Question

Nitrogen fixer soil organism belong to
(a) Mosses
(b) Bacteria
(c) Green algae
(d) Soil fungi

Answer 1

Hint: These are microscopic, single-celled organisms that exist in every environment, both inside and out of every organism. Some of them are harmful, but most of them serve as a useful purpose. They support many kinds of life, both in plant and animal, and they are utilized in industrial and medicinal processes.

Complete answer:
The atmospheric nitrogen isn't available for the plants. By the method of nitrogen fixation, atmospheric N is regenerated into ammonia and made available for plants. The nitrogen fixer soil organisms belong to the bacterium. Samples of free-living nitrogen-fixing bacterium include blue-green algae, Azotobacter, eubacterium. Examples of symbiotic nitrogen-fixing bacteria include Rhizobium, Bradyrhizobium.

Nitrogen is a very important macronutrient because it's a part of nucleic acids and proteins.
Atmospheric nitrogen, which is a diatomic molecule N$_2$, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot cash in this nitrogen because they did not have the required enzymes to convert it into biologically useful forms. However, nitrogen is often “fixed.” It is often converted to ammonia (NH$_3$)) through biological, physical, or chemical processes. Biological nitrogen fixation is the conversion of atmospheric nitrogen (N$_2$)) into ammonia (NH$_3$), which is exclusively administered by prokaryotes, like soil bacteria or cyanobacteria. Biological processes contribute to 65 percent of the nitrogen utilized in agriculture.

The most important source of Biological Nitrogen Fixation is the symbiotic interaction between soil bacteria and legume plants, including many crops important to humans. The ammonia resulting from fixation is often transported into the plant part and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, like soybeans and peanuts, contain high levels of protein and are among the most important agricultural sources of protein within the world.

So, the correct answer is, “Bacteria.”

Note: Nitrogen fixation by the Bacteria:
The symbiotic nitrogen-fixing bacteria invades into the hairs of host plants and they get multiplied
which will stimulate the formation of root nodules, enlargements of plant cells. Within the nodules,
the bacteria convert free nitrogen to ammonia, which the host plant utilizes for its development and
from here the plants get the nitrogen.