Survival of Azorhizobium caulinodans in the Soil and Rhizosphere of Wetland Rice under Sesbania rostrata-Rice Rotation.

The survival of indigenous and introduced strains of Azorhizobium caulinodans in flooded soil and in the rice rhizosphere, where in situ Sesbania rostrata was incorporated before the rice crop, is reported. The azorhizobia studied were both root and stem nodulating. In a pot experiment, two crop cycles each of inoculated and noninoculated Sesbania-rice were compared with two crop cycles of flooded fallow-rice. In a field experiment, the effect of repeated incorporation of in situ S. rostrata in the Sesbania-rice sequence was studied. Soils in which inoculated S. rostrata was incorporated contained about 3,000 times more azorhizobia than did soils in the flooded fallow treatment and about 50 times more azorhizobia than did soils in the noninoculated Sesbania treatment. Azorhizobial numbers in the inoculated Sesbania treatment declined toward rice harvest but remained much higher than in the flooded fallow-rice treatment. Repeated incorporation of S. rostrata increased the population density of indigenous soil azorhizobia, whereas the population of inoculated strain ORS571 (Str Spc) declined to an undetectable level; this finding suggested low competitiveness by the introduced strain. In the incorporated Sesbania treatment, the rice rhizosphere harbored significantly more A. caulinodans and supported higher nitrogenase activity per plant than did the rhizosphere of the flooded fallow-rice treatment. Sterile rice seedlings inoculated with A. caulinodans showed nitrogenase activity comparable to that of seedlings inoculated with Azospirillum lipoferum 34H, a rice root isolate. Rhizobia from Sesbania aculeata, Sesbania sesban, a Trifolium sp., and Vigna unguiculata did not support appreciable nitrogenase activity.