Q.(a) Explain the role of bacteria in soil fertility. OR

The Nitrogen Cycle: Why Life Needs a Round Trip

Imagine you are a nitrogen atom. You spend most of your life floating lazily in the air as part of a nitrogen molecule ($N_2$). It's comfortable, but boring. The problem is that you are locked in a triple bond with another nitrogen atom — one of the strongest bonds in chemistry. Almost nothing can break it. And yet, every living thing desperately needs you. Proteins, DNA, ATP — all of them contain nitrogen. So how does a lazy, unreactive gas become the building block of life?

The answer is a cycle — a series of transformations that move nitrogen from the air into the soil, into living things, and eventually back into the air. That is the nitrogen cycle.

The Precise Statement

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted between its various chemical forms — from atmospheric dinitrogen ($N_2$) to biologically usable compounds like ammonia ($NH_3$) and nitrate ($NO_3^-$), and then back to $N_2$ — through the processes of nitrogen fixation, nitrification, assimilation, ammonification, and denitrification.

Step-by-Step Walkthrough

1. Nitrogen Fixation — Breaking the Unbreakable

Only a few organisms can crack the $N \equiv N$ triple bond. These are nitrogen-fixing bacteria — Rhizobium (living in root nodules of legumes), Azotobacter (free-living in soil), and Anabaena (a cyanobacterium). They use an enzyme called nitrogenase to convert $N_2$ into ammonia ($NH_3$).

Lightning also does this — the immense energy splits $N_2$ and $O_2$, forming nitrogen oxides that dissolve in rain and fall as nitrates. But lightning contributes only about 10% of fixed nitrogen. The rest is biological.

Notice the ATP cost — fixation is energetically expensive. That's why only a few organisms do it.

2. Nitrification — Making It Plant-Friendly

Plants cannot absorb ammonia directly from the soil in large amounts. They prefer nitrate ($NO_3^-$). So soil bacteria convert ammonia into nitrate in two steps:

• Step 1: Nitrosomonas oxidises $NH_3$ to nitrite ($NO_2^-$)
• Step 2: Nitrobacter oxidises $NO_2^-$ to nitrate ($NO_3^-$)

3. Assimilation — Plants Eat Nitrate

Plants absorb nitrate ($NO_3^-$) through their roots and reduce it back to ammonia inside their cells, then use that ammonia to make amino acids, nucleotides, and chlorophyll. Animals get their nitrogen by eating plants (or other animals).

4. Ammonification — Recycling Dead Matter

When plants and animals die, or when animals excrete waste, decomposer bacteria and fungi break down the organic nitrogen (proteins, nucleic acids) and release ammonia ($NH_3$) back into the soil. This process is called ammonification. The ammonia can then re-enter the nitrification pathway.

5. Denitrification — Closing the Loop

Finally, some bacteria — Pseudomonas and Clostridium — use nitrate as an electron acceptor in place of oxygen (anaerobic respiration). They convert $NO_3^-$ back into $N_2$ gas, which escapes into the atmosphere. This is denitrification. …