Azolla is a free-floating aquatic fern that has been used for centuries in Asia as a biofertilizer of rice paddies (see Fig. 33.13b). Its success as a fertilizer is due to its symbiotic association with nitrogen-fixing cyanobacteria. Azolla is common worldwide on still water such as ponds and quiet streams. Its leaves contain an internal cavity in which colonies of nitrogen-fixing cyanobacteria become established. The cyanobacteria provide Azolla with a source of nitrogen, while the fern provides shelter and carbohydrates. The cyanobacteria are reported to have higher rates of nitrogen fixation when they are housed within an Azolla leaf than when they live freely in the environment.
When added to newly planted rice fields, this fast-growing fern suppresses weed growth by forming a dense, floating layer. At the same time, it provides organic nitrogen that eventually finds its way into the developing rice plants. The floating fertilizer factories provided by Azolla may have made possible the long history of rice cultivation in Asia.