Biogeochemical cycles overview — water, carbon, nitrogen, phosphorus compared

Question

Compare the four major biogeochemical cycles — water, carbon, nitrogen, and phosphorus. Which cycles have a gaseous phase and which are sedimentary? What are the key reservoirs and processes in each?

(NEET 2023 had a question on nitrogen cycle steps; CBSE 12 boards regularly ask comparisons)

Solution — Step by Step

The first thing to establish: gaseous cycles have their main reservoir in the atmosphere, while sedimentary cycles have their reservoir in rocks/soil.

Gaseous: Water cycle, Carbon cycle, Nitrogen cycle
Sedimentary: Phosphorus cycle (no significant gaseous phase)

This classification is a direct NEET favourite.

The water cycle is powered by solar energy and gravity. Key processes: evaporation, transpiration, condensation, precipitation, and runoff.

The ocean is the largest reservoir (97% of water). The cycle has no chemical transformation — water just changes physical state.

Carbon moves between atmosphere ( $\text{CO}_2$ ), oceans (dissolved $\text{CO}_2$ ), biomass (organic compounds), and fossil fuels.

Key processes: photosynthesis (fixes $\text{CO}_2$ ), respiration (releases $\text{CO}_2$ ), decomposition, and combustion. The ocean is actually the largest carbon sink.

Atmosphere has 78% $\text{N}_2$ , but organisms cannot use it directly. The cycle involves:

Nitrogen fixation: $\text{N}_2 \rightarrow \text{NH}_3$ (by Rhizobium, Azotobacter, lightning)
Nitrification: $\text{NH}_3 \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^-$ (by Nitrosomonas, Nitrobacter)
Assimilation: Plants absorb $\text{NO}_3^-$
Ammonification: Organic N $\rightarrow$ $\text{NH}_3$ (decomposers)
Denitrification: $\text{NO}_3^- \rightarrow \text{N}_2$ (by Pseudomonas)

Phosphorus exists mainly in rock and soil as phosphate ( $\text{PO}_4^{3-}$ ). It enters the biotic system through weathering of rocks, is absorbed by plants as phosphate ions, passes through the food chain, and returns to soil through decomposition.

No gas phase means this cycle is slow and often the limiting nutrient in ecosystems.

flowchart LR
    subgraph Gaseous["Gaseous Cycles"]
        W[Water Cycle] --> WR[Reservoir: Oceans]
        C[Carbon Cycle] --> CR[Reservoir: Atmosphere + Oceans]
        N[Nitrogen Cycle] --> NR[Reservoir: Atmosphere 78%]
    end
    subgraph Sedimentary["Sedimentary Cycle"]
        P[Phosphorus Cycle] --> PR[Reservoir: Rocks/Soil]
    end
    W -.->|Solar energy driven| WP[Evaporation - Condensation - Precipitation]
    C -.->|Photosynthesis/Respiration| CP[CO2 fixation and release]
    N -.->|Bacterial processes| NP[Fixation - Nitrification - Denitrification]
    P -.->|Weathering| PP[Rock weathering - Absorption - Decomposition]

Why This Works

All biogeochemical cycles maintain a balance between inputs and outputs in each reservoir. When we burn fossil fuels, we add carbon to the atmosphere faster than photosynthesis and ocean absorption can remove it — that is the climate change problem in one sentence.

The nitrogen cycle is the most exam-relevant because it involves specific bacteria at each step. NEET loves asking which bacterium does what. Phosphorus is important because it is the limiting factor in most freshwater ecosystems — eutrophication happens when excess phosphorus enters water bodies.

Alternative Method

A consolidated comparison table for quick revision:

Feature	Water	Carbon	Nitrogen	Phosphorus
Type	Gaseous	Gaseous	Gaseous	Sedimentary
Main reservoir	Oceans	Atmosphere + Oceans	Atmosphere	Rocks
Key process	Evaporation	Photosynthesis	N-fixation	Weathering
Limiting?	No	No	Yes (for some)	Yes (freshwater)
Human impact	Dam, irrigation	Fossil fuel burning	Fertilizers	Mining, fertilizers

Common Mistake

Students frequently confuse nitrification and nitrogen fixation. Nitrogen fixation converts atmospheric $\text{N}_2$ into ammonia ( $\text{NH}_3$ ) — done by Rhizobium and Azotobacter. Nitrification converts ammonia to nitrites and then nitrates ( $\text{NH}_3 \rightarrow \text{NO}_3^-$ ) — done by Nitrosomonas and Nitrobacter. These are completely different steps with different bacteria.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

Try These Next