Effects of inoculation of cyanobacteria on nitrogen status and nutrition of rice (Oryza sativa L.) in an Entisol amended with chemical and organic sources of nitrogen

A field experiment was conducted with wetland rice (Oryza sativa cv. IR-36) in a sandy clay loam soil (Entisol) to study the effect of inoculation with a soil-based mixed culture of four diazotrophic cyanobacteria,Aulosira fertilissima, Nostoc muscorum, N. commune andAnabaena spp., on the N-flux in inorganic NH₄⁺+NO₃^–+ NO₂^–), easily oxidizable, hydrolysable and non-hydrolysable forms of N in soil during vegetative growth periods of the crop. Effects on grain and straw yield and N uptake by the crop were estimated. The effects of applying urea N and N as organic sources, viz.Sesbania aculeata, Neem (Azardirachta indica) cake and FYM, each at the rate of 40 kg N ha^–1, to the soil were also evaluated. Inoculation significantly increased the release of inorganic N, evidenced by its increased concentrations either in soil or in soil solution. However, such increases rarely exceeded even 4% of total N gained in different froms in the soil system by inoculation during the vegetative growth stages of the rice plant, when the nutritional requirement of the plants is at a maximum. Most of the N₂ fixed by cyanobacteria remained in the soil as the hydrolysable form (about 85%) during this period. Inoculation caused an insignificant increase in grain (8%) and straw (11%) yield, which was, however, accompanied by a significant increase in N uptake by the grain (30%) and an increase in total uptake of 15.3 kg N ha 1. Such beneficial effects of inoculation varied in magnitude with the application of organic sources, with farmyard manure (FYM) being the most effective. Application of urea N, on the other hand, markedly reduced such an effect.