Nutrient Cycles

The cycle:

1. PHOTOSYNTHESIS — plants take CO₂ from air, fix into glucose.
2. FEEDING — animals eat plants, taking in carbon.
3. RESPIRATION — all organisms break down glucose, releasing CO₂.
4. DECOMPOSITION — when organisms die, decomposers break them down, releasing CO₂.
5. FOSSILISATION — over millions of years, dead organisms become coal, oil, gas (carbon stored).
6. COMBUSTION — burning fossil fuels and wood releases stored CO₂ rapidly.

The balance.

• Without human activity, in/out are balanced.
• HUMANS BURN fossils faster than photosynthesis can absorb → CO₂ rising in atmosphere → GLOBAL WARMING.

Worked qualitative. Why does deforestation make global warming worse?

• Trees absorb CO₂ via photosynthesis.
• Cutting trees REMOVES this CO₂ sink.
• BURNING the wood RELEASES the stored carbon.
• Double effect: less absorption + more release.
• Replanting trees is one strategy to mitigate.

Cambridge tip. Memorise the 6 steps. Always include both PHOTOSYNTHESIS and RESPIRATION (the main flux).

Steps:

1. EVAPORATION — water from oceans, lakes, rivers turns to vapour (sun's heat).
2. TRANSPIRATION — plants release water vapour through stomata.
3. CONDENSATION — vapour rises into cooler air → forms clouds (tiny water droplets).
4. PRECIPITATION — droplets combine, fall as rain, snow, hail, sleet.
5. RUNOFF — rain runs into rivers → returns to ocean.
6. GROUNDWATER — some water seeps into soil, replenishes aquifers, slowly returns to oceans.

Sometimes called the HYDROLOGICAL CYCLE.

Worked qualitative. Why does cutting forests sometimes change LOCAL rainfall?

• Trees pump huge amounts of water from soil to atmosphere via transpiration.
• This water often falls back as RAIN nearby.
• Cutting trees → less transpiration → less local rainfall.
• Amazon rainforest 'creates' much of its own rain.

Cambridge tip. Don't forget TRANSPIRATION as a source of water vapour, alongside evaporation. Cambridge marks both.

The challenge. Air is 78% N₂, but plants can't use N₂ directly. They need NITRATE (NO₃⁻). Bacteria do the conversions.

Steps in the cycle:

1. Nitrogen fixation (atmosphere → soil).

• NITROGEN-FIXING BACTERIA in soil + ROOT NODULES of legumes (peas, beans, clover).
• Convert N₂ → ammonia (NH₃) or ammonium (NH₄⁺).
• Now usable by plants.

2. Decomposition (dead matter → soil).

• DECOMPOSERS (saprotrophs — bacteria + fungi) break down dead organisms + animal waste.
• Release ammonium back to soil.

3. Nitrification (ammonium → nitrate).

• NITRIFYING BACTERIA in soil.
• Two-step: ammonium → nitrite → NITRATE.
• Plants absorb nitrate.

4. Plant uptake.

• Roots absorb nitrate via active transport.
• Nitrate used to make amino acids → proteins.

5. Animal feeding.

• Animals eat plants → get nitrogen as proteins.

6. Denitrification (soil → atmosphere).

• DENITRIFYING BACTERIA, especially in waterlogged anaerobic soil.
• Convert nitrate back to N₂ → released to atmosphere.
• Returns nitrogen to start of cycle.

Why farmers add NITRATE FERTILISER:

• Crop plants remove nitrate from soil.
• If not replaced, soil becomes nitrogen-depleted.
• Adding nitrate (or planting legumes) replenishes.

Why LIGHTNING also fixes nitrogen:

• High-energy electrical discharge breaks N₂ apart.
• Nitrogen oxides form → dissolve in rain → reach soil as nitrate.
• A small natural fix.

Worked qualitative. Why do farmers ROTATE crops, including LEGUMES (e.g. beans)?

• Other crops deplete soil nitrogen.
• Legumes (peas, beans) have root nodules with nitrogen-fixing bacteria → ADD nitrogen to soil.
• Rotating: grow legumes one year, other crops next → soil refilled.
• Practised for thousands of years.

Cambridge tip. Memorise the FOUR bacteria types. Cambridge marks each name + its conversion.