Non-symbiotic nitrogen-fixing bacteria influencing mineral and hydrolysable organic nitrogen in rhizosphere soils of rice (Oryza sativa)

A field experiment was conducted during the year 2002 to find out the effect of inoculation with 2 non-symbiotic N-2-fixing bacteria, viz Azotobacter (strain D S-3) and Azospirillum (strain DM10) in presence of 50 kg urea N/ha on growth and activities of diazotrophs in relation to availability of inorganic and organic nitrogen in rhizosphere soils of 'IET 4786' rice (Oryza sativa L). In most cases, successful inoculation of diazotrophs was recorded with proliferation of Azotobacter and Azospirillum either alone or in combination, in rhizosphere soils and nitrogenase activity (C2H2 reduction) of rice roots. Inoculation with diazotrophs significantly increased the availability of inorganic (exchangeable NH4+ and soluble NO3-) (29.7%) and organic (hydrolysable NH4+, amino acid, hexosamine, serine + threonine and unidentified fraction) nitrogen content (23.3%) in rhizosphere soils, resulting in greater yield (23.7%). In general, combined inoculation of diazotrophs was more stimulative than their single inoculation in augmenting nitrogen mineralization in soil. The uninoculated soil series receiving 100 kg N/ha also recorded similar response in nitrogen mineralization and crop yield.