copyright © 2009 pearson education, inc. chapter 22 biogeochemical cycles lecture prepared by aimee...

Copyright © 2009 Pearson Education, Inc.

Chapter 22

Biogeochemical Cycles

Lecture prepared byAimee C. Wyrick


Many chemical reactions take place in abiotic components of the ecosystem– Atmosphere– Water– Soil– Parent material

The biogeochemical cycle is the cyclic flow of nutrients from the nonliving to the living and back to the nonliving components of the ecosystem

Chapter 22 Biogeochemical Cycles


Two types of biogeochemical cycles In gaseous biogeochemical cycles, the

main pools of nutrients are the atmosphere and the oceans– Global– Nitrogen, carbon dioxide, oxygen

22.1 There Are Two Major Types of Biogeochemical Cycles


In sedimentary biogeochemical cycles, the main pool of nutrients is the soil, rocks, and minerals– Inorganic sources of minerals are released to

living animals through weathering and erosion– Phosphorus



Hybrid of gaseous and sedimentary cycles occur– Sulfur



Both gaseous and sedimentary cycles– Involve biological and nonbiological processes– Are driven by the flow of energy through the

ecosystem– Are tied to the water cycle

Biogeochemical cycles could not exist without the water cycle



All biogeochemical cycles have a common structure– Inputs– Internal cycling– Outputs



22.2 Nutrients Enter the Ecosystem via Inputs

The input of nutrients depends on the cycle– Nutrients with a gaseous cycle enter the

ecosystem via the atmosphere– Nutrients with a sedimentary cycle enter the

ecosystem via weathering of rocks and minerals


22.2 Nutrients Enter the Ecosystem via Inputs

Supplementing soil nutrients (of terrestrial habitats) are carried by rain, snow, air currents, and animals– Wet fall are those nutrients supplied by

precipitation– Dry fall are the nutrients brought in by airborne

particles and aerosols

The sources of nutrients for aquatic ecosystems – From the surrounding land in the form of

drainage water, detritus, sediment– Form the atmosphere in precipitation


22.3 Outputs Represent a Loss of Nutrients from the Ecosystem

The output (export) of nutrients depends on the cycle– Release of CO2 from expiration of heterotrophic

organisms

Organic matter can be carried out of an ecosystem– Through surface flow of water or underground

flow of water– By herbivores

Nutrients are released slowly from organic matter as it is decomposed


22.3 Outputs Represent a Loss of Nutrients from the Ecosystem

Human harvesting (farming and logging)– Nutrient loss must be replaced by fertilizers

Fire converts a portion of the standing biomass and soil organic matter to ash– Leaching and erosion of soil


22.4 Biogeochemical Cycles Can Be Viewed from a Global Perspective

Often, the output from one ecosystem represents an input to another

The exchange of nutrients among ecosystems requires us to view the biogeochemical processes on a broad spatial scale– This is particularly true of nutrients that go

through a gaseous cycle


22.5 The Carbon Cycle Is Closely Tied to Energy Flow

Carbon is so closely tied to energy flow that the two are inseparable– Ecosystem productivity = grams C

fixed/m2/year

Inorganic carbon dioxide is the source of all carbon– The inorganic carbon is fixed into the living

component through photosynthesis– Carbon dioxide is again released following

respiration



Terrestrial cycling of carbon– Input: photosynthesis– Output: respiration, decomposition,

combustion

Net primary productivity = carbon uptake (photosynthesis) – carbon loss (respiration)– Net ecosystem productivity = difference in

rates



The rate of carbon cycling is determined by the rates of primary productivity and decomposition

The rates of primary productivity and decomposition are directly affected by temperature and precipitation– In warm, wet ecosystems (e.g., tropical rain forest),

production and decomposition rates are high and carbon cycles through the ecosystem quickly

– When dead material has not completely decomposed in the past (e.g., in swamps) the matter has formed fossil fuels



Aquatic cycling of carbon– Input: photosynthesis, diffusion, transport– Output: respiration, decomposition, diffusion

Significant amounts of carbon can be bound as carbonates incorporated into exoskeletons (e.g., shells) of many aquatic organisms


22.6 Carbon Cycling Varies Daily and Seasonally

Carbon dioxide concentration fluctuates throughout the day– This is a function of the difference in

photosynthetic activity in response to sunlight and temperature


22.6 Carbon Cycling Varies Daily and Seasonally

The production and use of carbon dioxide fluctuates with the seasons– This is a function of temperature and timing of

the growing and dormant seasons With the onset of the growing season, the

atmospheric concentration begins to drop as plants withdraw carbon dioxide through photosynthesis– The fluctuations are greater in terrestrial

environments as compared to aquatic ecosystems

– Fluctuations are much greater in the Northern Hemisphere due to the larger land area


22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land

Earth’s carbon budget is linked to the atmosphere, land, and oceans and to the mass movements of air currents

The Earth contains 1023 grams (or 100 million gigatons) of carbon!– All but a small fraction of this carbon is buried

in sedimentary rock and is not actively involved in the global carbon cycle



Carbon pool involved in the global carbon cycle amounts to 55,000 gigatons (Gt)– Fossil fuels: 10,000 Gt– Oceans: 38,000 Gt (mostly as bicarbonate and

carbonate ions)– Dead organic matter: 1650 Gt– Living matter (mostly phytoplankton): 3 Gt

– Terrestrial – Dead organic matter (in soil): 1500 Gt– Living matter: 560 Gt

– Atmosphere: 750 Gt



The surface water acts as the site of main exchange of carbon dioxide between atmosphere and ocean – Uptake of CO2 depends on its reaction with

carbonate ions (CO32–) to form bicarbonates (HCO3

–)

Carbon circulates physically by means of currents and biologically through photosynthesis and movement through the food chain– Net uptake of carbon in oceans = 1 Gt/year– Net loss of carbon in oceans (due to sedimentation)

= 0.5 Gt/year



Recent studies suggest that the terrestrial surface is a carbon sink, with a net uptake of CO2 from the atmosphere

– Uptake of CO2 from the atmosphere by terrestrial systems is determined by photosynthesis

– CO2 losses from terrestrial systems are a function of respiration (especially decomposition)



More carbon is stored in soils than in living matter

The average carbon/volume of soil increases from the tropical regions poleward to the boreal forest and tundra– The greatest accumulation of organic matter

occurs in areas where decomposition is inhibited (e.g., frozen or waterlogged soils)


22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen

Nitrogen is an essential constituent of protein, a building block of all living tissue

Nitrogen is available to plants in two forms– Ammonium (NH4

+)

– Nitrate (NO3–)

The Earth’s atmosphere is 80 percent nitrogen in the form of N2

– This form is unavailable to plants for assimilation



Nitrogen enters the ecosystem via two pathways

Atmospheric deposition via wetfall and dryfall provides nitrogen in a form already available for plant uptake– High-energy fixation occurs when gaseous

nitrogen (N2) is converted to ammonia and nitrate by energy from cosmic radiation, meteorite trails, or lightning — this accounts for only 0.4 kg N/ha annually



Atmospheric nitrogen can be converted into a usable form biologically — this accounts for 10 kg N/ha annually

This fixation is carried out by:– Symbiotic bacteria living in mutualistic

associations with plants– Free-living aerobic bacteria– Cyanobacteria (blue-green algae)

Nitrogen fixation requires considerable energy– To fix 1 g of nitrogen, nitrogen-fixing bacteria

must expend about 10 g of glucose!



Rhizobium bacteria are symbiotic organisms and form nodules in the roots of host plants– Associated with leguminous plants

Free-living soil bacteria (Azotobacter, Clostridium) are prominent in converting nitrogen into a usable form

Cyanobacteria (Nostoc, Calothrix) fix nitrogen in terrestrial and aquatic ecosystems

Certain lichens may also fix nitrogen



Ammonification occurs when ammonium (NH4

+) is converted to NH3 as a waste product of microbial activity– Loss of gaseous NH3 from the soil to the

atmosphere is influenced by soil pH



Nitrification is the stepwise conversion of NH4

+ to NO2– (by Nitrosomonas) and then

conversion of NO2– to NO3

– (by Nitrobacter)– The nitrate may be taken up by plant roots or

returned to the atmosphere



Denitrification is the chemical reduction of NO3

– to N2O and N2 (by Pseudomonas) which are then returned to the atmosphere– This reduction requires anaerobic conditions– This process is common in wetland ecosystems

and bottom sediments of aquatic ecosystems



Nitrate is the most common form of nitrogen exported from terrestrial ecosystems in stream water

The amount of nitrogen recycled is usually much greater than inputs or outputs of nitrogen



Nitrogen fixation and nitrification are influenced by environmental conditions

Bacterial activity is affected by temperature, moisture, and soil pH– In highly acidic soils, bacterial action is

inhibited



The internal cycling of nitrogen is fairly similar from ecosystem to ecosystem– Assimilation of NH4

+ and NO3– by plants

– Return of nitrogen to the soil, sediments, and water via decomposition



The nitrogen pool– Atmosphere: 3.9 × 1021 g– Terrestrial

– Biomass: 3.5 × 1015 g– Soils: 95 × 1015 to 140 × 1015 g



Nitrogen loss– Terrestrial and aquatic denitrification: 200 ×

1012 g/yr– Sedimentation

Nitrogen input– Freshwater drainage: 36 × 1012 g/yr– Precipitation: 30 × 1012 g/yr– Biological fixation: 15 × 1012 g/yr



Human activity has significantly influenced the global nitrogen cycle– Conversion of native forests and grasslands to

agricultural fields– Application of chemical fertilizers to

agricultural fields

– Auto exhaust and combustion add N2O, NO, and NO2 to the atmosphere, which leads to an increase in ozone concentration of the stratosphere



Animation: Energy Flow and Nutrient Cycling – Part 1



Animation: Energy Flow and Nutrient Cycling – Part 2


22.9 The Phosphorus Cycle Has No Atmospheric Pool

Phosphorus (P) can only be cycled from land to sea and is not returned via the biogeochemical cycle

The main reservoirs of P are rock and natural phosphate deposits

Phosphorus is released by weathering, leaching, erosion, and mining– In most soils, only a small fraction of total

phosphorus is available to plants



In freshwater and marine ecosystems, the phosphorus cycle moves through three states– Particulate organic phosphorus (PP)

– Dissolved organic phosphates (PO)

– Inorganic phosphates (Pi)



Organic phosphates are taken in by all forms of phytoplankton, which are eaten by zooplankton

Zooplankton may excrete as much phosphorus daily as it stores in its biomass

Some phosphorus is deposited in sediments surface waters may become depleted while the deep waters become saturated– This phosphorus can be returned to the surface

waters and accessed by organisms when upwelling occurs



The phosphorus cycle – Little atmospheric component although

airborne transport of ~1 × 1012 g P/yr– River transport = 21 × 1012 g P/yr (only 10

percent is available for NPP)– Ocean waters are a significant global pool of P

simply due to large volume– Organic phosphorus in the surface waters is

recycled very rapidly– The phosphorous deposited in sediments or

deep waters is unavailable to phytoplankton until upwelling


Ecological Issues Nitrogen Saturation

NPP in most terrestrial forest ecosystems is limited by soil nitrogen availability.

Anthropogenic activity and high-intensity agriculture have increased inputs of nitrogen oxides in the atmosphere far above natural inputs– Generally, nitrogen is deposited in the region

where it originated and thus will vary with geography and human population density



Soil nitrogen concentration influences the rate of N uptake and plant tissue concentration

Up to a point, as nitrogen concentration increases, net primary productivity increases



As the ecosystem approaches “nitrogen saturation,” the soil and plant community suffer negative impacts– Release of other important soil cations (e.g.,

Mg) as ammonium concentration increases– Soil acidification that may contribute to toxic

levels of aluminum ions


22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous

The sulfur cycle has both sedimentary and gaseous phases– In the long-term sedimentary phase, sulfur is

tied up in organic and inorganic deposits and is released by weathering and decomposition

– The gaseous phase permits sulfur to circulate on a global scale



Atmospheric sulfur sources (as H2S)– Combustion of fossil fuels– Volcanic eruptions– Ocean surface exchange– Decomposition

Atmospheric sulfur dioxide (SO2) is carried back to the surface in rainwater as weak sulfuric acid (H2SO4)



Sulfur is incorporated into plants via photosynthesis and building of sulfur-bearing amino acids

Excretion and death return sulfur from living material back to the soil and sediments– Bacteria release it as hydrogen sulfite or

sulfate– Colorless, green, and purple bacteria each

have a unique interaction with sulfur



Pyritic rocks (those that contain FeS) can be a source of sulfur if weathered or uncovered by humans (during coal mining)– These products (e.g., sulfuric acid) can be

extremely detrimental to aquatic ecosystems


22.11 The Global Sulfur Cycle Is Poorly Understood

The annual flux of sulfur compounds (SO2, H2S, sulfate particles) through the atmosphere ~300 × 1012 g– Wetfall and dryfall of sulfate particles



Oceans are a large source sulfate aerosols, though most are redeposited in precipitation and dryfall– Dimethylsulfide [(CH3)2S] is the major sulfur

gas emitted (16 × 1012 g S/yr) from the oceans and is generated by biological processes

H2S is the dominant sulfur form emitted from freshwater wetlands and anoxic soils



Forest fires emit 3 × 1012 g S annually Volcanic activity contributes to the global

cycle of sulfur


22.12 The Oxygen Cycle Is Largely Under Biological Control

The major source of oxygen (O2) that supports life is the atmosphere and may originate from two processes– Breakup of water vapor = 2 H2O O2 + 4 H+

– Photosynthetic production

The input of oxygen must have exceeded its loss (due to respiration) for an overall abundance of oxygen



Water and carbon dioxide are other sources of oxygen

Oxygen is also biologically exchangeable in various molecules that are transformed by living organisms (e.g., hydrogen sulfide to sulfates)



Due to oxygen’s reactivity, its cycling in the ecosystem is complex– Carbon dioxide + calcium carbonates– Nitrogen compounds nitrates– Iron compounds ferric oxides



Ozone (O3) is an atmospheric gas– In the stratosphere (10 to 40 km above Earth)

it acts as a UV shield– Close to the ground, it is a pollutant



In the stratosphere, O2 is freed by solar radiation and freed oxygen atoms rapidly combine with O2 to form O3 (this reaction is reversible)– Under natural conditions, a balance exists

between ozone formation and destruction– Human activity has interrupted this balance,

and various molecules (e.g., CFCs) reduce the production of O3


22.13 The Various Biogeochemical Cycles Are Linked

The biogeochemical cycles are linked through their common membership in compounds that form an important component of their cycles– Nitrate and oxygen in nitrate

Autotrophs and heterotrophs require nutrients in different proportions for different processes– Stoichiometry is the branch of chemistry that

deals with the quantitative relationships of elements in combination


22.13 The Various Biogeochemical Cycles Are Linked

The limitation of one nutrient can affect the cycling of all the others (e.g., macro and micro plant nutrients)– Nitrogen availability will influence a plant’s

rubisco concentration– Rubisco concentration affects photosynthetic

rate and carbon assimilation– The carbon cycle is directly affected by

nitrogen availability

copyright © 2009 pearson education, inc. chapter 22 biogeochemical cycles lecture prepared by aimee...

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