🌿 Chapter 01 · Topic 05 · Biogeochemical Cycles

Biogeochemical Cycles — Carbon, Nitrogen, Water

Cycling of nutrients between biotic and abiotic components; gaseous vs sedimentary cycles; key bacteria in nitrogen cycle; phosphorus cycle.

What are Biogeochemical Cycles?

A biogeochemical cycle (nutrient cycle) is the cyclic movement of chemical elements between the living (biotic) components and the non-living (abiotic) environment — atmosphere, hydrosphere, and lithosphere.

Bio = living organisms; Geo = earth/rocks/soil; Chemical = chemical elements
Unlike energy, nutrients are recycled and reused in ecosystems
Driven by biological processes (photosynthesis, respiration, decomposition) and physical processes (weathering, precipitation)

Type	Reservoir	Examples
Gaseous Cycle	Atmosphere or hydrosphere	Carbon cycle, Nitrogen cycle, Oxygen cycle, Water cycle
Sedimentary Cycle	Earth’s crust/rocks/soil	Phosphorus cycle, Sulphur cycle, Calcium cycle

Key Difference: Gaseous cycles have atmospheric reservoir → more rapid and self-regulating. Sedimentary cycles have rock/soil reservoir → slower, more prone to disruption.

Carbon Cycle

Carbon is the backbone of all organic molecules. The carbon cycle involves the movement of carbon between the atmosphere (CO₂), living organisms, oceans, and geological formations.

Key Processes:

Photosynthesis — plants absorb CO₂ from atmosphere; convert to organic carbon (glucose)
Respiration — organisms release CO₂ back to atmosphere during cellular respiration
Decomposition — decomposers break down dead organic matter; release CO₂
Combustion — burning of fossil fuels and biomass releases stored carbon as CO₂
Ocean absorption — oceans absorb CO₂; phytoplankton fix carbon via photosynthesis

Carbon Sinks (absorb more carbon than they release):

Forests (especially tropical rainforests)
Oceans (largest carbon sink)
Soil (humus, peat bogs)
Wetlands and mangroves

Greenhouse Effect Link: Burning fossil fuels increases atmospheric CO₂ → enhanced greenhouse effect → global warming. CO₂ is the primary greenhouse gas responsible for climate change.

Nitrogen Cycle

Nitrogen makes up ~78% of the atmosphere but most organisms cannot use atmospheric N₂ directly. The nitrogen cycle converts N₂ into usable forms.

Key Processes:

Nitrogen Fixation — conversion of atmospheric N₂ to ammonia (NH₃) or nitrates; by bacteria or lightning
Nitrification — conversion of ammonia (NH₃) → nitrites (NO₂⁻) → nitrates (NO₃⁻); by nitrifying bacteria
Assimilation — plants absorb nitrates from soil; incorporate into proteins and nucleic acids
Ammonification — decomposers break down dead organic matter; release ammonia (NH₃)
Denitrification — conversion of nitrates back to N₂ gas; returns nitrogen to atmosphere

Process	Key Bacteria	Conversion
Nitrogen Fixation	Rhizobium (symbiotic in legume roots), Azotobacter, Anabaena, Nostoc (free-living)	N₂ → NH₃
Nitrification (Step 1)	Nitrosomonas, Nitrosococcus	NH₃ → NO₂⁻
Nitrification (Step 2)	Nitrobacter	NO₂⁻ → NO₃⁻
Denitrification	Pseudomonas, Thiobacillus	NO₃⁻ → N₂
Ammonification	Bacillus, Clostridium, fungi	Organic N → NH₃

Exam Favourite: Rhizobium = symbiotic nitrogen fixer in legume root nodules. Azotobacter = free-living nitrogen fixer in soil. Nitrosomonas → nitrites. Nitrobacter → nitrates. Pseudomonas = denitrifier.

Water Cycle (Hydrological Cycle)

The water cycle describes the continuous movement of water through the Earth’s systems — atmosphere, land surface, and underground.

Evaporation — water from oceans, lakes, rivers converts to water vapour (driven by solar energy)
Transpiration — water released by plants through stomata as water vapour
Evapotranspiration — combined evaporation + transpiration
Condensation — water vapour cools and forms clouds/droplets
Precipitation — water falls as rain, snow, sleet, hail
Surface Runoff — water flows over land surface into rivers and oceans
Infiltration — water seeps into soil; replenishes groundwater
Groundwater flow — water moves through aquifers; feeds springs and rivers

Key Fact: ~97% of Earth’s water is in oceans (saline). Only ~3% is freshwater, of which ~2% is locked in glaciers and ice caps. Only ~1% is accessible freshwater.

Phosphorus Cycle

Phosphorus is essential for DNA, RNA, ATP, and cell membranes. Unlike carbon and nitrogen, phosphorus has no gaseous phase.

Type: Sedimentary cycle — reservoir is rocks and soil (not atmosphere)
Weathering — phosphate rocks are broken down by rain and erosion; release phosphate ions (PO₄³⁻) into soil
Uptake — plants absorb phosphates from soil; pass to consumers through food chain
Decomposition — decomposers release phosphates back to soil from dead organic matter
Sedimentation — phosphates wash into oceans; settle as sediment; eventually form new rocks (very slow)

Why is phosphorus a limiting nutrient? Because it has no atmospheric reservoir and cycles very slowly. Phosphorus availability often limits plant growth in terrestrial and aquatic ecosystems. Excess phosphorus from fertilisers causes eutrophication in water bodies.

Cycle	Type	Reservoir	Gaseous Phase	Key Feature
Carbon	Gaseous	Atmosphere	Yes (CO₂)	Linked to greenhouse effect
Nitrogen	Gaseous	Atmosphere (78% N₂)	Yes (N₂)	Requires bacteria for fixation
Water	Gaseous	Oceans	Yes (H₂O vapour)	Driven by solar energy
Phosphorus	Sedimentary	Rocks/soil	No	Slowest cycle; limiting nutrient

Revision Checklist

✅ Biogeochemical cycles = cyclic movement of nutrients between biotic and abiotic components
✅ Gaseous cycles: Carbon, Nitrogen, Oxygen, Water (atmospheric reservoir)
✅ Sedimentary cycles: Phosphorus, Sulphur (rock/soil reservoir)
✅ Carbon cycle: photosynthesis (absorb CO₂), respiration/combustion (release CO₂)
✅ Carbon sinks: forests, oceans, soil, wetlands
✅ Nitrogen fixation: Rhizobium (symbiotic), Azotobacter (free-living)
✅ Nitrification: Nitrosomonas (NH₃→NO₂⁻), Nitrobacter (NO₂⁻→NO₃⁻)
✅ Denitrification: Pseudomonas (NO₃⁻→N₂)
✅ Water cycle: evaporation → condensation → precipitation → runoff/infiltration
✅ Phosphorus cycle = sedimentary; NO gaseous phase
✅ Phosphorus = limiting nutrient; excess causes eutrophication
✅ ~97% Earth’s water = saline oceans; only ~1% accessible freshwater