A field experiment using three strains of Azotobacter chroococcum and two of Azospirillum brasilense was carried out at two elevations in Mamlay Watershed, Sikkim, India. A local maize variety was used as the test crop. The treatments (applied by seed inoculation) resulted in statistically significant improved plant performance at the subtropical location where yield enhancement of 1.15-fold over control was recorded with one of the bacteria. Azotobacter chroococcum W5 (originally a wheat isolate) gave the best performance. Observations recorded for various microbial communities indicated stimulation of some of the native, beneficial groups of microorganisms. During the middle of growing period there was a 2–5-fold increase in the population of actinomycetes, and a group of bacteria able to grow on N-free medium. This suggests that the observed effects of seed inoculation on plant growth may in part be due to the stimulation of already existing plant growth-promoting rhizobacteria in and around roots. Bacterial inoculation also resulted in significantly higher values for nitrogen and phosphorus content of plant components. Seed inoculation did not result in improvement of plant performance at the temperate location. This was probably due to the inability of the introduced bacteria to establish or survive at lower temperatures.
