How is nitrogen incorporated into the soil? why do animals need to get nitrogen from plants?

Question

How is nitrogen incorporated into the soil? why do animals need to get nitrogen from plants?

Answer 1

Nitrogen is taken from the atmosphere by bacteria. These can be the rhizobacteria living in root nodules, free living azobacteria or actinomycetes. In root nodules they provide nitrogen directly to the leguminous plants that host them but the other bacteria must be consumed by protozoans to release the nitrogen to plants. The bacteria protozoa eat contain more nitrogen than the protozoa can use, so they excrete the excess as ammonium. Why protozoa release excess nitrogen is that they need less than bacteria in cell construction. They release the excess nitrogen in the form of ammonium (NH4+) where some is taken by other bacteria but some is taken by the plants.

The bacteria are the only source of organic nitrogen, neither plants nor animals can fix nitrogen from the atmosphere for themselves so all their nitrogen can be traced back to the bacteria. These bacteria's numbers fluctuate but are frequently in the range of one million CFU per gram of soil.

Different organisms use different ratios of carbon to nitrogen but none need as much as the bacteria that first fix it into a usable organic.

Carbon C:N nitrogen ratios:

bacteria are 3:1 - 7:1 so use the most nitrogen

protozoa are 10:1 and up

Fungi are 7:1 - 25:1

Plants are 40:1 and use the least nitrogen

Animals are 14:1

Answer 2

[Answer: Nitrogen from ammonium (NH3/NH4+ [in solution]) is present in soils and fertilizers.  Animals require nitrogen because it s a major biomolecule required to sustain life of multi-cellular heterotrophs -- they can't take in N2 (gas), NO3-, or NH4+ as their source of nitrogen.  Their only option is to take their nitrogen from CxHyNz (organic) molecules, which in this case will be from the plants.]

Answer 3

In addition to the above answers which are quite explanatory, I would like to add more to this.  The nitrogen is also fixed  by the lightening that takes place during thunder storms.  At very very high temperature during lightening the atmospheric nitrogen reacts with the water molecules and forms NH3 i.e.  ammonia. This is brought down to soil along with rain water and it forms ammonium salts when reacts with soil. This occurs in a very less amount as compared to fixation of nitrogen by soil bacteria.