Nitrogen (N2) is essential for all living things. Amino acids and proteins, as well as a variety of other chemical molecules, are produced during the nitrogen fixation process. Biological nitrogen fixation is an important aspect of microbial activity
(Pereira et al., 2017). Only prokaryotes, which can be symbiotic or free-living in nature, can fix nitrogen biologically. Biological nitrogen fixation, mediated by nitrogenating enzymes, is a well-known and important biological activity mechanism.
Azolla is an aquatic fern in the Salviniaceae family that grows in a symbiotic relationship with the blue-green algae Anabaena azollae. The name Azolla is derived from the Greek words azo (to dry) and allyo (to kill), implying that the plant will die if allowed to dry. Azolla contains about seven or eight living species and forty extinct species
(Small et al., 2011). The two subgenera of the genus Euazolla are Euazolla and Rhizosperma. Euazolla is made up of five species from the New World:
A.
caroliniana,
A.
filiculoides,
A.
mexicana,
A.
microphylla and
A.
rubra. The presence of three megaspore swimmers distinguishes it.
The small, free-swimming, fast-growing fern can be found all over the world. Azolla is a dichotomously branching, free-swimming aquatic fern that grows in moist soil, ditches and swampy ponds. Indian species are triangular in shape, measuring 1.5 to 3.0 cm long and 1 to 2 cm wide. Fronds have small roots that are frequently associated with microphylla
(Ahmed et al., 2021), as well as a short branched stem called the rhizome that is covered in small, alternately overlapping leaves. The sporophyte is dorsiventrally organised, with dorsal and abdominal lobes on each leaf
(Pratte et al., 2021). The ventral lobe is narrow and colourless, with only one cell thick distal half. The aerial dorsal leaflet lobe is composed of layered mesophylladaxial and abaxial epidermal tissue, numerous stomata and unicellular papillae. An elliptical cavity is formed by the folding of the adaxial epidermis in the dorsal leaf lobe. The cavity, which is mostly gas-filled, is lined with mucus and contains the cyanobiont Anabaena azollae as well as a gram-positive, non-nitrogen-fixing bacterium identified as an Arthrobacter species
(Sebastian et al., 2021). These green plants are considered high in proteins, fats, pharmacologically active secondary metabolites and antioxidants. As a result, their extracts were used in biocomposite films
(Eltabakh et al., 2021). It also contains macronutrients like calcium, magnesium and potassium, as well as vitamins like A (precursor beta-carotene) and B12. It can be used as a food supplement due to its ease of cultivation, high productivity and high nutritional value
(Sreenath et al., 2016).
Azolla is a free-floating aquatic fern that fixes atmospheric nitrogen in collaboration with the nitrogen-fixing cyanobacterium Anabaena azollae. This is found endophytically in Azolla leaves’ dorsal lobes
(Ghadimi et al., 2021). Nitrogen is obtained for the rice crop from this source. The only existing cyanobacterial plant symbiosis is that between Azolla and Anabaena. It is used as organic fertiliser in agriculture. The primary aquatic symbiotic system is Azolla. The environment contains seven different species. Azolla is a nitrogen-rich plant that was once used as a green. Wetland rice obtains organic matter from manure and organic matter. In North Vietnam and China, After 20 days of inoculation, approximately 70% of the Azolla nitrogen content was mineralized to NH+4. The rice plant readily absorbs NH+4
(Maham et al., 2020). Excess Azolla is used as compost and is fed to dryland crops and vegetables to increase crop yield. Azolla has a high protein content of about 23-27% on a dry weight basis, followed by 3-6% nitrogen, 0.5-0.9% phosphorus and 2-4.5% potassium, among other majors and micronutrients present
(Sonowal et al., 2021). Azolla can reclaim soil and reduce weed infestation in rice crops and it has also been used in wastewater treatment and heavy metal degradation. Aside from these applications, it can also be used directly as aquaculture feed, which is known as azobiofer
(Kumar et al., 2021).
Azolla is grown as a cover crop in flood plains for two to three weeks. The water is then drained and the Azolla fern is planted in the field before the rice is planted. Otherwise, 4-5 quintal of fresh Azolla are added to stagnant water a week after planting rice. Green Azolla makes excellent fish feed, while dry Azolla flakes make excellent chicken feed. It is a bio-fertilizer, insect repellent, salad dressing and most importantly, a bio-scavenger because it removes all heavy metals
(Mlbm et al., 2021).
Azolla’s possible benefits
The overall Azolla biosytem was depicted in Fig 1, with the following potential benefits:
1. It grows easily in the wild and under controlled conditions.
2. It is simple to produce in large quantities as green manure.
3. It can fix atmospheric CO2 and nitrogen to produce carbohydrates and ammonia and after decomposition, it provides soil accessible nitrogen for plant uptake as well as organic carbon content.
4. The oxygen produced by oxygenated photosynthesis helps plant root systems and other soil microbes respire.
5. It dissolves Zn, Fe and Mn in water, making them available to rice.
6. Azolla can be used to supplement chemical nitrogen fertilisers (20 kg/ha) and improve crop yield and quality. It improves the efficiency with which chemical fertilizers are used.
7. It reduces the rate of evaporation in irrigated paddy fields.
8. Lower disease prevalence.
9. Increase the number of flowers and fruits.
10. Improve seedling or transplanted plant establishment and survival.
11. Improve drought tolerance by watering less frequently.
Advantages of Azolla
Azolla as a biofertilizer for rice production
Azolla is a bio-fertilizer that produces approximately 300 tonnes of green bio-hectares per year in a typical subtropical environment, equivalent to 800 kg of nitrogen (1800 kg of urea). The rapid decomposition of Azolla in soil and the efficient availability of its nutrients are critical factors in its use as a nitrogen-fixing biofertilizer for rice crops
(Adhikari et al., 2021). The rapid reproduction rate of Azolla and its ability to decompose have become critical factors in its use as a green manure and organic fertiliser in paddy fields.
The Azolla benefits listed below are suitable for use as organic rice fertiliser:
1. Azolla is a nitrogen-fixing plant with a high nitrogen fixation rate.
2. Azolla is rapidly expanding.
3. Because azolla floats on the water’s surface, it cannot compete with rice for light and space.
4. Azolla thrives in partial shade, which a rice canopy can easily provide in its early and mid-growing stages in most regions.
5. Azolla begins to die and decompose when the rice reaches maturity due to low light intensity under the canopy and a lack of nutrients, releasing nutrients into the medium.
6. Because Azolla decomposes quickly, the nitrogen it has absorbed, as well as phosphorus and other nutrients it may have noticed in the water, possibly in competition with the rice, are quickly released back into the medium and made available to the rice to grow during crop development
(Thapa et al., 2021).
7. Azolla accumulates more potassium in its tissues than rice in low-potassium environments, making this nutrient available to rice after decomposition
(Bin et al., 2021).
8. A thick Azolla mat provides additional weed suppression in a paddy field.
Azolla as a green fertilizer
Azolla as a cover crop can benefit water bamboo, arrowhead, taro, wheat and rice
(Kimani et al., 2021). Incubating Azolla in moist soil as a green manure resulted in rapid mineralization, with 60-80% of the nitrogen released after two weeks
(Ahmad et al., 2021).
Azolla: Beneficial effects on crops
Azolla is beneficial to wheat when grown in a rice-wheat rotary cultivation system
(Lubis et al., 2021). Azolla increases mung bean production by increasing water holding capacity, organic carbon, ammoniacal nitrogen, nitrate nitrogen and accessible phosphorus, potassium, calcium and magnesium while decreasing pH and bulk density
(Vat et al., 2021).
Azolla: Beneficial effects on soil physiological and chemical properties
Azolla is a herb that increases soil fertility. The application of Azolla, which improves soil fertility by increasing total soil nitrogen, organic carbon and accessible phosphorus, confirmed these findings
(Youssef et al., 2021).
Azolla in reclamation of saline soils
Although Azolla is salt sensitive, growing it in a saline environment for two years reduced salinity from 0.35 to 0.15 and the desalination rate (71.4%) was 1.8 times faster than water leaching and 2.1 times faster than Sesbania, as well as decreased electrical conductivity, pH of acidic soil and increased soil calcium content
(Shariffi et al., 2021).
Azolla for bioremediation
The heavy metals iron and copper can be removed from contaminated water by A.pinnata and Lamna minor. Water. Pollutants can be harmful even in low concentrations. It is processed through ponds and can be reused for a variety of purposes
(Annisa et al., 2021). This is done for agricultural purposes. Tolerance and acceptance have proven to be advantageous. Three Azolla species have chromium accumulation in their leaves
(Goala et al., 2021). Azolla biomass also acts as a pollutant biofilter. Azolla has a remarkable ability to directly concentrate metals from impurities such as Cu, Cd, Cr, Ni, Pb and nutrient effluents
(Prabhakaran et al., 2022).
Azolla as a mosquito repellent
By laying a thick mat of Azolla on the water’s surface, you can prevent mosquito hatching and adult emergence
(Hossain et al., 2021). According to a study of ponds, ponds, wells, paddy fields and sewers,
Anoplieles spp. brood was almost completely suppressed in waters completely covered with Azolla.
A.
pinnata significantly reduced oviposition and adult emergence in
Culex quinquefasciatus and
Anopheles culicifacies giles, but larval survival was unaffected
(Sumitha et al., 2021). The hatchability of the eggs was reduced in part.
Azolla for weed control
Weeds have also been found to be inhibited by Azolla. Our findings were supported by the fact that an Azolla cover significantly reduced the total amount of weeds, particularly the most common weed, Monochoria vaginalis, which grasses and hedges could not always control
(Singh et al., 2021).
Azolla in biogas production
The anaerobic fermentation of Azolla (or a combination of Azolla and rice straw) produces methane gas, which can be used as fuel and the remaining effluent, which contains all of the nutrients originally incorporated into plant tissues with the exception of a small percentage of lost nitrogen
B.
ammonia, can be used as fertiliser
(Thiruvenkatachari et al., 2021).
Azolla and bioenergy
Nitrogenization in the symbionts evolves hydrogen rather than nitrogen fixation when
Azolla anabaena is grown in a nitrogen-free environment and/or a nitrate-rich aquatic medium
(Hamdan et al., 2021).
Controlling the spread of aquatic weeds
Weed growth is stifled when Azolla forms a thick, almost opaque mat. There are most likely two mechanisms at work here, the most efficient of which is light deprivation for immature weed seedlings due to sunlight obstruction
(Ouedraogo et al., 2021). The other is a dense, interlocking Azella mat that provides physical resistance to the emergence of weed seedlings
(Marzvan et al., 2021). Rice seedlings are not harmed by Azolla’s weed suppression function because they are planted above the Azolla mat.
Azolla for human consumption
A few researchers have experimented with preparing Azolla in soups or “Azolla meatballs as human food.”
Sjodin and colleagues (2012) Traditional cough treatment has been reported to be effective in Tanzania when Azolla is used
(Rahal et al., 2019). Azolla is widely used as a food fortifier due to its high protein content
(Divya et al., 2020).
Azolla as a space diet ingredient
Azolla has been proposed as part of the space diet while on Mars in collaboration with the Space Agriculture Task Force and it has been discovered that Azolla meets human nutritional needs on Mars
(Katayama et al., 2008). They used combination of Azolla, spirulina and carbohydrate.
Azolla as a livestock nutritional supplement
Azolla is consumed as a dietary supplement by pigs, rabbits, chickens, ducks and fish
(Verma et al., 2021). Azolla is harvested in large quantities and fed to cattle and pigs as feed
(Tarrif et al., 2021). It was also demonstrated that feeding Azolla to broilers produced growth and body weight values comparable to corn-soybean meal
(Ouedraogo et al., 2021). Digested Azolla manure left over from biogas production was suitable as fish pond fertiliser and in a study of lactating cows
(Abdelatty et al., 2021), it was discovered that Azolla could be used as a feed ingredient, with milk yield and fat percentage remaining at the same levels as when fed conventional feed.
Present status of Azolla
Azolla is already grown commercially in India and China, where its utility has long been recognised. The fern’s use was once limited due to propagation issues, but it is now used in larger growing areas. The Chinese have been using azolla for hundreds of years. It was first used in India in the 11
th century. These are the only two countries that have long been cultivating Azolla. The practise most likely began when it was discovered that growing wild Azolla in paddy fields was beneficial for harvesting. However, until reliable methods of overwintering and oversummering the fern were developed, organised use of the fern would be impossible. Because Azolla can only be grown from vegetative material, it requires protection during seasons when the weather is too harsh to grow.
