Please have a look of the attached files

Generally the Cyanobacteria grow well under P and N fertilization upto a certain limit and excess of P also, but not under excess of N in any case as thy fix nitrogen. However these all also depends upon the cyanobacterial species. In your case as you have not defined your dose and genus you are growing, there may be many reason for the same.

Cyanobacteria require excess phosphorus for nitrogen fixation, but they don't need excess nitrogen. Presence of excess nitrogen in the habitat reduces their ability to fix atmospheric nitrogen. In filamentous cyanobacteria the frequency of heterocyst formation decreases under high nitrogen conditions, while the frequency of akinete formation increases under phosphorus deficient conditions.

Please have a look at the following articles on researchgate

1. AL Singh, PK Singh 1986. Comparative effects of Azolla and blue-green algae in combination with chemical N fertilizer on rice crop. Proceedings Indian Academy of Sciences : Plant Sciences 96 (2), 147-152

2. AL Singh, PK Singh1990. Phosphorus fertilization and the growth and N2-fixation of Azolla and blue-green algae in rice field. Indian Journal of Plant Physiology 33, 21-26.

Also have have a look at the following articles on researchgate

1. AL Singh, PK Singh, P Lata 1988. Effect of split application of phosphorus on the growth of Azolla and low land rice. Nutrient Cycling in Agroecosystems/ Fertilizer Research 16 (2), 109-117

2.AL Singh, PK Singh, P Lata 1988. Effects of different levels of chemical Nitrogen (urea) onAzolla and Blue-green algae intercropping with rice. Nutrient Cycling in Agroecosystems/ Fertilizer Research 17 (1), 47-59

Some N2 fixing cyanobacteria grow better with more fixed N as N2 fixation is an energy- and reductant-intensive process Not having to invest in N2 fixation frees up resources for faster growth.

Ms Prabha Kumari Tiwari please letus know the Cyanobacteria which you are studying and the purpose of study. Be sue that it is not contaminated by green algae

Thank you all for your valuable answers.