1. Biogeochemical Cycles Lecture prepared by Aimee C. Wyrick
Chapter 22
Biogeochemical Cycles
Lecture prepared by
Aimee C. Wyrick

3. Chapter 22 Biogeochemical Cycles
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

4. 22.1 There Are Two Major Types of 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

5. 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

6. 22.1 There Are Two Major Types of Biogeochemical Cycles
Hybrid of gaseous and sedimentary cycles occur
Sulfur

7. 22.1 There Are Two Major Types of Biogeochemical Cycles
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

8. 22.1 There Are Two Major Types of Biogeochemical Cycles
All biogeochemical cycles have a common structure
Inputs
Internal cycling
Outputs

9. Figure 22.1

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 22.5 The Carbon Cycle Is Closely Tied to Energy Flow
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

17. 22.5 The Carbon Cycle Is Closely Tied to Energy Flow
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

18. 22.5 The Carbon Cycle Is Closely Tied to Energy Flow
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

19. Figure 22.2

20. 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

21. Figure 22.3

22. 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

23. Figure 22.4

24. 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

25. 22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land
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

26. Figure 22.5

27. 22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land
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

28. 22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land
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)

29. 22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land
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)

30. 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

31. Figure 22.6

32. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

33. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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!

34. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

35. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric 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

36. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

37. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

38. Figure 22.7

39. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

40. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric 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

41. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

42. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
The nitrogen pool
Atmosphere: 3.9 × 1021 g
Terrestrial
Biomass: 3.5 × 1015 g
Soils: 95 × 1015 to 140 × 1015 g

43. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

44. Figure 22.8

45. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
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

46. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
Animation: Energy Flow and Nutrient Cycling – Part 1

47. 22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen
Animation: Energy Flow and Nutrient Cycling – Part 2

48. 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

49. Figure 22.9

50. 22.9 The Phosphorus Cycle Has No Atmospheric Pool
In freshwater and marine ecosystems, the phosphorus cycle moves through three states
Particulate organic phosphorus (PP)
Dissolved organic phosphates (PO)
Inorganic phosphates (Pi)

51. 22.9 The Phosphorus Cycle Has No Atmospheric Pool
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

52. 22.9 The Phosphorus Cycle Has No Atmospheric Pool
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

53. Figure 22.10

54. 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

55. Figure 1

56. Ecological Issues Nitrogen Saturation
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

57. Ecological Issues Nitrogen Saturation
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

58. Figure 2

59. 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

60. Figure 22.11

61. 22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous
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)

62. 22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous
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

63. 22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous
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

64. 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

65. 22.11 The Global Sulfur Cycle Is Poorly Understood
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

66. 22.11 The Global Sulfur Cycle Is Poorly Understood
Forest fires emit 3 × 1012 g S annually
Volcanic activity contributes to the global cycle of sulfur

67. Figure 22.12

68. 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

69. 22.12 The Oxygen Cycle Is Largely Under Biological Control
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)

70. Figure 22.13

71. 22.12 The Oxygen Cycle Is Largely Under Biological Control
Due to oxygen’s reactivity, its cycling in the ecosystem is complex
Carbon dioxide + calcium  carbonates
Nitrogen compounds  nitrates
Iron compounds  ferric oxides

72. 22.12 The Oxygen Cycle Is Largely Under Biological Control
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

73. 22.12 The Oxygen Cycle Is Largely Under Biological Control
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

74. 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

75. 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