Chapter 3Biogeochemical Cycles

Big QuestionWhy Are Biogeochemical Cycles Essential

to Long-Term Life on Earth?

• A biogeochemical cycle is the complete path a

chemical takes through the Earth’s four major

reservoirs:

– atmosphere

– hydrosphere (oceans, rivers, lakes, groundwaters,

and glaciers)

– lithosphere (rocks and soils)

– biosphere (plants and animals).

• Chemicals enter storage compartments - sinks

• Amount that moves between compartments is the flux

Essential Elements

• 24 elements are required for life

• Macronutrients are required in large quantities

– carbon, hydrogen, nitrogen, oxygen, phosphorus,

and sulfur.

• Micronutrients are required in small/medium

quantities, or not at all in some organisms

– Copper, sodium, iodine

Geological Cycle

• The formation and change of Earth materials

through physical, chemical, and biological

processes

The Tectonic Cycle

• Lithosphere is comprised of several plates

floating on denser material

• Plates move slowly relative to each other –

plate tectonics

• Divergent plate boundaries occur at spreading

ocean ridges

• Convergent plate boundaries occur when

plates collide

Biogeochemical Cycles in Ecosytems

• Begins with inputs from reservoirs such as

atmosphere, volcanic ash, stream runoff, ocean

currents, submarine vents

• Chemicals cycle through physical transport

and chemical reactions (e.g. decomposition)

• All ecosystems “leak” chemicals to other

ecosystems.

Carbon Cycle

• Carbon is vital for

life but is not

abundant

• Enters biological

cycles through

photosynthesis to

produce organic

forms of carbon

Carbon Cycle in a Pond

• Large inorganic carbon reservoir in oceans

• Dissolved CO2 is converted to carbonate and

bicarbonate

• Transferred from land by rivers and wind

Fossil Fuels

• Decomposition of dead organisms may be

prevented by lack of oxygen or low

temperatures

• Burial in sediments over thousands or millions

of years transforms the stored organic carbon

into coal, oil or natural gas

Global Carbon Cycle

Global Carbon Cycle

Case of the missing carbon!

– Analysis shows contribution of 8 .5 bill. tons

into the atmosphere but less than ½ stays

there…where does it go?

– 7 billion from fossil fuels and 1.5 billion from

deforestation

Case of the missing carbon!

– Appears oceans are acting as carbon sinks as are

forests and grasslands.

– But which area is more critical, and which one

dominates.

– Will these blessings last?

• If they stop functioning we could face drastic changes

even before 2050.

Case of the missing carbon!– Global tests of CO2 show less in the north than the south

despite larger northern outputs

– Why is this the case?

– If land plants are doing the work then there should be a

corresponding oxygen increase.

– If it is dissolving in the oceans then there should be no

added oxygen.

Case of the missing carbon!

– Results (best guess):

• Ocean is soaking up 2.4 billion tons globally

• Land plants do the most work in the northern

hemisphere

– Forests literally breath in the carbon but appetite changes

dramatically due to season, amount of sunlight, rainfall, and

age of forests

• Marine organisms undergo photosynthesis as well

• So that leaves about 2.9 units unaccounted for between

these groups.

Case of the missing carbon!

– Biggest threats:

• Decline in forest growth

• Killing of ocean phytoplankton due to rising sea temperatures

• Death of forests due to spread of disease and insects

• Melting permafrost layer

• Land clearing for development and agriculture

• Ofcourse continued output of carbon from fossil fuel burning

Nitrogen Cycle

• Essential for manufacturing proteins and DNA

• Although 80% of atmosphere is molecular

nitrogen, it is unreactive and cannot be used

directly

• Nitrogen fixation converts nitrogen to

ammonia or nitrate

Nitrogen Fixation

• Some organisms have a symbiotic relationship

with nitrogen fixing bacteria

• Found in root nodules in some plants, or in the

stomach of some herbivores

• Nitrogen fixation also occurs through lightning

and industrial processes

Denitrification

• When organisms die, denitrifying bacteria

convert organic nitrogen to ammonia,

nitrate, or molecular nitrogen

Global Nitrogen Cycle

Phosphorus Cycle

• No gaseous phase

• Slow rate of transfer

• Released by erosion of exposed rock

• Absorbed by plants, algae, and some bacteria

• Exported from terrestrial ecosystems by runoff to oceans

• May be returned through seabird guano

Global Phosphorus Cycle

Phosphate Mining

• Impact on

landscape by

open-pit mining