Azolla – Anabaena SYMBIOSIS

Azolla – Anabaena SYMBIOSIS

Azolla

• Azolla is a free-floating water fern (only know Pteridophyte that lives in Symbiosis) that floats in water and fixes atmospheric nitrogen in association with nitrogen fixing Blue Green Algae Anabaena azollae.
• Fern is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via Spores and have neither Seeds nor Flowers.
• Azolla is also called as Mosquito fern, Duckweed fern, Fairy moss or Water fern.
• Azolla belongs to the family Salviniaceae.
• Azolla is Omnipresent in nature that means it is present in all places at all times.
• Azolla is found on water in ponds, lakes, swamps, ditches and paddy fields of temperate and tropical regions. Because of its rapid growth, high Nitrogen content and ability to grow in water, it has been exploited as a fertilizer for Rice.

Figure – 1: Azolla

• Azolla is used as biofertilizer for wetland rice and it is known to contribute 40 -60 kg N ha^-1 per rice crop.
• The agronomic potential of Azolla is quite significant particularly for rice crop and it is widely used as biofertilizer for increasing rice yields.
• The important factor in using Azolla as a biofertilizer for rice crop is its quick decomposition in soil and efficient availability of its nitrogen to rice.
• In tropical rice soils the applied Azolla mineralizes rapidly and its nitrogen is available to the rice crop in very short period. 

Common Species of Azolla

There are 7 living species of Azolla

Azolla pinnata (native to India)

Azolla caroliniana

Azolla rubra

Azolla filiculoides

Azolla nilotica

Azolla Mexicana

Azolla microphylla.

Azolla – Anabaena Symbiosis

• Each leaf of Azolla consists of two lobes, a (i) Thick Aerial dorsal lobe and (ii) Thin ventral lobe occasionally of a slightly larger size. The dorsal lobe is green and has a blue green algal symbiont (Anabaena azollae) within a central cavity.
• All the species of the genus Azolla harbour a filamentous N₂-fixing Cyanobacterium Anabaena until now referred as Anabaena azollae.
• The heterocyst of the symbiont Anabaena is the site of Nitrogen fixation (Symbiotic nitrogen fixation).
• Azolla provides nutrients and a protective leaf cavity for Anabaena, which in turn provided nitrogen for the Fern.
• The high N₂ fixing ability, rapid growth, high biomass accumulation and Nitrogen content determines the potential of Azolla as a biofertilizer for rice.
• Biological nitrogen fixation through Azolla - Anabaena complex is considered a potential biological system for increasing rice yield at comparatively low cost. The ability of Azolla to fix N₂ is about 1.1 kg N/ha/day. The doubling time varies between 2 and 10 days for most species and maximum biomass ranged between 0.8 to 5.2 t dry matter/ha with an average of 2.1 t/ha.

Mass multiplication of Azolla in Cement Tanks/Polythene lined pits

• Select a field and prepare thoroughly and level uniformly.
• Make the pit with the size of 10 sq. meter (12 feet × 9 feet) and make the floor of pit even by removing any roots and other plants.
• Spread out silpauline sheet of suitable size (for eg. 15 × 12 feet for a pit size of 12 × 9 feet) even without any holes and fix the edges with mud or bricks.
• Pour approximately 30 - 35 kg (@3 - 3.5 kg/sq.m) of sieved fertile soil or make a soil bed of 1 to 2 cm thick evenly.
• Maintain water level to a height of 10 cm.
• Mix 4 kg (@ 400 g/sq.m) of cattle dung in 20 litres of water and sprinkle in the field.
• Apply 25 g Super phosphate (@2.5 g/sq.m) as basal dose.
• Inoculate fresh Azolla biomass @ 1 kg to each pot.
• Apply super phosphate @ 25 g as top-dressing fertilizer on 4^th and 8^th day after Azolla inoculation.
• Apply Carbofuran (furadan) granules @ 25 g/pit (@2.5g/sq.m) on 7^th day after Azolla inoculation.

Figure – 2: Mass multiplication of Azolla in polythene lined pits

• Maintain the water level at 10 cm height throughout the growth period of two or three weeks.
• Under optimal maintenance and climatic condition Azolla grows rapidly and fill the pit within 10 - 15 days.
• There after the yield of Azolla is around 2 - 2.5 kg per day (@ 200 -250g/sq.m/day). Totally 8 - 10 kg of Azolla can be harvested.
• Wash properly with water to get rid of foul smell.
• The same procedure can be followed for multiplication in cement tanks also.

Mass multiplication of Azolla under Field condition

• A simple Azolla nursery method for large scale multiplication of Azolla in the field has been evolved for easy adoption by the farmers.
• Select a wetland field and prepare thoroughly and level uniformly.
• Mark the field into one cent plots (20 × 2 m) by providing suitable bunds and irrigation channels.
• Maintain water level to a height of 10 cm.
• Mix 10 kg of cattle dung in 20 litres of water and sprinkle in the field.
• Apply 100 g super phosphate as basal dose.
• Inoculate fresh Azolla biomass @ 8 kg to each pot.
• Apply super phosphate @ 100 g as top dressing fertilizer on 4^th and 8^th day after Azolla inoculation.
• Apply Carbofuran (furadan) granules @ 100 g/plot on 7^th day after Azolla inoculation.
• Maintain the water level at 10 cm height throughout the growth period of two or three weeks.
• Harvest the Azolla from 10 - 15 days at the rate of 8-10kg/plot daily, totally 50 kg of Azolla can be harvested.

Figure – 3: Mass multiplication of BGA under Rice field condition