RHIZOSPHERE ASSOCIATED BACTERIA AND SOIL PHYSICO- CHEMICAL PROPERTIES OF TEA GARDEN

In the rhizosphere, plant-microbe associations play critical part in major ecosystem processes. The pHs of the collected rhizosphere soil were acidic (4.24 4.77) and favorable for the established tea orchards. Soil texture was either sandy clay loam or sandy loam in nature. Organic carbon of the samples ranged from 0.91 1.19%, depicting the soil falling in very low to low category due to monoculture of tea for a long time. Total nitrogen content of the soil samples ranged in between 0.10 and 0.13%. Moreover, Phosphorus and important metal ions viz. potassium, calcium and magnesium were measured. Mean heterotrophic bacterial load of the soil samples ranged from 1.59 ± 0.22 × 10 to 10.88 ± 2.31 × 10 cfu/g and from 1.58 ± 0.29 × 10 to 6.93 ± 0.79 × 10 cfu/g on NA and PYG, respectively. Maximum bacterial count on both media was observed in samples of Saloon Section Area plot while the lowest counts were found in samples of D1 Section plot. Sixteen bacterial isolates were selected and purified for identification, which was conducted by amplifying ~ 600 bp fragments of 16S rDNA. All the isolates were Gram positive and rod shaped. Bacillus was found to be the dominant genus (63%) in the tea rhizosphere soil. Other isolates were identified as varied species of Lysinibacillus (19%), Paenibacillus (12%) and Brevibacterium (6%). A phylogenetic tree was generated which showed only one major cluster comprising of two sub-clusters grouping Paenibacillus sp. in one and Lysinibacillus along with Bacillus sp. in another. Present observation indicated that Bacillus sp. is a major organism and has a strong role in evolution among the tea rhizospheric bacteria. Introduction The amount of microbial species in the rhizosphere may fluctuate from thousands to millions and accordingly, the interactions between roots and soil microbes are often specialized on the basis of co-evolutionary pressures (Duffy et al. 2004, Morrissey et al. 2004). In the rhizosphere, plant-microbe interactions play important roles in many vital ecosystem processes, including carbon sequestration and nutrient cycling (Singh et al. 2004). Positive plant-microbe interactions include plant-microbe symbioses, such as plant associations with plant-growth promoting rhizobacteria (PGPR), epiphytes and mycorrhizal fungi. These interactions have been proved to have many beneficial impacts on plants, including disease suppression (Mendes et al. 2011), increased nutrient availability and uptake and increased immunity to abiotic (Zolla et al. 2013) and biotic stresses (Zamioudis and Pieterse 2012), each leading to increases in plant productivity. Many studies have revealed that soil microbial communities are affected by numerous factors i.e. plant species, soil types and agricultural management (Berg and Smalla 2009, Jangid et al. 2008). Other beneficial effects of soil microbes on plants include nutrient and water uptake enhancement, dinitrogen fixation, phytohormone production, symbiosis, disease and pest control and enhancement of soil quality. Thus, soil and its microbial communities are obligatory for Author for correspondence: <tahsin.khan@du.ac. bd>. Department of Botany, University of Barisal. Soil Science Division, Bangladesh Tea Research Institute (BTRI), Srimangal, Moulvibazar, Sylhet, Bangladesh.