietnamiensis was more efficient (1 900 nmol C2H4 tube(-1) day(-1)) than the others, with the exception of str ain MRB16 of A lipoferum (from Bangladesh). Consequently, strain TVV75 of B vietnamiensis has been retained for subsequent inoculation trial s in Vietnam where yield increases were obtained.