Contribution of non-leguminous biofertilizers to rice biomass, nitrogen fixation and fertilizer-N use efficiency under flooded soil conditions

A field experiment was conducted to study the effect of root-associated diazotrophic rhizobacteria and Azolla, alone and in combinations, on rice grown in a flooded saline soil. N-15 labelled ammonium sulphate at the rate of 30 kg N/ha was used to monitor its recovery and computation of biological nitrogen fixation (BNF) using an isotopic dilution method. The bacterial population of nitrogen-fixing bacteria generally increased more for inoculated treatments compared with uninoculated ones. The rice biomass (straw + grain) was maximum for Azolla cover followed by the bacterial treatment. The fertilizer-N recovery in rice was maximum for Azolla cover + bacteria followed by Azolla cover treatment, which may be due to lowering of floodwater pH by Azolla. The maximum amount of BNF (46 kg N/ha) was obtained for bacterial treatment, followed by bacteria + Azolla incorporated treatment. The study indicated that use of biofertilizers, along with a low input of chemical-N fertilizer, is useful for increasing rice yield, fertilizer-N use efficiency and BNF in rice grown in flooded soil conditions.