Nitrogen Fixation Efficiency of Azospirillum Largimobile in System of Rice Intensification: SRI.

Biological nitrogen fixation is an important process which contributes N to the plants. Azospirillum largimobile, an aerobic free living microorganism, has been reported to fix considerable amount of N in many crops. However, under conventional rice growing system its N fixation may be limited by oxygen availability. The system of rice intensification (SRI) is based on careful soil and water management. This system facilitates the oxygen translocation into the soil which may enhance soil microbial activities including N fixation. The objective of this study was to determine the effect of A. largimobile inoculation and rice systems on its population, nitrogen fixation efficiency and rice yield. In the first experiment, 3 methods of A. largimobile inoculation: 1) seed inoculation 2) seedling inoculation and 3) soil inoculation, were tested in rice under greenhouse conditions. The results showed that the method of soil inoculation had the highest A. largimobile population in the rhizophere. In the second experiment, the effects of rice growing systems (SRI and conventional) and A. largimobile soil inoculation on nitrogen fixation efficiency, rice yield and yield components were studied. The results indicated that there was a significant interaction between microbial inoculation and rice growing systems on nitrogen fixation efficiency. In general, nitrogen fixation efficiency increased with A. largimobile inoculation but the magnitude of the increase was greater under SRI than under conventional system. Dry matter, yield and yield components in the SRI were significantly higher than those in the conventional system. However, they were not significantly different between A. largimobile inoculation and uninoculation.

Keywords:  Nitrogen fixation, System of rice intensification (SRI), Azospirillum largimobile