Explain the nitrogen cycle with diagram

The atmosphere is 78% nitrogen ($\text{N}_2$), but plants and animals can’t use nitrogen gas directly. It must first be converted into usable forms like nitrates ($\text{NO}_3^-$) or ammonia ($\text{NH}_3$). The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, and living organisms.

Three ways nitrogen gets fixed:

1. Biological fixation — Rhizobium bacteria in root nodules of leguminous plants (like peas, beans) convert $\text{N}_2$ into ammonia. Free-living bacteria like Azotobacter also do this.
2. Lightning — the enormous energy in lightning bolts converts $\text{N}_2$ and $\text{O}_2$ into nitrogen oxides, which dissolve in rain and enter the soil as nitrates.
3. Industrial fixation — fertiliser factories convert $\text{N}_2$ into ammonia (Haber process) for use in agriculture.

Nitrifying bacteria (like Nitrosomonas and Nitrobacter) in the soil convert ammonia into nitrites and then nitrates. Plants absorb nitrates through their roots. This is the form plants actually use.

Plants absorb nitrates and build them into proteins and nucleic acids. Animals eat plants and get their nitrogen from plant proteins.

When plants and animals die, or when animals excrete waste, decomposers (bacteria and fungi) break down the organic matter and release ammonia back into the soil. This process is called ammonification.

Denitrifying bacteria (like Pseudomonas) convert soil nitrates back into $\text{N}_2$ gas, which returns to the atmosphere. This completes the cycle.

The cycle: Atmosphere $\rightarrow$ Fixation $\rightarrow$ Nitrification $\rightarrow$ Plant uptake $\rightarrow$ Animal consumption $\rightarrow$ Death/Decay $\rightarrow$ Ammonification $\rightarrow$ Denitrification $\rightarrow$ Atmosphere