Tolerance of Azolla to chemical nitrogen fertilizer in rice field

The fresh and dry matter yield, N.P and chlorophyll content of Azolla and heterocyst frequency of its algal symbiont, Anabaena azollae decreased gradually as N concentrations increased from zero to 60 kg N ha⁻¹ in fallow rice field treated with either urea or ammonium sulphate. On the contrary, the growth of Azolla in the planted rice field was not significantly affected by the application of graded levels of urea or ammonium sulphate upto 90 kg N ha⁻¹, but decline in growth was noticed in the treatments of 120 and 150 kg N ha⁻¹ as urea. Between the two forms of N fertilizers urea was more inhibitory for the growth and N₂ fixation of Azolla. Inoculation of Azolla alongwith graded levels of inorganic-N fertilizer linearly increased the rice yield up to 150 kg N ha⁻¹.