Biogeochemical Cycles

pete hamilton sandringham college

Fundamentals’ of biogeochemical cycles• All matter cycles...it is neither created nor destroyed...

• As the Earth is essentially a closed system with respect to matter, we can say that all matter on Earth cycles .

• Biogeochemical cycles: the movement (or cycling) of matter through a system

The Geological /Rock Cycle

• The transport and transformation of one type of rock/mineral into another.

• Processes involved include:– Volcanism– Uplifting– Weathering & Erosion– Transportation – Sedimentation– Burial , Heat & Pressure Time

Erosion

Oxygen-Ozone: A Chemical Cycle

The Water Cycle• Involves movement of water through the

atmosphere, the lithosphere and the biosphere.

• It is a true biogeochemical cycle• The processes involved include:

– Evaporation– Transpiration– Condensation,– Precipitation– Runoff– Infiltration– Discharge

The Water Cycle–All life depends on the presence of water–60% of the adult human body weight is

water–Amount of water available determines the

nature and abundance of organisms present

– It can be synthesized and broken down• Synthesized during cellular respiration• Broken down during photosynthesis 7

The Water Cycle

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The Water Cycle– Liquid water from the Earth’s surface

evaporates into the atmosphere– Occurs directly from the surfaces of oceans,

lakes, and rivers– Terrestrial ecosystems: 90% of evaporation

is through plants– Water in the atmosphere is a gas– Cools and falls to the surface as

precipitation10

The Water Cycle• Groundwater: under ground

water–Aquifers: permeable,

underground layers of rock, sand, and gravel saturated with water

–Two subparts: • Upper layers constitute the water table• Lower layer can be tapped by wells

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Biogeochemical Cycles• The carbon cycle

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Photosynthesis : Plants use the energy of sunlight to convert carbon dioxide and water into carbohydrates and oxygen.

6CO2 + 6H2O + energy → C6H12O6 + 6O2 Photosynthesizing organisms include the plant life of the land areas as well as the phytoplankton of the oceans.

The tiny marine cyanobacteria Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.

Most living organisms use oxygen to break carbohydrates down releasing energy in a process called cellular respiration. 

O2 + carbohydrates → CO2 + H2O + energy

Decomposition is a process in which microbes breakdown organic matter releasing energy. When this process uses oxygen it is called aerobic cellular respiration. 

O2 + carbohydrates → CO2 + H2O + energy

Combustion is the chemical oxidation of fuels (organic substances such as coal,oil etc) to release stored energy

O2 + FUEL → CO2 + H2O + energy

Biogeochemical Cycles• Carbon fixation: metabolic reactions that make

nongaseous compounds from gaseous ones• In aquatic systems inorganic carbon is present in

water as dissolved CO2 and as HCO3- ions

• CO2 is used by algae and aquatic plants for photosynthesis

• HCO3- ions can be used by some aquatic organisms

to make shells16

Biogeochemical Cycles• Methane producers

– Microbes that break down/ decompose organic compounds in the absence of oxygen by anaerobic cellular respiration provide an additional dimension to the carbon cycle

– Methanogens: produce methane (CH4)– Wetland ecosystems are a source of CH4

– CH4 is oxidized to CO2 but can remain as CH4 for a long time

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Biogeochemical Cycles• Human activities which have disrupted the

natural carbon cycle include:– Land Clearing – removal of vegetation which would

normally result in net removal of carbon dioxide from the atmosphere as consequence of photosynthesis

– Burning of fossil fuels which returns carbon dioxide that has been locked beneath the earths crust for millions of years to the atmosphere .

– Agricultural practices– Industrial processes– Increased population of people and agricultural

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The Keeling Curve

The major reservoirs or sinks of carbon include:

• Atmosphere• Plants• Soil/ Sediments /Rocks• Freshwater• Oceans

– The ocean contains the largest active pool of carbon near the surface of the Earth..

• The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs.

IN THE OCEAN:• The seas contain around 36000 gigatonnes of

carbon, mostly in the form of bicarbonate ion. • The level of bicarbonate can affect the oceans pH.• More carbon dioxide more bicarbonate

lower pH

• Carbon is readily exchanged between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean. When CO2 enters the ocean, carbonic acid is formed:

• CO2 + H2O ⇌ H2CO3• This reaction has a forward and reverse rate, that is it

achieves a chemical equilibrium. Another reaction important in controlling oceanic pH levels is the release of hydrogen ions and bicarbonate. This reaction controls large changes in pH:

• H2CO3 ⇌ H+ + HCO3−

Biogeochemical Cycles• Nitrogen Cycle

– Nitrogen is a component of all proteins and nucleic acids

– Atmosphere is 78% nitrogen N2

– The bonds of N2 are very strong and are not broken easily. Atmospheric nitrogen is inert.

– Availability to living organisms• Usually the element in shortest supply• Most plants and animals cannot use N2 (gas)• Use instead NH3, and NO3

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Biogeochemical Cycles• Nitrogen fixation: synthesis of

nitrogen containing compounds from N2–Nitrification: N2 --> NH3 --> NO3

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–Denitrification: NO3- --> N2

– Nitrogen can be fixed by lightening.– Both processes are carried out by

microbes: free or living on plant roots

During lightning strikes enough energy is supplied to break the bonds of the nitrogen molecule

and form nitrous oxide according to the equation below.

N2 + O2 => 2NO The nitrous oxide formed

combines with oxygen to form nitrogen dioxide according to the

equation below.2NO + O2 => 2NO2

Nitrogen dioxide readily dissolves in water to produce nitric and nitrous acids which provides a source of nitrates available to

plants.2 NO2 + H2O => HNO3 + HNO2

N2Root nodules of legumes containing bacteria

Soil bacteria

Biogeochemical Cycles• Nitrogen Cycle

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• When organisms excrete waste, the nitrogen is released back into the environment.

• When organisms die and decompose, the nitrogen is broken down and converted to ammonia.

• Nitrates may also be converted to gaseous nitrogen through a process called denitrification and returned to the atmosphere, continuing the cycle.

Biogeochemical Cycles• Phosphorus cycle

–Phosphorus is required by all organisms• Occurs in nucleic acids, membranes, ATP

–No significant gas form–Exists as PO4

3- in ecosystems–Plants and algae use free inorganic

phosphorus, animals eat plants to obtain their phosphorus

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Biogeochemical Cycles• Phosphorus cycle

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Biogeochemical Cycles• Limiting nutrient: weak link in an

ecosystem; shortest supply relative to the needs of organisms

• Iron is the limiting nutrient for algal populations• Nitrogen and phosphorus can also be

limiting nutrients for both terrestrial and aquatic ecosystems

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