The ability of Azolla’s symbiont, Anabaena, to sequester atmospheric nitrogen has been used for thousands of years in the Far East, where Azolla is extensively grown in rice paddies to increase rice production by more than to 50%.
Rice is an enormously important staple in many tropical and temperate regions of the world. Billions of people rely on the crop to live and hundreds of millions are now threatened by food shortages that are increasing each year.
World rice production was approximately 645 million tonnes in 2007. At least 114 countries grow rice and more than 50 have an annual production of 100,000 tonnes or more. Asian farmers produce about 90% of the total, with two countries, China and India, growing more than half the total crop.
Nitrogen is the single most limiting factor in rice cultivation, strongly affecting the crop yield. Azolla substantially increases the amount of nitrogen fertilizer available to growing rice and it is has been used for thousands of years as a ‘green’ nitrogen fertilizer to increase rice production.
Research into Azolla’s use in rice production has therefore grown over the past years, including the development of new hybrids.
Videos of Azolla‘s use to increase rice production are shown here.
Azolla’s increase in rice productivity
Less that 5% of the nitrogen sequestered by Azolla is available immediately to the growing rice plants. The remaining 95% remains in the Azolla’s biomass until the plant dies. As the plant decomposes, its organic nitrogen is rapidly mineralized and released as ammonia, which then becomes available as a biofertilizer for the growing rice plants.
Various techniques have therefore been developed to maximize Azolla’s nitrogen fertilization, with the result that Azolla now has enormous potential to increase rice production worldwide and hence alleviate food shortages.
These include methods to increase the availability of the nitrogen assimilated by Azolla-Anabaena to the growing rice plants.
In Tanzania, Wagner (1996) applied Azolla nilotica in various trials as an intercrop and obtained increases of up to 103% in rice grain yield.
Experiments at the University of California at Davis showed that Azolla increased rice yields by 112% over unfertilized controls when applied as a monocrop during the fallow season, by just 23% when applied as an intercrop with rice. However, the amount increased by 216% when Azolla was applied both as a monocrop and an intercrop (Peters, 1978).
Azolla’s nitrogen release into water
Most of the nitrogen fixed becomes available to rice only after the Azolla has decomposed, although a small amount of ammonium is released into the water by Azolla during growth (Watanabe, 1984)
This was confirmed by Chung-Chu (1984), who determined that only 3 to 4% of the total nitrogen fixed by Azolla is excreted into the water medium during its growth.
During decomposition, organic nitrogen is mineralized rapidly during the first two weeks and then at a more gradual rate (Watanabe, 1984). Nitrogen is released mainly in the form of ammonium. Ammonium-nitrogen released was found to stabilize at about 1 mg ammonium-N g-1 of fresh Azolla, which was 26-28% of the total nitrogen content of Azolla (Tung & Shen, 1985).
Azolla’s incorporation of nitrogen into soil
Incorporation of Azolla into the soil improves the release of nitrogen (Tung & Shen, 1985). If Azolla is grown as a monocrop and the field should be drained several days in advance of incorporation. The last mat should be incorporated and the field kept drained for 4 or 5 days before transplanting rice in order to speed decomposition (Lumpkin, 1987a).
Azolla’s use as an intercrop
Azolla incorporated 78 days after transplanting rice was shown to contribute a greater amount of nitrogen to rice grain than was contributed by earlier incorporation (30-53 days after transplanting) (Ito & Watanabe, 1985). Since it has been found that the optimal stocking density for Azolla, with respect to area-specific nitrogenase activity, is approximately 50 to 100 g dry weight m-2 (Hechler & Dawson, 1995), nitrogen inputs may be best maximized by frequent but partial incorporations of Azolla.
Other benefits of incorporating Azolla in rice cultivation
As well as its nitrogen biofertilization, Azolla provides a variety of benefits for rice production and grows in a way that is complementary to rice cultivation: The thick Azolla mat in rice fields suppresses weeds.  Since Azolla floats at the water surface, it does compete with rice for light and space.  In most climates, Azolla grows best under a partial shade of vegetation which is provided by the rice canopy during early and intermediate stages of growth  When the rice approaches maturity, Azolla begins to die and decompose due to low light intensities under the canopy and a depletion of nutrients, thus releasing its nutrients into the water.  Because Azolla decomposes rapidly, its nitrogen, phosphorus and other nutrients are rapidly released into the water and made available for uptake by rice during grain development.  Azolla has a greater ability than rice to accumulate potassium in its tissues in low-potassium environments, providing rice with potassium after Azolla’s decomposition  In contrast with chemical nitrogenous fertilizers, Azolla has various positive long-term effects, including the improvement of soil fertility by increasing total nitrogen, organic carbon, plus phosphorus, potassium, other nutrients and organic matter.  If chemical nitrogenous fertilizers are applied, the presence of an Azolla mat reduces ammonia volatilization that would normally occur.  When grown in a rice field, Azolla reduces the ammonia volatilization that occurs following the application of inorganic nitrogen fertilizers by 20% to 50%. This is due to the fact that the Azolla cover reduces light penetration into the floodwater, thus hindering the rise of pH which normally stimulates ammonia volatilization in an Azolla-free rice field (Watanabe & Liu, 1992).