Biogeochemical Cycles

WaterNitrogen

Carbon DioxidePhosphorus

SulfurBy Laiba Sarwar

Biogeochemical Cycle :

• Chemical elements are required by life from the living and

nonliving parts of the environment.

• These elements cycle in either a gas cycle or a sedimentary cycle

• In a gas cycle elements move through the atmosphere.

• Main reservoirs are the atmosphere and the ocean.

• Sedimentary cycle elements move from land to water to

sediment.

Components of a biogeochemical cycle

• Source = a pool that releases more nutrients than it accepts• Sinks = a pool that accepts more nutrients than it releases

Carbon Cycle

Sediment storage of carbon

• Decomposition returns carbon to the sediment– The largest reservoir of carbon– May be trapped for hundreds of millions of years

• Aquatic organisms die and settle in the sediment– Older layers are buried deeply and undergo high

pressure– Ultimately, it may be converted into fossil fuels

• Oceans are the second largest reservoir of carbon

http://www.buzzle.com/images/diagrams/carbon-cycle.jpg

Carbon (C) enters the biosphere during photosynthesis:

CO2 + H2O (carbon dioxide+ water)--->

C6H12O6 + O2 + H2O (sugar+oxygen+water)

Carbon is returned to the biosphere in cellular respiration:

O2 +H2O + C6H12O6 ---> CO2 +H2O + energy

Carbon Facts

• Every year there is a measurable difference in the concentration of atmospheric CO2 with changes in the seasons.– For example, in winter there is almost no

photosynthesis ( higher CO2 )– During the growing season there is a

measurable difference in the concentration of atmospheric CO2 over parts of each day.

The Hydrologic cycle

• Water is essential for biochemical reactions

– It is involved in nearly every environmental system

• Hydrologic cycle = summarizes how liquid, gaseous and solid water flows through the environment

–Oceans are the main reservoir

•Evaporation = water moves from aquatic

and land systems into the atmosphere

•Transpiration = release of water vapor by

plants

•Precipitation, runoff, and surface water =

water returns to Earth as rain or snow and

flows into streams, oceans, etc.

Evaporation•The process by which liquid water is transformed into a gaseous state•Evaporation into a gas ceases when the gas reaches saturation•The molecules that escape the condensed stage have above-average energies.

•Those left behind have below-average energies

•Manifested by a decrease in the temperature of the condensed phase.

Evaporation

• Energy breaks bonds that hold molecules together• Net evaporation occurs when the rate of

evaporation exceeds the rate of condensation• Removes heat from the environment:

Net Cooling

Primary mechanism for surface-to-atmosphere water transport

Evaporation v. Precipitation• About equal on a global scale• Evaporation more prevalent over the oceans than

precipitation• Over land, precipitation exceeds evaporation• Most water evaporated from the oceans falls back into

the ocean as precipitation• 10% of water evaporated from the ocean is transported

over land and falls as precipitation• Once evaporated, a water molecule spends ~ 10 days

airborne

The process of water loss from plants through stomata.

•passive process that depends on:~humidity of the atmosphere ~the moisture content of the soil

•only 1 % of the water transpired used for growth

•transports nutrients from the soil into the roots and carries them to the various cells of the plant

•keeps tissues from becoming overheated

(Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues.)

Transpiration

TranspirationAccounts for ~ 10% of the moisture in the atmosphere

Depends on:

» Temperature» Humidity» Insolation» Precipitation» Soil type and saturation» Wind» Land slope

Transpiration

Precipitation

• The vapor that accumulates or freezes on condensation nuclei is acted on by gravity and falls to Earth’s surface.

rain, freezing rain, sleet, snow, or hail

primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth

Groundwater

• Aquifers = underground reservoirs of sponge-like regions of rock and soil that hold…– Groundwater = water found underground beneath layers

of soil• Water table = the upper limit of groundwater in an

aquifer– Water may be ancient (thousands of years old)

• Groundwater becomes exposed to the air where the water table reaches the surface– Exposed water runs off to the ocean or evaporates

The hydrologic cycle

The Nitrogen Cycle

Nitrogen Facts

• Nitrogen (N) is an essential constituent of protein, DNA, RNA, and chlorophyll.

• Nitrogen is the most abundant gas in the atmosphere.

• Nitrogen must be fixed or converted into a usable form.

Sources

• Lightning

• Inorganic fertilizers

• Nitrogen Fixation

• Animal Residues

• Crop residues

• Organic fertilizers

Forms of Nitrogen• Urea CO(NH2)2

• Ammonia NH3 (gaseous)• Ammonium NH4

• Nitrate NO3

• Nitrite NO2

• Atmospheric Dinitrogen N2

• Organic N

• Nitrogen comprises 78% of our atmosphere– It is contained in proteins, DNA and RNA

• Nitrogen cycle = describes the routes that nitrogen atoms take through the environment– Nitrogen gas cannot be used by organisms

• Nitrogen fixation = lightning or nitrogen-fixing bacteria combine (fix) nitrogen with hydrogen – To form ammonium– Which can be used

by plants

Nitrification and Denitrification

• Nitrification = bacteria convert ammonium ions first into nitrite ions then into nitrate ions– Plants can take up these ions

• Animals obtain nitrogen by eating plants or other animals• Decomposers get it from dead and decaying plants or

other animals– Releasing ammonium ions to nitrifying bacteria

• Denitrifying bacteria = convert nitrates in soil or water to gaseous nitrogen– Releasing it back into the atmosphere

Nitrification

R-NH2

NH4 NO2

NO3NO2

NO

N2O

N2

NitrificationTwo step reactions that occur together :

• 1rst step catalyzed by Nitrosomonas2 NH4

+ + 3 O2 2 NO2- +2 H2O+ 4 H+

• 2nd step catalyzed by Nitrobacter• 2 NO2

- + O2 2 NO3-

Denitrification

R-NH2

NH4 NO2

NO3NO2

NO

N2O

N2

Denitrification• Removes a limiting nutrient from the

environment• 4NO3

- + C6H12O6 2N2 + 6 H20• Inhibited by O2

• Not inhibited by ammonia• Microbial reaction• Nitrate is the terminal electron

acceptor

The nitrogen cycle

Sources of Oxygen:

• Phototosynthes and Photo disassociation of H2O vapor • CO2 and circulates freely throughout the biosphere. • Some CO2 combines with Ca to form carbonates. • O2 combines with nitrogen compounds to form nitrates. • Oxygen combines with iron compounds to form ferric

oxides. • Oxygen2 in the troposphere is reduced to O3 (ozone). • Ground level O3 (ozone) is a pollutant which damages

lungs.

Oxygen Cycle (Photosynthesis)

Step One of Oxygen Cycle• Plant release oxygen into the atmosphere as a

by-product of photosynthesis.

oxygen

Step Two of Oxygen Cycle

• Animals take in oxygen through the process of respiration.

• Animals then break down sugars and food.

Step Three in Oxygen Cycle

• Carbon dioxide is released by animals and used in plants in photosynthesis.

• Oxygen is balanced between the atmosphere and the ocean.

Phosphorus (P) Cycle

Component of DNA, RNA, ATP, proteins and enzymes - Cycles in a sedimentary cycle - A good example of how a mineral element becomes part of an organism. - The source of Phosphorus (P) is rock. - Phosphorus is released into the cycle through erosion or mining. - Phosphorus is soluble in H2O as phosphate (PO4) -Phosphorus is taken up by plant roots, then travels through food chains. - It is returned to sediment

• The phosphorus cycle, is the circulation of phosphorous among the rocks, soils, water, and plants and animals of the earth.

• Human beings and all other organisms must have phosphorus to live.

• In nature, most phosphorus occurs in phosphate rock, which contains phosphate ions combined with calcium, magnesium, chlorine, and fluorine.

• It cannot be found in air in the gaseous . This is because phosphorous is usually liquid at normal temperatures pressures.

• This cycle is the slowest of the matter cycles. • Phosphorus is most commonly found in rock formations and

ocean sediments as phosphate salts. Phosphates are also limiting factors for plant-growth in marine ecosystems, because they are not very water-soluble.

• Another example of the phosphorus cycle is when rocks are created.

• The phosphate in the soil moves on and transfers its phosphate to the rocks underwater. When the uplifting of the rocks occurs it takes the phosphate along with it. After that the weathering of rocks occur and the rocks begin to break down into the soil and the phosphate in the rocks ends up in the soil again and the cycle repeats.

Uplifting of rocks

Weathering of rock

Runoff

Phosphates in rock

Phosphates in soil

(inorganic)Phosphates in solution

Precipitated (solid) phosphates

RockDecomposition

Phosphates in organic

compounds

PlantsAnimals

Detritus

Detritivores in soil

All these examples of phosphates are inorganic (white boxes). However, the Phosphorus Cycle is also organic (yellow boxes).

Not all phosphates in the runoff make it to the water; others sink into the soil. These inorganic phosphates are transformed into organic ones by plants, which are in turn eaten by animals.

The dead animals, retain their internal phosphorus stores and detritivores (scavengers which feed on dead plants and animals or their waste) change the organic phosphates back to inorganic ones.

Phosphorus (P) Cycle

Sulfur (s) Cycle

• Component of protein

• Cycles in both a gas and sedimentary cycle.

• The source of Sulfur is the lithosphere (earth's crust)

• Sulfur (S) enters the atmosphere as  hydrogen

sulfide (H2S) during fossil fuel combustion, volcanic

eruptions, gas exchange at ocean surfaces, and decomposition.

• SO2 and water vapor makes H2SO4 ( a weak sulfuric

acid), which is then carried to Earth in rainfall.

• Sulfur in soluble form is taken up by plant roots and

incorporated into amino acids such as cysteine. It

then travels through the food chain and is eventually

released through decomposition.

 

Summary

• The building blocks of life :Water ,Nitrogen, Carbon Dioxide, Phosphorus, Sulfur

• Continually cycle through Earth's systems, the atmosphere, hydrosphere, biosphere, and lithosphere, on time scales that range from a few days to millions of years.

• These cycles are called biogeochemical cycles, because they include a variety of biological, geological, and chemical processes.

Human impacts on the hydrologic cycle

• Removing forests and vegetation increases runoff and erosion, reduces transpiration and lowers water tables

• Irrigating agricultural fields depletes rivers, lakes and streams and increases evaporation

• Damming rivers increases evaporation and infiltration • Emitting pollutants changes the nature of precipitation• The most threatening impact: overdrawing

groundwater for drinking, irrigation, and industrial use– Water shortages create worldwide conflicts

Humans affect the carbon cycle

• Burning fossil fuels moves carbon from the ground to the air

• Cutting forests and burning fields moves carbon from vegetation to the air

• Today’s atmospheric carbon dioxide reservoir is the largest in the past 800,000 years– It is the driving force behind climate change

• The missing carbon sink: 1-2 billion metric tons of carbon are unaccounted for– It may be taken up by plants or soils of northern temperate

and boreal forests

Humans affect the nitrogen cycle

• Haber-Bosch process = production of fertilizers by combining nitrogen and hydrogen to synthesize ammonia– Humans overcame the limits on crop productivity

• Fixing atmospheric nitrogen with fertilizers– Increases emissions of greenhouse gases and smog– Washes calcium and potassium out of soil– Acidifies water and soils– Moves nitrogen into terrestrial systems and oceans– Reduces diversity of plants adapted to low-nitrogen

soils– Changed estuaries and coastal ecosystems and fisheries

Humans affect the phosphorus cycle

• Mining rocks for fertilizer moves phosphorus from the soil to water systems

• Wastewater discharge also releases phosphorus • Runoff containing phosphorus causes eutrophication

of aquatic systems– Produces murkier waters– Alters the structure and function of aquatic systems– Do not buy detergents that contain phosphate

Cutting and burning of tropical rain forests affects the phosphorus cycle. As the forest is cut and/or burned, nutrients originally stored in plants and rocks are quickly washed away by heavy rains, causing the land to become unproductive.

Agricultural runoff provides much of the phosphate found in waterways. Crops often cannot absorb all of the fertilizer in the soils, causing excess fertilizer runoff and increasing phosphate levels in rivers and other bodies of water. The phosphate in the water is eventually precipitated as sediments at the bottom of the body of water. In certain lakes and ponds this may be re-dissolved and recycled as a problem nutrient.

Animal wastes or manure may also be applied to the land as fertilizer. If misapplied on frozen ground during the winter, much of it may lost as run-off during the spring thaw. In certain area very large feed lots of animals, may result in excessive run-off of phosphate and nitrate into streams.

Solutions to the dead zone

• The Harmful Algal Bloom and Hypoxia Research and Control Act (1998) – Called for an assessment of hypoxia in the dead zone

• Solutions outlined included:– Reduce nitrogen fertilizer use in Midwestern farms– Apply fertilizer at times which minimize runoff– Use alternative crops and manage manure better– Restore wetlands and create artificial ones– Improve sewage treatment technologies– Evaluate these approaches

Decreasing pollution

• Scientists, farmers and policymakers are encouraged to – Decrease fertilizer use– While safeguarding agriculture

• Offering insurance and incentives• Using new farming methods• Planting cover crops• Maintaining wetlands• There have been some successes

– Despite a lack of funding