Nitrogenous agrochemicals inhibiting native diazotrophic cyanobacterial contribution in wetland rice ecosystem

Abstract

The diazotrophic cyanobacteria are known to play an important role in nitrogen economy of wetland rice ecosystem. Due to intensive use of nitrogenous fertilizers in rice cultivation, maintenance of soil health has become a challenging task. In this context, a systematic study was conducted in flooded rice field to determine the effects of different doses of commonly used urea and ammonium sulphate nitrogenous fertilizers on native cyanobacterial biomass, N2-fixation and N-yield at various stages of rice plants. The results clearly demonstrated that both urea and ammonium sulphate inhibited cyanobacterial growth and N2-fixation at different stages of rice crop. Maximum growth and N2-fixation were observed at heading stage of rice plant at 60 days after transplantation when compared with early and late stages of rice crop. Of the two fertilizers, urea was found more inhibitory to cyanobacteria and among the two seasons, these fertilizers were more inhibitory in the dry than in the wet season. This study suggests that N fertilizers need to be used judiciously in rice cultivation so that both cyanobacterial contribution and high rice yield are ensured.