Question

which one of the following microbes does not fix atmospheric nitrogen
1) Azospirillum
2) Azotobacter
3) Anabena
4) Glomus​

Answer 1

azospirillum



Photosynthetic nitrogen-fixing microorganisms help maintain the nitrogen level

of soil in rice paddies when environmental factors favor the growth of micro·

organisms. Our studies showed that blue-green algae in particular have a signifi·

cant role in nitrogen-fixation in light. The most active nitrogen-fixation by micro-

organisms occurred in the soil shortly after it had been submerged under light.

The longer the submergence, the less nitrogen microorganisms were fixed. In a

greenhouse experiment, the fixed nitrogen appeared not to be immediately available

to the rice plant. The amount of nitrogen that can be fixed in the field by

nitrogen-fixing microorganisms in paddy water was estimated using the acetylene

reduction method during the rice-growing period. The amount of nitrogen fixation

by these microorganisms is not sufficient to account for the amount of nitrogen

uptake by rice during the rice-growing period.

In many parts of Asia, fertilizers are not used, and rice production depends on the

natural fertility of the soil. A continuing supply of nitrogen over the years, despite

removal of this element by rice crops, is claimed to result from the fixation of atmos-

pheric nitrogen by microorganisms in paddy fields ( 1, 2 ). If the source of nitrogen

for rice crops were only soil organic matter, it is estimated that the total nitrogen

present in the soil would not last £or many years. Thus, the maintenance of soil

fertility in rice paddies for years can be explained only by microbial nitrogen fixation

in the soil, since its possible sources, i.e. rain or irrigation water and rice plant res-

idues, are not significant enough to bring about a nitrogen balance.

Various kinds of microorganisms fix atmospheric nitrogen, but it is not known which

of these actually take part in nitrogen fixation in paddy soils. This paper reports a

possible effect of photosynthetic microorganisms on the fertility of a Philippine soil.