Biogeochemical Cycles

You’ll be able to

Trace carbon, nitrogen, and phosphorus atoms through their major reservoirs
Explain why the phosphorus cycle has no significant atmospheric phase
Connect the water cycle’s processes to the movement of nutrients

Matter cycles between reservoirs

Unlike energy, the atoms that make up living things are never lost — they cycle endlessly through reservoirs (the atmosphere, oceans, rock, soil, and organisms). These biogeochemical cycles move an element between its living (biotic) and nonliving (abiotic) forms. A reservoir where an element sits for a long time, like carbon in limestone, is called a sink.

The carbon and nitrogen cycles — the atmospheric ones

Both carbon and nitrogen have huge atmospheric reservoirs. Carbon moves out of the air when producers fix CO₂ into sugars during photosynthesis, and returns through cellular respiration, decomposition, and combustion; buried organic carbon becomes fossil fuels, and burning them returns that long-stored carbon far faster than nature removes it. Nitrogen is 78% of air as N₂, but that triple-bonded gas is useless to most life until nitrogen fixation (by bacteria or lightning) turns it into ammonia, nitrification converts that to nitrates plants absorb, assimilation builds it into proteins and DNA, ammonification returns it to soil as organisms decay, and denitrification sends N₂ back to the atmosphere.

The phosphorus and water cycles

Phosphorus is the odd one out: it has no significant atmospheric (gas) phase. It is released only by the slow weathering of rock, taken up by plants as phosphate (PO₄³⁻), passed through food webs, and returned to soil and sediment by decomposition — which makes it slow-moving and often the limiting nutrient in ecosystems. The water cycle ties everything together: water moves by evaporation and transpiration into the air, condensation into clouds, precipitation back to the surface, and runoff or infiltration into rivers and groundwater — and along the way it is the vehicle that carries dissolved nitrogen and phosphorus through soils and into waterways.

Key difference among the cycles

C and N: large atmospheric reservoir · P: no atmospheric phase

Carbon (CO₂) and nitrogen (N₂) both cycle through the air; phosphorus moves through rock, water, and soil only — which makes it move slowly.

Worked example

A farmer notices that adding phosphate fertilizer dramatically boosts crop growth, while adding extra nitrogen fertilizer does almost nothing. What does this reveal, and why is phosphorus so often the culprit?

1.A nutrient that boosts growth when added was the one in short supply — it was the limiting nutrient.
2.Adding nitrogen did little, so nitrogen was already adequate; adding phosphorus helped, so phosphorus was limiting here.
3.Phosphorus has no atmospheric reservoir — it enters ecosystems only through slow weathering of rock.
4.With such a slow natural resupply, phosphorus is commonly the scarcest nutrient, so adding it removes the bottleneck on growth.

Answer: Phosphorus was the limiting nutrient; because it has no gas phase and comes only from slow rock weathering, it is frequently the growth-limiting element.

Checkpoint

Which characteristic makes the phosphorus cycle different from the carbon and nitrogen cycles?

Watch out

Nitrogen fixation and nitrification are easy to confuse. Fixation converts inert atmospheric N₂ into ammonia; nitrification then converts that ammonia into nitrates that plants can absorb. Different steps, different bacteria — keep the order straight: N₂ → ammonia → nitrite → nitrate.

Checkpoint

Burning fossil fuels affects the carbon cycle primarily by:

On the exam

Expect a question that asks you to name the process at a specific arrow in a cycle diagram. Memorize the direction of each: photosynthesis pulls CO₂ down, respiration/combustion push it up; fixation pulls N₂ down, denitrification pushes it up.

Answer the 2 checkpoints as you read.