Effect of Nitrogen Fertilizers on Nitrogen Fixation and Heterocyst Frequency of Cyanobacterium Anabaena azollae in 7 Species of Azolla

Abstract

Summary The effect of nitrogen fertilizers ammonium sulphate (825 mg pot −1 ) and urea (395 mg pot −1 ) corresponding to 40 kg N ha −1 was studied on acetylene reduction activity (ARA) and heterocyst frequency (HF) in the cyanobacterium Anabaena azollae occurring in successive leaves of Azolla caroliniana Willdenow , Azolla microphylla Kaulfuss , Azolla filiculoides Lamark , Azolla mexicana Presl ., Azolla nilotica Decaisne , Azolla rubra R. Brown and Azolla pinnata R. Brown (the last from both Malaysia and India) grown on flooded soil in pots under net house conditions. Both HF and ARA increased from first leaf (shoot apex) to the last one (before branch) in all the species, although these were highest with no nitrogen treatment followed by that with ammonium sulphate and urea treatments. The two nitrogen fertilizers had little effect on the relative growth rate of A. microphylla and A. mexicana . The relative growth rates of the remaining Azolla species were also unaffected by ammonium sulphate, but were significantly lower with urea than with no nitrogen treatment.