15.3 Biogeochemical Cycles | Ecology | Biology 11

Matter, unlike energy, is recycled within ecosystems. The pathways through which chemical elements move between the living (biotic) and non-living (abiotic) components of the environment are called Biogeochemical Cycles (bio = living; geo = earth/geological; chemical = elements involved).

Key Concepts

Reservoir Pool vs. Exchange Pool

Feature	Reservoir Pool	Exchange Pool
Size	Large	Small
Movement	Slow	Rapid
Nature	Usually abiotic	Biotic + abiotic
Example	Atmosphere ( $N_{2}$ ), Rocks (P)	Soil nutrients, plant biomass

The reservoir pool stores nutrients for long periods, while the exchange pool is where active cycling between organisms and environment occurs.

1. The Carbon Cycle

Carbon is the backbone of all organic molecules. Its primary reservoir is the atmosphere (as $C O_{2}$ ) and the oceans (as dissolved $C O_{2}$ and carbonates).

Key Processes:

Photosynthesis: Producers fix atmospheric $C O_{2}$ into organic compounds $6 C O_{2} + 6 H_{2} O light C_{6} H_{12} O_{6} + 6 O_{2}$
Respiration: All organisms release $C O_{2}$ back to the atmosphere
Decomposition: Decomposers break down dead organic matter, releasing $C O_{2}$
Combustion: Burning of fossil fuels and biomass releases stored carbon as $C O_{2}$
Sedimentation: Carbon becomes locked in fossil fuels and limestone over geological time

Human Impact: Burning fossil fuels and deforestation have increased atmospheric $C O_{2}$ , contributing to the greenhouse effect and global warming.

2. The Nitrogen Cycle

Nitrogen ( $N_{2}$ ) makes up ~78% of the atmosphere but most organisms cannot use it directly. It must be converted into usable forms.

Steps of the Nitrogen Cycle

1. Nitrogen Fixation Conversion of $N_{2}$ → $N H_{3}$ (ammonia) or $N O_{3}^{-}$ (nitrates)

Biological: Rhizobium (in root nodules of legumes), Azotobacter (free-living)
Physical: Lightning converts $N_{2}$ to $N O_{3}^{-}$
Industrial: Haber process (fertilizer production)

2. Ammonification Decomposers break down dead organic matter and excretory products → releasing $N H_{4}^{+}$ (ammonium) into soil.

3. Nitrification Aerobic bacteria convert ammonium to nitrates (plant-usable form):

Nitrosomonas: $N H_{4}^{+}$ → $N O_{2}^{-}$ (nitrites)
Nitrobacter: $N O_{2}^{-}$ → $N O_{3}^{-}$ (nitrates)

4. Assimilation Plants absorb $N O_{3}^{-}$ from soil and incorporate nitrogen into proteins, nucleic acids, and chlorophyll.

5. Denitrification Anaerobic bacteria (e.g., Pseudomonas) convert $N O_{3}^{-}$ → $N_{2}$ , returning nitrogen to the atmosphere.

3. The Phosphorus Cycle

Phosphorus is essential for ATP, DNA, RNA, and cell membranes (phospholipids).

Key Features:

Primary reservoir: Sedimentary rocks (lithosphere)
No gaseous phase — it is a purely sedimentary cycle
Slowest of the major nutrient cycles

Pathway:

Weathering of rocks releases phosphate ( $P O_{4}^{3 -}$ ) into soil and water
Absorption by plants and algae
Consumption by animals through food
Decomposition returns phosphate to soil
Sedimentation — phosphate washes into oceans and becomes locked in sediment/rock

Note: Because phosphorus has no atmospheric reservoir, it is often a limiting nutrient in ecosystems.

4. Reservoir vs. Exchange Pool — Summary

Summary Table: Comparing Major Biogeochemical Cycles

Cycle	Primary Reservoir	Gaseous Phase?	Key Organisms
Carbon	Atmosphere, Oceans	Yes ( $C O_{2}$ )	Plants, decomposers
Nitrogen	Atmosphere ( $N_{2}$ )	Yes ( $N_{2}$ )	Rhizobium, Pseudomonas
Phosphorus	Sedimentary rocks	No	Plants, decomposers
Water	Oceans, glaciers	Yes ( $H_{2} O$ vapour)	All organisms

Key Principle: Unlike energy (which flows in one direction and is lost as heat), matter is recycled — the same atoms have been cycling through ecosystems for billions of years.

Key Concepts

Reservoir Pool vs. Exchange Pool

Feature	Reservoir Pool	Exchange Pool
Size	Large	Small
Movement	Slow	Rapid
Nature	Usually abiotic	Biotic + abiotic
Example	Atmosphere ( $N_{2}$ ), Rocks (P)	Soil nutrients, plant biomass

The reservoir pool stores nutrients for long periods, while the exchange pool is where active cycling between organisms and environment occurs.

1. The Carbon Cycle

Carbon is the backbone of all organic molecules. Its primary reservoir is the atmosphere (as $C O_{2}$ ) and the oceans (as dissolved $C O_{2}$ and carbonates).

Key Processes:

Photosynthesis: Producers fix atmospheric $C O_{2}$ into organic compounds $6 C O_{2} + 6 H_{2} O light C_{6} H_{12} O_{6} + 6 O_{2}$
Respiration: All organisms release $C O_{2}$ back to the atmosphere
Decomposition: Decomposers break down dead organic matter, releasing $C O_{2}$
Combustion: Burning of fossil fuels and biomass releases stored carbon as $C O_{2}$
Sedimentation: Carbon becomes locked in fossil fuels and limestone over geological time

Human Impact: Burning fossil fuels and deforestation have increased atmospheric $C O_{2}$ , contributing to the greenhouse effect and global warming.

2. The Nitrogen Cycle

Nitrogen ( $N_{2}$ ) makes up ~78% of the atmosphere but most organisms cannot use it directly. It must be converted into usable forms.

Steps of the Nitrogen Cycle

1. Nitrogen Fixation Conversion of $N_{2}$ → $N H_{3}$ (ammonia) or $N O_{3}^{-}$ (nitrates)

Biological: Rhizobium (in root nodules of legumes), Azotobacter (free-living)
Physical: Lightning converts $N_{2}$ to $N O_{3}^{-}$
Industrial: Haber process (fertilizer production)

2. Ammonification Decomposers break down dead organic matter and excretory products → releasing $N H_{4}^{+}$ (ammonium) into soil.

3. Nitrification Aerobic bacteria convert ammonium to nitrates (plant-usable form):

Nitrosomonas: $N H_{4}^{+}$ → $N O_{2}^{-}$ (nitrites)
Nitrobacter: $N O_{2}^{-}$ → $N O_{3}^{-}$ (nitrates)

4. Assimilation Plants absorb $N O_{3}^{-}$ from soil and incorporate nitrogen into proteins, nucleic acids, and chlorophyll.

5. Denitrification Anaerobic bacteria (e.g., Pseudomonas) convert $N O_{3}^{-}$ → $N_{2}$ , returning nitrogen to the atmosphere.

3. The Phosphorus Cycle

Phosphorus is essential for ATP, DNA, RNA, and cell membranes (phospholipids).

Key Features:

Primary reservoir: Sedimentary rocks (lithosphere)
No gaseous phase — it is a purely sedimentary cycle
Slowest of the major nutrient cycles

Pathway:

Weathering of rocks releases phosphate ( $P O_{4}^{3 -}$ ) into soil and water
Absorption by plants and algae
Consumption by animals through food
Decomposition returns phosphate to soil
Sedimentation — phosphate washes into oceans and becomes locked in sediment/rock

Note: Because phosphorus has no atmospheric reservoir, it is often a limiting nutrient in ecosystems.

4. Reservoir vs. Exchange Pool — Summary

Summary Table: Comparing Major Biogeochemical Cycles

Cycle	Primary Reservoir	Gaseous Phase?	Key Organisms
Carbon	Atmosphere, Oceans	Yes ( $C O_{2}$ )	Plants, decomposers
Nitrogen	Atmosphere ( $N_{2}$ )	Yes ( $N_{2}$ )	Rhizobium, Pseudomonas
Phosphorus	Sedimentary rocks	No	Plants, decomposers
Water	Oceans, glaciers	Yes ( $H_{2} O$ vapour)	All organisms

Key Principle: Unlike energy (which flows in one direction and is lost as heat), matter is recycled — the same atoms have been cycling through ecosystems for billions of years.