Seasonal Population Dynamics of Rhizosphere and Non-rhizosphere Soil Microorganisms of Chir Pine Seedlings (Pinus roxburghii Sarg.)

Seasonal population dynamics of bacteria, actinomycetes and fungi was studied in rhizosphere and non-rhizosphere soil of chir pine (Pinus roxburghii Sarg.) seedlings growing in polybags and in situ (forest). In greenhouse experiment bacteria and actinomycetes were present in higher numbers and their populations fluctuated with season. Fungal population, although lower in numbers, remained stable throughout the year. Population fluctuations with lower numbers were more prominent in rhizosphere and non-rhizosphere soils of forest plants. Differential bacterial population characteristics viz. sporeformers, fluorescent colony producers, methylene blue reducers, ammonifiers and glucose fermenters were also taken into account. The population of sporeformers was comparable with methylene blue reducers, which was higher than fluorescent colony producers, ammonifiers and glucose fermenters, respectively. The rhizosphere soil bacterial count of nursery seedlings ranged from 4.36 x 106 – 6.37 x 106 g-1 dry soil weight and from 8.8 x 105 – 2.64 x 106 g-1 soil on dry weight basis in forest plants during various seasons. Sporeformers were a magnitude lower than total bacterial population and fluorescent colony producers were magnitude lower than sporeformers. Actinomycetes count ranged from 6.0 x 105 – 3.02 x 106 g-1 dry soil weight in the nursery plants and from 6.4 x 105 – 1.17 x 106 g-1 dry soil weight in forest plants. Fungal population was a magnitude lower than bacterial and actinomycetes population, which ranged from 9.0 x 104 – 2.9 x 105 g-1 dry soil weight in the nursery plants and from 9.0 x 104 – 3.1 x 105 g-1 soil in forest plants. A similar trend of microbial population fluctuation but with lower numbers was observed in forest non-rhizosphere soil.
Aims: To compare the seasonal population fluctuations of rhizosphere and non-rhizosphere soil for better understanding of population dynamics of soil microorganisms.
Study Design: The observations were taken from nursery grown and forest grown seedlings. The microbial populations of pine seedling rhizosphere and non-rhizosphere soil were seasonally enumerated for one year at the intervals of three months for four times.
Place and Duration of Study: The study was conducted at Sardar Bhagwan Singh (PG) Institute of Biomedical Sciences and Research (S.B.S.P.G.I.), Dehradun, Uttarakhand, India, situated at foothills of central Himalayas, for one year during January to December, 2005.
Methodology: The seeds of Pinus roxburghii Sarg. were collected from natural chir pine forest from one healthy plant to minimize genetic variability in the experiment. The seeds were germinated on water agar medium and saplings were planted in polybags and kept in greenhouse nursery. Microbial colony forming units (CFUs) of the rhizosphere and non-rhizosphere soil of nursery grown and forest seedlings were enumerated for one year. Their populations were correlated with the meteorological data of the Dehradun valley.
Results: The total bacterial population in terms of CFUs was comparatively higher in all seasons followed by actinomycetes, both these populations fluctuated with season. Fungal population, although lower in numbers, was consistent throughout the period. Microbial populations were found to be dependent on environmental factors like soil and air temperature, relative humidity and precipitation. The population of each microbial type reached maximum during third trimester, just after the end of monsoon season.
Conclusion: The microbial population of chir pine rhizosphere soil and non-rhizosphere soil fluctuates seasonally. Microbial populations were found to be dependent on soil temperature, air temperature, precipitation and relative humidity.