Cyanobacteria as biofertilizers
Heterocystous cyanobacteria and several nonheterocystous cyanobacteria are known for their ability to fix atmospheric nitrogen (Capone et al. 2005). The fertility of many tropical rice field soils has been mainly attributed to the activity of nitrogen-fixing cyanobacteria. An estimation showed that more than 18 kg N ha−1 year−1 was added to the soils by cyanobacteria (Watanabe and Cholitkul 1979). Inoculation of cyanobacteria to increase the fertility of soils has been successfully attempted. For example, Azolla was used as an organic fertilizer in rice cultivation in many countries (Kaushik and Venkataraman 1979). Addition of Azolla was found to support the growth of soil micro-organisms including heterotrophic N2 fixers. Recently, nitrogen-fixing cyanobacteria have been reported to dominate desert crusts worldwide (Garcia-Pichel and Pringault 2001). This is believed to contribute significantly to the fertility of desert soils and may eventually facilitate vegetation of deserts.