Repeated beneficial effects of rice inoculation with a strain of Burkholderia vietnamiensis on early and late yield components in low fertility sulphate acid soils of Vietnam

TVV75, a strain of Burkholderia vietnamiensis, was isolated from an acid su lphate soil of south Vietnam, and selected for its high in vitro nitrogen f ixation potential. This plant growth-promoting rhizobacterium (PGPR) had be en used in a previously reported pot experiment. It was used in two new pot experiments and four field experiments to inoculate lowland rice at sowing and at transplanting, in three different South Vietnam acid sulphate soils . We first studied the effect of inoculation during early plant growth in n urseries. Seedlings were then transplanted both to field and pots. Treatmen ts included two levels of inoculation (inoculated vs uninoculated) and thre e levels of N fertilizer (0, recommended rate and half this rate), in a ran domized block design with six replicates. In all four experiments nitrogen appeared to be the limiting factor for yield. Inoculation had already had a strong beneficial effect at the transplanting stage (day 24), as measured by shoot weight (+33%) root weight (+57%), and leaf surface (+30% at day 14 ). Final results indicated that inoculation of rice with B. vietnamiensis T VV75 significantly increased several yield components, resulting in a final 13 to 22% increase in grain yield. A late yield component, 1,000 grain wei ght, was significantly increased by inoculation, but not by nitrogen fertil izers, in all pot and field experiments, indicating a long-lasting effect o f the inoculated bacteria. It was possible to evaluate the nitrogen fertili zer equivalent of inoculation (NFEI): at the medium rate of N fertilizer, i noculation ensured a yield equivalent to that obtained in the uninoculated control with 25 to 30 kg more nitrogen fertilizer. Comparison of the local cost of NFEI kg N-fertilizer and the cost of inoculation would help in maki ng the decision to inoculate.