Nitrogen-Fixing Activity of Paddy Soil in Pots Under Various Treatments

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Nitrogen-Fixing Activity of Paddy Soil in Pots Under Various Treatments

Nitrogen-fixing activity of paddy soil in pots under various treatments (Kalininskyaya et al., 1973)

Treatment	Atom % excess 15N in soil sample after incubation*	Mean nitrogen fixed in m5l/kg soil/month
1. Control	0.183	8065
2. 50 kg (NH4)2SO₄/ha	0.095	4.22
3. Rice straw (5 t/ha)	0.221	11.92
4. Rice straw (5 t/ha) + 50 kg (NH4)2SO₄/ha	0.226	11.73
5. Rice straw (10 t/ha)	0.770	39.34
6. Rice straw (10 t/ha) +50 kg (Nli4hSO₄lha	0.780	50.00

From green house pot-culture experiments, using rice plants grown under flooded conditions the International Rice Research Institute, Manila (IRRI Annual Report, 1978) has brought out a nitrogen balance sheet which indicates that photosynthetic nitrogen-fixing microorganisms increase the accretion of nitrogen to rice plat1ts in flooded soil. In terms of N in four crops (mg/pot), the amount of nitrogen removed by the crop in standard pots where blue-green algae were spontaneously growing was 967.

When pots were covered with black cloth so as to exclude blue-green algae, the amount of nitrogen removed by the crop was reduced to 915 whereas in fallow-flooded pots without plants no significant amount of nitrogen was fixed.

In pots containing blue-green algae plus P and Fe, the amount of nitrogen recovered from the crop increased to 1093 and this was further increased to 1316 in the presence of Azolla.

C₂H₂ reducing activity of bacteria isolated from waterlogged paddy soil (from V.R. Rao, 1973)

Bacteria	N₂[C₂H₂C₂H₄] fixed in* μg/ml medium/day
Bacteria	Aerobic system	Anaerobic system
Mycobacterim flavum	5.72	7.70
Pseudomonas sp.	3.79	5.32
Bacillus sp.	0.084	0.140
Bacillus polymyxa	7.88	7.88
Bacillus polymyxa	3.65	5.36
Mycobacterium sp.	5.30	13.65
Pseudomonas sp.	0.14	1.00
Bacillus polymyxa	11.38	11.38
Bacillus polymyxa	6.83	6.83
Bacillus polymyxa	5.32	9.10
Bacillus polymyxa	3.50	3.15
Mycobacterium sp.	15.17	15.93

From the foregoing account, it is clear that the waterlogged rice ecosystem provides a congenial microhabitat for nitrogen-fixing bacteria and blue-green algae to thrive. The current knowledge indicated that photosynthetic blue-green algae are the dominant nitrogen fixers in rice fields.

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Nitrogen Fixing Activity of Paddy Soil in Pots under Various Treatments Some New Nitrogen Fixing Bacterial Associations Several Nitrogen Fixing Bacteria Response of Plants Azospirillum Inoculation The Concept of C4 Dicarboxylic Acid Pathway Nitrogen Fixation by Rhizobium in a Free Living State

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