Cyanobacteria possess the ability to bind atmospheric nitrogen. For this they use two different methods. First is the nitrogen adjustment in Heterocysten. These are cells, which specialize in the binding of nitrogen. The particularly thick cell wall of the Heterocysten are remarkable. Inside the cell is an oxygen free environment which is needed by the enzyme Nitrogenase for the reduction of N2 into ammonium NH3+. This enzyme is very oxygen sensitive and can not function in presence of O2.The thick cell wall reduces the diffusion atmospheric gases so that the penetration of oxygen is prevented to a large extent, while however sufficient nitrogen for the saturation of the Nitrogenase is allowed penetrate the wall.However, the Heterocysten lack the Photo System II, by means of which cells containing Chlorophyl fix CO2 and build organic compounds like sugar. This is the classical version of photosynthesis which is described in our text books. In order to overcome a sugar deficit the Heterocysten are supplied with simple sugars called Disaccharides from neighboring vegetative cells. The Heterocysten are connected to the vegetative cells through fine channels which make nutrient transport possible. Photo System I is however confined to the Heterocysten. These build ATP, using light as their energy source, which is an important actor in the reduction of nitrogen into NH4+ . Fixed atmospheric nitrogen is stored after nitrification in the form of Cyanophycin. Cyanophycin is stored as an easily accessible protein reserve, which enables growth when nitrogen is absent from the surrounding environment. Cyanophycin is also converted into Glutamin. Glutamin is exchanged along the same connecting channels to neighboring cells, to supply them with nitrogen. The formation of Heterocysten is

stimulated by a lack of bound nitrogen in the environment. When nitrogen is in a constant supply, the formation of Heterocysten is slowed or completely stopped.

There are also forms of blue agae which do not form specialized cells, but can fix nitrogen. These forms contrary to those using heterocysten, separate the processes of binding nitrogen and CO2 during the daylight hours they operate standard photosynthesis for sugar production. However, during the night the Phycobiliprotein, which is used in photosynthesis, are reduced and in their place Nitrotrogenase is formed. Thus the functions of the oxygen producing photo system II is stopped, and N2 fixing becomes possible. When sufficient nitrogen is stored as Cyanophycin, the Nitrogenase is reduced by CO2, which leeches into the cell over the course of the night. At the same time Phycobiliprotein is again synthesized, so that the photo system II can again function.
(Translators Note:That is way-cool)

Availability of Nitrogen in the ground

How quickly the nitrogen in the Cyanobacteria becomes effective, depends on the composition of nitrogen.
Organically bound N is slower but also usable for longer periods. This is the type of nitrogen constructed by Cyanobacteria in the form of Glutamin and Cyanophycin. The presence of amino acids like Glutamin serve to enrich humus, and lead to improved ground fertility. Also through these processes nitrogen is gradually released, and the total nitrogen content of the soil increased. This means that with the constant and long term presence of Cyanobacteria in the soil, the humus content and quality will be continually increased/replenished. Over years as a pool of available nitrogen forms, CO2 is also bound. Additionally a small part of the Cyano nitrogen content contains 4+ which though found only is small quantities can be used by plants at short notice.

Christiaan van den Hoek, Hans Martin Jahns, David G. Mann (1993). Algen, 3. Neubearbeitete Auflage, Georg Thieme Verlag Stuttgart, New York