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Cyanobacteria are oxygen evolving, nitrogen fixing prokaryotes occur in every conceivable habitats but abundantly in water logged rice fields in several rice growing countries. Nitrogen fixing cyanobacteria play a vital role in the maintanence of soil fertility and sustainability in rice field ecosystems (Roger & Reynaud, 1979). Cyanobacterial inoculation to rice crop was found to be effective in different agroclimatic conditions like tropical or temperate climates and soil types such as saline soils, phosphorous rich soils, alkaline or acidic soils etc. (Singh et al., 2017). The paddy field ecosystem provides an environment favourable for the growth of cyanobacteria with respect to their requirement for light, water, temperature, humidity and nutrient availability. In submerged soil system, biological nitrogen fixation contributes 25 to 30 kg nitrogen per hectre for one cropping season (Saexena et al., 2007; Kaushik, 2001). They are cosmopolitan in distribution and occur in every conceivable habitat where life is possible. They are reported to occur in extreme climatic conditions such as hot springs, polar deserts and Antartic regions (Halder, 2015, 2016). Even though they are ubiquitious, they prefer to grow in rice fields because the soil pH, temperature, humidity, crop canopy and soil moisture are favourable for its growth in paddy fields. It releases a large number of secondary metabolites into the paddy fields which also influences the growth of paddy (Wilson, 2006). The beneficial effect of cyanobacterial biofertilizer for paddy such as increase in number of tillers, seeds, length of the plant and leaf, yield etc. has been well documented (Karthikeyan et al., 2009; Sao & Samual, 2018; Radhakrishnan & Venkitaraman, 2005). Cyanobacterial biomass is also used for the production of various bioactive compounds, food items and biofuels (Hall et al., 1995; Malik et al., 2001; Paumann et al., 2005). They can control the deficiency of nitrogen in the soil and also improves its properties. Apart from nitrogen fixation, it also influences the overall growth of paddy including the grain yield (Roger et al., 1980; Singh, 1981; Alam et al., 2014). Since the urea and ammonium sulphate are the easily available and commonly used inorganic nitrogen fertilizer in the paddy fields of India, this made the the author to study how much these synthetic nitrogen fertilizers such as urea and ammonium sulphate influences the growth of paddy field cyanobacterium, Westiellopsis prolifica.

Response of paddy field cyanobacterium, Westiellopsis prolifica Janet to the inorganic nitrogenous fertilizers