Physical Geography 101
Biogeochemical Cycles

Biogeochemical Cycles

Circuits or pathways by which chemical elements or molecules move through both biotic (“bio-”) and abiotic (“geo-”) reservoirs of an ecosystem
Hydrologic Cycle
Carbon Cycle
Nitrogen Cycle
Hydrogen Cycle
Oxygen Cycle
Phosphorus Cycle
Sulfur Cycle

Global Carbon Cycle

Carbon is exchanged between active areas and reservoirs

Carbon Movement

From atmosphere to plants:
From atmosphere/plants to animals:
From atmosphere to surface:
From oceans to rock:
From ocean to rock:
From ocean to rock:
From land to rock:
From rock to air:
From plants to air:
From rock to air:

Carbon Storage

Reservoir: Amount, Billions of Metric Tons
Terrestrial Plants: 540 – 610
Atmosphere: 578 – 766 (from 1700 to 1999)
Soil Organic Matter: 1500 - 1600
Fossil Fuel Deposits: 4,000
Oceans: 38,000 – 40,000
Marine Sediment and Sedimentary Rocks: 66,000,000 – 100,000,000


Nitrogen is critical in many biologic processes and for life on Earth
It is found in amino acids, in proteins and nucleic acids such as DNA and RNA
Nitrogen is used by plants in chlorophyll molecules for photosynthesis

Nitrogen Cycle

Nitrogen is exchanged between active areas and reservoirs.

Nitrogen Movement

Fixation – Gaseous N2 converted to ammonia (NH3)
Ammonification – Decomposition by microorganisms converts nitrogen compounds to ammonia (NH3)
Assimilation – plants absorb nitrogen, nitrate (NO3-), and ammonia (NH3) from soil
Nitrification – soil bacteria convert ammonia to nitrate (NO3-),
Denitrification – soil bacteria return nitrogen to atmosphere as N2 gas.