Biogeochemical Cycles — Carbon, Nitrogen, Water

Every atom that makes up your body has been cycled through countless organisms before you. The carbon in your proteins was once $\text{CO}_2$ in the atmosphere. The nitrogen in your DNA was once in the soil, or in rain, or in a bacterium. These elements never disappear — they cycle through living systems, the atmosphere, water, and rock.

Biogeochemical cycles track how essential elements move between the biotic (living) and abiotic (non-living) components of an ecosystem. They are called “biogeochemical” because they involve biological organisms, geological processes, and chemical transformations.

For CBSE Class 12 and NEET, these cycles carry high weightage in the Ecosystem chapter. Expect 3–5 marks on the carbon cycle, nitrogen cycle, or water cycle in most board exams.

Key Terms & Definitions

Biogeochemical Cycle: The pathway by which a chemical element or compound moves through the biotic and abiotic components of Earth.

Reservoir: A large pool where an element is stored — atmosphere, ocean, lithosphere (rocks/soil), biosphere.

Flux: The rate of transfer of an element between reservoirs.

Carbon Fixation: Conversion of atmospheric $\text{CO}_2$ into organic carbon compounds by photosynthesis.

Decomposition: Breakdown of organic matter by decomposers (bacteria, fungi) releasing elements back into the environment.

Nitrification: Conversion of $\text{NH}_3/\text{NH}_4^+$ → $\text{NO}_2^-$ → $\text{NO}_3^-$ by nitrifying bacteria.

Denitrification: Conversion of $\text{NO}_3^-$ → $\text{N}_2$ gas by denitrifying bacteria.

Nitrogen Fixation: Conversion of atmospheric $\text{N}_2$ into ammonia ( $\text{NH}_3$ ) by nitrogen-fixing bacteria.

Transpiration: Water loss from plant leaves through stomata.

Evapotranspiration: Combined evaporation from soil and transpiration from plants.

The Carbon Cycle

Carbon is the backbone of all life. It exists as $\text{CO}_2$ in the atmosphere, as dissolved carbonate in oceans, as organic molecules in living organisms, and as fossil fuels and limestone in rocks.

Major Carbon Fluxes

Into the biosphere (fixation):

Photosynthesis: $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ — land plants and phytoplankton fix ~120 Gt C/year.
Chemosynthesis: Some bacteria fix carbon using chemical energy (deep sea vents).

Out of the biosphere (release):

Respiration: All organisms release $\text{CO}_2$ during aerobic respiration.
Decomposition: Decomposers break down organic matter, releasing $\text{CO}_2$ and methane.
Combustion: Burning wood, coal, petroleum releases stored carbon rapidly.
Volcanism: Volcanic eruptions release $\text{CO}_2$ from the lithosphere.

Ocean’s Role

Oceans absorb ~30% of anthropogenic $\text{CO}_2$ emissions. The dissolved $\text{CO}_2$ forms carbonic acid ( $\text{H}_2\text{CO}_3$ ), acidifying the ocean — a process called ocean acidification — which threatens coral reefs and shellfish.

Human Impact

Burning fossil fuels has increased atmospheric $\text{CO}_2$ from ~280 ppm (pre-industrial) to ~420 ppm today. Deforestation removes carbon sinks. Together, these amplify the greenhouse effect, driving climate change.

NEET/CBSE Favourite: “Name the process by which carbon enters the biotic component” → Photosynthesis. “How is carbon returned to atmosphere from dead organisms?” → Decomposition + Respiration.

The Nitrogen Cycle

Nitrogen is 78% of the atmosphere, yet most organisms cannot use atmospheric $\text{N}_2$ directly. Life depends on specialised bacteria to “fix” nitrogen into usable forms.

Stages of the Nitrogen Cycle

1. Nitrogen Fixation

Atmospheric $\text{N}_2$ is converted to $\text{NH}_3$ (ammonia):

Biological fixation: Rhizobium (in legume root nodules), Azotobacter, Anabaena (cyanobacteria). These use the enzyme nitrogenase.
Atmospheric fixation: Lightning converts $\text{N}_2$ to $\text{NO}_x$ , which dissolves in rain as nitrate.
Industrial fixation: Haber process — $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ (used to make fertilisers).

2. Nitrification

$\text{NH}_4^+$ → $\text{NO}_2^-$ → $\text{NO}_3^-$

Performed by:

Nitrosomonas: converts $\text{NH}_4^+$ → $\text{NO}_2^-$
Nitrobacter: converts $\text{NO}_2^-$ → $\text{NO}_3^-$

Plants absorb nitrate ( $\text{NO}_3^-$ ) from soil.

3. Assimilation

Plants absorb $\text{NH}_4^+$ or $\text{NO}_3^-$ and incorporate nitrogen into amino acids, proteins, DNA, chlorophyll.

4. Ammonification

Decomposers break down organic nitrogen (dead organisms, excreta) back into $\text{NH}_3/\text{NH}_4^+$ . This returns nitrogen to the soil.

5. Denitrification

Pseudomonas and related bacteria convert $\text{NO}_3^-$ → $\text{N}_2$ gas. This returns nitrogen to the atmosphere, completing the cycle.

Nitrogen fixation: Rhizobium, Azotobacter, Anabaena
Nitrification: Nitrosomonas, Nitrobacter
Denitrification: Pseudomonas
Ammonification: various decomposer bacteria and fungi

Why Legumes Improve Soil

Rhizobium lives in root nodules of legumes (peas, beans, soybean, groundnut) in a mutualistic symbiosis — bacteria get carbohydrates from the plant; the plant gets fixed nitrogen. Crop rotation with legumes traditionally enriched soil nitrogen naturally.

The Water (Hydrological) Cycle

Water is unique — it cycles through all three states (solid, liquid, gas) and is essential for all life processes.

Main Processes

Evaporation: Water from oceans, lakes, soil → water vapour. Driven by solar energy. Oceans contribute ~86% of atmospheric water vapour.

Transpiration: Plants release water vapour through stomata. A single large tree can transpire 400 litres of water per day.

Condensation: Water vapour cools, forms clouds (liquid droplets on condensation nuclei).

Precipitation: Rain, snow, hail falls when droplets become heavy enough. About 80% falls back into oceans.

Infiltration: Water seeps into soil → replenishes groundwater (aquifers).

Surface runoff: Water flows into rivers, streams → back to oceans.

Transpiration + Evaporation = Evapotranspiration: The combined flux from land surface to atmosphere.

Key Numbers (for NEET)

Average residence time of water molecule in atmosphere: ~9 days
In ocean: ~3,200 years
In glaciers: 20,000–100,000 years

Solved Examples

Example 1 — CBSE Level (3 marks)

Q: Explain the role of microorganisms in the nitrogen cycle.

Solution: Microorganisms play four critical roles:

Nitrogen fixation — Rhizobium (symbiotic in legumes), Azotobacter (free-living) fix atmospheric $\text{N}_2$ → $\text{NH}_3$ .
Nitrification — Nitrosomonas converts $\text{NH}_4^+$ → $\text{NO}_2^-$ ; Nitrobacter converts $\text{NO}_2^-$ → $\text{NO}_3^-$ (plant-usable form).
Ammonification — Decomposer bacteria convert organic nitrogen in dead matter → $\text{NH}_4^+$ , returning it to soil.
Denitrification — Pseudomonas converts $\text{NO}_3^-$ → $\text{N}_2$ , returning nitrogen to atmosphere.

Without these microorganisms, the nitrogen cycle would stop — fixed nitrogen would accumulate and atmospheric nitrogen couldn’t be used.

Example 2 — NEET Level

Q: In which step of the nitrogen cycle does energy fixation occur?

Answer: Nitrogen fixation. The nitrogenase enzyme (in Rhizobium etc.) requires significant energy (16 ATP per $\text{N}_2$ fixed) to break the very stable $\text{N}\equiv\text{N}$ triple bond.

Exam-Specific Tips

CBSE Class 12 Ecosystem chapter: Carbon cycle, nitrogen cycle, and water cycle are standard diagram questions. For nitrogen cycle, always label all five steps AND the bacteria responsible. Diagrams fetch marks even if one step is unclear — show you know the sequence.

NEET 2023 had a question: “Which of the following bacteria is NOT involved in the nitrogen cycle?” — knowing all four types of nitrogen-cycle bacteria (fixing, nitrifying, ammonifying, denitrifying) and representative species is essential.

A memory aid for the nitrogen cycle order: Fix-Nitrify-Assimilate-Ammonify-Denitrify → FNAAD. Or think of the fate of nitrogen: from air → soil (fix) → soil oxidised (nitrify) → plant (assimilate) → decomposed (ammonify) → back to air (denitrify).

Common Mistakes to Avoid

Mistake 1: Confusing nitrification and nitrogen fixation. Nitrogen fixation = $\text{N}_2$ → $\text{NH}_3$ (bringing N from atmosphere). Nitrification = $\text{NH}_3$ → nitrate (in soil). Students mix these up in MCQs.

Mistake 2: Saying plants directly use atmospheric nitrogen. Only prokaryotes (bacteria, cyanobacteria) can fix atmospheric $\text{N}_2$ . Plants absorb $\text{NH}_4^+$ or $\text{NO}_3^-$ from soil, not $\text{N}_2$ directly.

Mistake 3: Forgetting the ocean in the carbon cycle. The ocean is a massive carbon reservoir and the largest carbon sink. Ignoring it gives an incomplete answer in 5-mark questions.

Mistake 4: Listing only one organism for nitrogen fixation. Rhizobium is the most famous, but free-living fixers (Azotobacter, Anabaena) are equally important for NEET/CBSE.

Mistake 5: Forgetting that transpiration is part of the water cycle. Students focus on evaporation and precipitation but omit transpiration. Transpiration accounts for ~10% of all moisture in the atmosphere — significant enough to be a standard exam point.

Practice Questions

Q1. Name the process by which atmospheric $\text{N}_2$ is converted to a plant-usable form. Name two bacteria involved.

Process: Nitrogen fixation. Bacteria: Rhizobium (symbiotic in legumes), Azotobacter (free-living in soil).

Q2. What is ammonification? When does it occur?

Ammonification is the conversion of organic nitrogen (from dead organisms and excreta) into ammonia/ammonium by decomposer bacteria and fungi. It occurs whenever organisms die and their proteins and nucleic acids are broken down by decomposers.

Q3. Why is the carbon cycle particularly important to study in the context of climate change?

Burning fossil fuels and deforestation are adding carbon to the atmosphere faster than natural sinks (forests, oceans) can absorb it. This increases atmospheric $\text{CO}_2$ , enhancing the greenhouse effect and driving global warming. Understanding the carbon cycle helps quantify these fluxes and design mitigation strategies.

Q4. What is the role of Nitrosomonas in the nitrogen cycle?

Nitrosomonas is a nitrifying bacterium that oxidises ammonium ( $\text{NH}_4^+$ ) to nitrite ( $\text{NO}_2^-$ ). This is the first step of nitrification.

Q5. Differentiate between the gaseous and sedimentary types of biogeochemical cycles.

Gaseous cycles (e.g., carbon, nitrogen, oxygen, water) have their major reservoir in the atmosphere or ocean. They are self-regulating and global in reach. Sedimentary cycles (e.g., phosphorus, sulphur partially, calcium) have their major reservoir in rocks/soil. They are slower, more localised, and lack a significant atmospheric phase. Phosphorus has no gaseous form, so it cycles only through soil, water, and organisms.

FAQs

What is the difference between a biogeochemical cycle and a food chain?

A food chain traces the flow of energy and matter through trophic levels (producer → herbivore → carnivore). A biogeochemical cycle traces a single element (C, N, P, etc.) through living and non-living components over time. A food chain is one-directional and ends with decomposers; a cycle is circular and continues indefinitely.

Which biogeochemical cycle is most affected by human activity?

The carbon cycle — through fossil fuel combustion and deforestation — and the nitrogen cycle — through synthetic fertilisers (Haber process nitrogen) — are most significantly altered. Excess nitrogen in ecosystems causes eutrophication of water bodies.

Why is the phosphorus cycle called a sedimentary cycle?

Phosphorus has no significant gaseous phase. It moves from rocks (via weathering) to soil to water to organisms, and back to sediment when organisms die and phosphate precipitates. There is no atmospheric reservoir, making it much slower and more localised than gaseous cycles.

What happens to the water cycle in a deforested area?

Without trees, transpiration drops dramatically, reducing local rainfall. Surface runoff increases (soil cannot absorb water without root systems), leading to soil erosion and flooding. Groundwater replenishment decreases. The local water cycle is severely disrupted.

Why is lightning important for the nitrogen cycle?

Lightning provides enough energy to break the $\text{N}\equiv\text{N}$ triple bond and oxidise $\text{N}_2$ to $\text{NO}_x$ (nitrogen oxides), which dissolve in rain as $\text{HNO}_3$ (nitric acid) and fall to the soil as $\text{NO}_3^-$ — plant-usable nitrate. Lightning contributes an estimated 5–8% of global nitrogen fixation.