In which of the following form, most plants absorb nitrogen?
1. Proteins
2. Nitrates and nitrites
3. Urea
4. Atmospheric nitrogen

A. 1 and 2
B. 2 and 3
C. 3 and 4
D. 1 and 4

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Hint: Nitrogen is an essential nutrient for plant growth, development, and reproduction. The cyclic movement of the nitrogen from the atmosphere to the soil and back into the atmosphere through plants, animals, and microorganisms is termed as the nitrogen cycle. The nitrogen cycle is a continuous pathway and it contains five steps: nitrogen fixation, nitrogen assimilation, ammonification, nitrification, and de-nitrification.

Complete answer:
-The gaseous di-nitrogen is fixed into inorganic nitrogenous substances by a process known as nitrogen fixation. It occurs in two methods:
a) The abiological method occurs in the atmosphere due to UV radiations and lightening that converts di-nitrogen into nitric oxide which is further oxidized to nitrogen dioxide. These oxides dissolve in rainwater and reach soil as nitrous and nitric acid. These acids react with alkali radicals of soil and form nitrates. The soluble nitrates are directly absorbed by plants.
The reaction of the physical nitrogen fixation is as follows:
1) ${ N }_{ 2 }\quad +\quad { O }_{ 2 }\quad \rightarrow \quad 2NO$
2) $2{ NO }\quad +\quad { O }_{ 2 }\quad \rightarrow \quad 2N{ O }_{ 2 }$
b) The biological method involves the conversion of di-nitrogen into nitrogenous compounds by prokaryotes and is known as biological nitrogen fixation. Nitrogen assimilation is a process of absorbing nitrates and ammonia to produce organic nitrogen.
- Thus, the answer is B: Plant absorbs nitrogen in the form of nitrates, nitrites, and urea (ammonia form).

Additional information:
1) Nitrates and ammonia are absorbed by plants and converted into constituents of the organic nitrogen. When plants are eaten by animals this organic nitrogen is passed on into the animal body.
2) The process of conversion of organic nitrogenous compounds from the dead bodies of the plant and animals into ammonia is called Ammonification. It is a mineralization process. The bacteria responsible for this are called ammonifying bacteria.
3) Nitrification is an oxidative and an exergonic process. The conversion of ammonia into nitrates and nitrites by bacteria is called nitrification. Such bacteria are called nitrifying bacteria. Ammonia is converted into nitrites by bacteria like nitrosamines and nitro-coccus.
4) Nitrogen is vital because it is the major component of chlorophyll, the compound by which the plants use sunlight energy to produce sugars from the water and carbon dioxide. It is also a major component of amino acids, the building blocks of protein without protein plants withers and dies. Some proteins act as structural units in plant cells while others act as enzymes making possible many of the biochemical reactions on which life is based. Nitrogen is the component of the energy transfer compounds such as ATP. A very small amount of organic nitrogen may exist as soluble organic compounds such as urea that may be slightly available to plants.

Note: Most plants fulfill their need for nitrogen requirements during growth from simple nitrogenous compounds such as urea as well as nitrates and nitrites. Plants are unable to take atmospheric nitrogen and cannot utilize complex proteins. The bacterial association of plants helps them to acquire atmospheric nitrogen in root nodules, which fixes the nitrogen and converts it to a utilizable form to plants such as nitrates and nitrites.