Microbial Diversity Of Rhizospheric Soil With Special Reference To Plant Growth Promoting Isolates Of Azotobacter

Iqbal Ahmad*, M. Yaqub Shiekh and Farah Ahmad

Department of Agricultural Microbiology, Faculty of Agricultural Science, Aligarh Muslim University, Aligarh - 202 002 (India)

ABSTRACT: Soil microbial populations are the key element in recycling the plant nutrients and thus play a major role in the maintenance of soil fertility and soil health. Various rhizospheric and non-rhizospheric soil samples of agricultural fields of Aligarh revealed the significant viable plate count of various functional groups of soil microorganisms. The population density of aerobic heterotrophic bacteria, actinomycetes, filamentous fungi did not significantly differ in the different field soil irrespective of crop under cultivation. Rhizospheric soil yielded almost ten times more microbial biomass as compared to non-rhizospheric soils. On the other hand population density of aerobic asymbiotic nitrogen fixers particularly Azotobacter chroococcum was poor in soil receiving long-term application of wastewaters. A total of 48 isolates of Azotobacter recovered from different crop rhizospheric soils were tentatively grouped into three morphological types on the basis of pigmentation and cultural characteristics. Isolates of group -A was tentatively identified as A. chroococcum and group B & C as the other species of Azotobacter. Carbohydrates like glucose, fructose, lactose, maltose, adonitol and rhamnose were commonly utilized by the fair number of isolates. Antibiotic resistance profile against 8 antibiotics revealed 60.4% strains resistant to nitrofurantoin followed by nalidixic acid (58.3%), co-trimazine (37.5%), novabiocin (31.2%), cloxacillin (20.8%) and least to chloramphenicol (12.5%). All test strains were found sensitive to streptomycin and doxycycline. All isolates produced ammonia, and showed varying degree of positive influence on germination of moong seeds in vitro. HCN production could be detected in five isolates. IAA production in the selected isolates varied from 3.0 to 7.6 μg/ml of culture filtrate. These isolates showed tolerance to salt (2.5-3.5% NaCl conc.) and pH value (7-10). It is expected that isolates exhibiting multi-PGP activities and elevated tolerance to environmental factors may be suited for further assessment and development as the effective PGPR- inoculants.

KEYWORDS: Azotobacter; antibiotic resistance; plant growth promoting activity

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