抄録
In South Kalimantan, Indonesia, a large area of rice paddy field is suffered with acid sulfate soil and the productivity of rice is supposed to be low. While in some paddy fields we have found that the yield of rice is larger than 5 ton ha-1 without any fertilizer. There assumed to be the contribution of nitrogen fixing bacteria for rice production by biological nitrogen fixation (BNF) under those low pH condition. We have screened bacteria from the rhizosphere and rhizoplane of local rice plant growing in natural field based on their growth on nitrogen free medium. We have obtained 26 isolates, and the majority of isolated bacteria belonged to the β-proteobacteria (34.6 %) and others are α-proteobacteria (19.2 %), γ-proteobacteria (0.38 %), Bacilli (19.2 %), and Actinobacteria (23.1 %) based on their 16S rDNA sequence. We have determined their nitrogenase activity and the existence of nifH gene by using PCR technique. Though all of the isolates can grow on nitrogen free medium, only five isolates belong to β-proteobacteria have high nitrogenase activity. The existence of nifH gene was also confirmed in theses five isolates by using PCR technique. We presume that one of the mechanisms of the adaptation of these bacteria under acid sulfate soil is changing the surrounding pH. In bacterial culture medium, the change of pH during the incubation of these isolates was deeply regulated by the existence of several organic acid anions. In rice culture with soil experiments, under initial soil pH 3.5 condition, though rice growth was inhibited with the addition of 500 μM Al, inoculation with obtained isolate significantly alleviated Al stress. It is suggested that beneficial interactions of rice and free living nitrogen fixing bacteria might be operating to reducing root Al stress in acid sulfate soil.
