The Biogeochemical Cycles

The biogeochemical cycle is the pathwayan element takes through the Earth system.

This cycle includes the atmosphere, oceansoil, living organisms, etc…

The cycle includes bio, because chemicalscycle through living things.

The cycle includes geo because the Earth itself is part of the cycle. (air water, land)

The 4 key components of the cycle are:

Solid Earth (rocks, soils) Air (atmosphere)

Water (both fresh and salt) Living systems ( organisms )

Chemicals cycle within an ecosystemand are exchanged with the biosphere.

Organisms exchange chemicals with the nonliving environment.

Chemicals are transferred among differentpools, and remain in those pools for varyinglengths of time, depending on the ART.

Biogeochemical Cycles and Life

Of the 109 known elements, only 24 are necessary for life.

These 24 are divided into macronutrients, which are needed in large amounts, and micronutrients, which are needed in small amounts.

Macronutrients

The macronutrients mostly consist of 6elements; Carbon, Hydrogen, NitrogenOxygen, Phosphorous, and Sulfur.

Other elements are also macronutrients,but play a lesser role. They are; CalciumSodium, and Potassium

If a particular element is not present in theproper amounts, it can become a limiting factor.

A limiting factor is the one key element, which if supplied in the proper amountwould allow the organism or population to grow.

There will always be a limiting factor ona population.

MicronutrientsMany of the micronutrients are necessary in low concentrations. Micronutrients may become toxic if the concentration is too high. Ex: Copper is an essential micronutrientbut is toxic at higher levels.

High concentrations of Cu are sometimes used as a pesticide because of its toxicity

Geologic CyclesCycles responsible for formation of Earthare referred to as geologic cycles.

1)The Tectonic Cycle: creation, destruction, and re-creation of outer crust of the Earth.

The outer crust of the Earth is called theLithosphere.

Lithospheric plates move, on average, 2 to 15 cm/year.

Environmental Effects:

Physical: determines the physical makeup of continents, locations of oceans, changeocean and atmospheric currents, creation of islands (evolution) . Ex: Movement of S. America away from Africa

Chemical: At plate boundaries, new materials are created from within the Earth, materials are buried and become oiland coal deposits.Ex: deep sea hydrothermal vents that support varied life, spreading sea floorcreated from magma from within the mantle of Earth.

2) The Hydrologic Cycle: movement of water from oceans to atmosphere, atmosphere to land, land to ocean,

The total volume of water on Earth is about 1.3 billion km3.

Major storage pools include the oceans, glaciers and ice caps, ground and surfacefresh water, and the atmosphere.

Important Environmental Impact:

Most of the water is stored in the oceans.As water evaporates from the oceans, mostis returned directly via precipitation.

458/505 km3/year, or 90.6%

47km3/year is moved onto land, and fallsas precipitation.

119km3/year falls as precipitation on land surfaces.

Of that total, 60%, or 72km3/year, is lost to evaporation back to the atmosphere.

Only 40%, or 47 km3/year, is sent into surface and sub-surface storage and runoff.

What are the implications for future population growth, based on the hydrologic cycle?

Remember the concept of a limiting factor.How will the availability of water, (especially clean water) effect the growth of the human population?

Are there any answers?

3) The Rock Cycle: depends on both theTectonic cycle ( for energy) and the Hydrologic cycle (for water).

The rock cycle produces both rock and soil,and recycles minerals.

Three types of rock are produced: igneoussedimentary, and metamorphic.

Igneous rocks are formed via molten material from Earth’s mantle.

Weathering (erosion) of these rocks produces sediments, including sand, silt,clay, gravel, and pebbles. Weathering also produces dissolved minerals and elements.

Ex: Granite, obsidian, lava

Weathered materials accumulate, and become sedimentary rock when theconditions are correct.

Heat, pressure, and chemistry all lead tosediments becoming rock.

Some sedimentary rock is biologicallyproduced, by coral reefs. Ex: Limestone

Ex: sandstone, shale, chalk

Metamorphic rock is formed from both igneous and sedimentary rock.

Immense pressure and heat formmetamorphic rock. The pressure creates rock that is unlikethe parent rock, hence the name.

Ex: chalk (S) into marble (M), Shale (S)into slate (M), Granite (I) into gneiss (M)

Different Types of CyclesMaterials and elements cycle into and out of an Ecosystem at varying rates.One of the largest factors in determiningthe rate of cycling is whether or notthe element has a phase that is taken into the atmosphere.

Metals as a rule are not moved into the atmosphere, therefore they cycle slowly

For example, Calcium does not form a gas,so does not have a large component in the atmosphere.

Sulfur ( a non metal) does form a gas, so does have a large pool in the atmosphere (hydrogen sulfide, sulfur dioxide)Elements that do not pool in the atmospheretend to be limiting factors, especially in land ecosystems.

Chemical Cycling and Natural Balance

For an ecosystem to sustain life, energy must be continuously added, and storage ofessential elements must not decline.

Elements are not always in a steady state,but must be replaced as they are used up or lost. Rate of gain must at least equal rate of loss. G > L, or G = L

Unfortunately, mankind has disrupted thenatural flow of many elements, or raisedthe rate of loss until it is much greater than the rate of gain. (G < L)-Removal of tropical rainforest trees,which are the main pool of elements inthe tropical forest. (minerals, N,P,K)-Increased erosion of all sorts leads to loss of essential nutrients from soil, muchfaster than they are replaced.

Some Major Chemical Cycles1)Carbon Cycle: an important element, the basic building block of all life on Earth.

Although basic for life, it is one of the least abundant elements by weight.It constitutes 0.032% of the weight of the Earth’s crust

Carbon has a gas phase (CO2, CH4), so is present in the atmosphere.

Carbon enters the atmosphere via the respiration of organisms, the burning oforganic material, and via diffusion fromthe oceans. It is removed from the atmosphere viaphotosynthesis of green plants and photosynthetic bacteria. The cycling of carbon is fairly rapid, with15% of the total carbon in the atmospherebeing taken up and released annually.

Inorganic carbon occurs as several forms:-as carbonate and bicarbonate (limestone,shells of marine animals)Carbon is taken into the ocean via diffusionof CO2 from the atmosphere. It is then taken up by organisms and converted intocarbonate and bicarbonate.

Aquatic and marine plants also use CO2 directly in photosynthesis.

Carbon is transferred from land to ocean byerosion and movement in rivers.

Carbon compounds moved this way are inorganic (CO2) and many types of organicmaterials (living matter, fine particles oforganic matter)

Winds also blow organic materials into theocean.

Human Effect on Carbon CycleIncreased burning of fossil fuels has addedabout 3 billion metric tons/year to the atmosphere. (Mostly CO2)

Deforestation leads to conversion of organiccarbon into inorganic CO2. (burning anddecomposition).

Carbon-Silicate CycleAlthough carbon does cycle fairly rapidlyfrom atmosphere to ocean and organic life,it does get tied up for geologically long periods in the carbon-silicate cycle.

1)Carbon dioxide in the atmosphere dissolves in water, forming carbonic acid.H2CO3. This is why all rain is slightly

acidic.

2) Chemical weathering of silicate rocksin the Earth’s crust releases bicarbonate ions (HCO3-) as well as calcium ions (Ca++)

3) Eventually these ions are transported to the ocean, where marine organisms use the bicarbonate and calcium to construct new shells.

4) When these organisms die, they fall to the bottom, and are incorporated into the sediment.

5) These sediments eventually become rock, and are transported via plate tectonics, and the cycle starts over.

The carbon thus trapped is held for a very long time as rock.

The Nitrogen CycleNitrogen is a biologically important elementbut cannot be used directly by organisms. It must first be converted to nitrate (NO3

-)or the ammonium ion (NH4

+).

This is accomplished either by lightning, which oxidizes atmospheric N into nitrate,or by bacteria, which fix N into ammonium.

Nitrogen FactsAbout 79% of the atmosphere consistsof nitrogen gas (N2)Nitrogen is virtually impossible to metabolize by itself, because the gasis held by a triple bond. Because of this, nitrogen is often a limiting factor in nature.

The 5 Steps of the Nitrogen Cycle

1)Nitrogen Fixation2) Nitrification3)Assimilation4)Ammonification5)Denitrification

1)Nitrogen Fixation -converting molecular nitrogen (N2) to Ammonia (NH3) Most is converted via biological activity.

-fixation by beneficial bacteria onplant roots, and in soil.

Rhizobium (bacteria) is associated withroots of many plants, most notably soy beans. -symbiotic relationshipFarmers often alternate soy beans with other crops that have high nitrogen needs,because the soybeans (and their associatedbacteria) add nitrogen to the soil.

2) Nitrification: when water is added to ammonia (NH3), it is converted tonitrate ions (NO3

-).

3) Assimilation: Plants absorb NH4+,

NH3 and NO3- to form amino acids

and proteins.

4) Ammonification: Plants convert NO3- into ammonia (NH3).

Ammonia is the required form of Nfor plants to use.Ammonium Nitrate fertilizer (NH4NO3) is often used because of itshigh Ammonia content.

5) Denitrification: some bacteria useNitrogen compounds (NH3, NO3)as a part of their metabolism, and give off N2 as waste. These bacteria (Pseudomonas, Enterobacter) are particularly useful in sewage treatment to lower Nitrogenoutput in wastewater.