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

• net sink - when input exceeds output

• net source - if output exceeds input.

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

• Plate movements change the location of continents and alter atmospheric and ocean circulation patterns

• Plate boundaries are geologically active, producing volcanoes and earthquakes

Hydrologic Cycle

• Evaporation

• Precipitation

• Runoff

• Groundwater

• 97% of water is stored in oceans, 2% in glaciers and ice caps, 1% as freshwater on land or atmosphere

• Drainage basins or watersheds are the area contributing runoff to a stream or river

• Vary in size from a hectare to millions of square miles (e.g. Mississipi River drainage basin)

• Human impacts include dam construction, irrigation, stormwater runoff

Rock Cycle• Igneous rocks form from molten material such

as lava. Broken down by physical and chemical weathering

• Sedimentary rocks form from accumulation of weathered material in depositional basins

• Metamorphic rocks are formed from sedimentary rocks exposed to heat, pressure or chemically active fluids

Rock Cycle

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.

Annual Calcium Cycle in a Forest Ecosystem

Soluble in water and easily lost through runoff

Annual Sulfur Cycle In a Forest Ecosystem

Includes gaseous forms (sulfur dioxide and hydrogen sulfide) and cycles much faster than calcium

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