• Nutrient cycles referred to as biogeochemical cycles • Gaseous forms of carbon, oxygen, and nitrogen occur in the atmosphere and cycle globally • Less mobile elements, including phosphorous, cycle on a more local level • Still, gains and losses from outside of the ecosystem are generally small when compared to the rate at which nutrients are cycled within the system.

1. Nutrient Cycling in Ecosystems

2. • With respect to energy, the biosphere is
an open system
– energy from the sun is constantly being
added
– energy is constantly lost to space
• For nutrients, the biosphere is a closed
system
– no external source of nutrients
– all nutrients ultimately are recycled

3. A little chemistry…
• Inorganic compounds – do not include
carbon (C) and hydrogen (H)
– CO2, NO2, H2S, PO4
• Organic compounds – include C and H
– sugars, proteins, amino acids, nucleic
acids (DNA & RNA)

4. 3 types of chemical changes in n
nutrient cycles:
1. Transformations between organic and
inorganic forms of elements
– Assimilation – incorporation of inorganic
chemicals into organic molecules
• photosynthesis: CO2 -> sugar
– Dissimilation – transformation of organic
chemicals into inorganic forms
• respiration: sugar -> CO2

5. 2. Transformations between different
inorganic forms of elements
– N2 -> NH3
3. Changes in the energy state of
chemicals…

6. • Each type of molecule has energy
stored in its chemical bonds
• Breaking bonds frees energy
• Creating new bonds uses energy

7. • Oxidized compounds (with oxygen) are
usually low energy
• Reduced compounds (usually with
hydrogen) have high energy

8. CH4 + O2  CO2 + energy
reduced oxidized
CO2 + H2O + energy  C6H12O6
oxidized reduced

10. CO2 CO2 PO4 PO4 NH3
sugar ATP protein
energy
energy

11. • Nutrients, unlike energy, are retained within the
ecosystem
Recall….Element Cycling through
Ecosystems
•There is continual recycling between organisms and
the physical environment
• In other words, elements alternate between living
and nonliving compartments of the ecosystem

12. Element Cycling: Overview
• Nutrient cycles referred to as biogeochemical cycles
• Gaseous forms of carbon, oxygen, and nitrogen occur
in the atmosphere and cycle globally
• Less mobile elements, including phosphorous, cycle on
a more local level
• Still, gains and losses from outside of the ecosystem
are generally small when compared to the rate at
which nutrients are cycled within the system.

13. Consumers
Producers
Nutrients
available
to producers
Abiotic
reservoir
Geologic
processes
Decomposers
rates of cycling in different ecosystems depends
mostly on differences in rates of decomposition
Generalized Biogeochemical Cycle

14. Ecosystem Compartments

15. The Four Main Element Cycles:
C, N, P and H20
•The biological importance of each chemical
• The inorganic and organic forms in which each
chemical is available or used
• The major repositories for each chemical
• The key processes in the biogeochemical cycle
of each chemical

16. Carbon Cycle
• Carbon forms the framework for all organic life
• Autotrophs convert CO2 (inorganic form) to organic sugars
used by all organisms-animals and prokaryotes
• Reservoirs of carbon in fossil fuels, soils, water
• CO2 taken up by plants = CO2 expired by plants, animals
and microbes
• In seawater, CO2 reacts with water to form carbonate
bicarbonate (HCO3-) and carbonate (CO3
2-) ions

17. The Carbon Cycle
• Carbon cycle is based on carbon dioxide
which makes up only about 0.03% of the
atmosphere.
– All terrestrial heterotrophic organisms obtain
carbon indirectly from photosynthetic organisms.
• Most organic compounds formed as a result of carbon
dioxide fixation are ultimately broken down and
released back into the atmosphere.

18. The Carbon Cycle
• Roughly 700 billion metric tons of carbon
dioxide are located in the atmosphere, and
approximately 1 trillion metric tons are
dissolved in the oceans.
– Fossil fuels contain another 5 trillion metric
tons.
• Increasing fuel consumption is liberating carbon at
an increasing rate.

19. Carbon Cycle
1. Photosynthesis/respiration
– carbon assimilation in photosynthesis =
respiration by animals
– most photosynthesis/respiration occurs in
oceans
– residence time = 31 yrs
2. Ocean-atmosphere interchange
– oceans absorb CO2 from air
– this buffers CO2 changes in the
atmosphere

20. 3. Precipitation of carbonates in water
CO2 + H2O  H2CO3 (carbonic acid)
Ca2+ + CO3
2- ↔ CaCO3

21. • CaCO3 precipitates to form limestone,
especially where plants take up CO2
• Ca is replaced by input from surface
waters

22. • Naturally, respiration balances
assimilation, so CO2 levels stay roughly
constant
• Burning of fossil fuels adds 6.5 GT,
clearing forests 1.5 GT each yr
• CO2 levels increase by 3.2 GT each yr
– missing sink?

23. Carbon Cycle

24. Carbon Cycle

25. Carbon Cycle (units = gigatons, 109 tons)

26. Nitrogen Cycle
• Nitrogen is a component of proteins (e.g.,
protein required for photosynthesis), and is in
nucleic acids
• Plants and algae can use two inorganic forms:
Ammonium (NH4
+) and Nitrate (N03
-) . Bacteria
can also use Nitrite (N02
-). And animals?
• Main reservoir of nitrogen is atmosphere, which
is 80 % nitrogen gas. Nitrogen also in soils and
water

27. • Nitrogen (N)
– essential for proteins, DNA/RNA
– 70% of the atmosphere
– atmospheric form (N2) can’t be used by
plants
– often the limiting resource for plants,
especially in oceans, deserts
• Nitrogen fixation – conversion of N2 to
NH3 (ammonia) by bacteria or lightning

28. 1. Biological N Fixation
a. What is it?
• Conversion of atmospheric N2 to NH4
+ (actually,
amino acids)
• Under natural conditions, nitrogen fixation is the
main pathway by which new, available nitrogen
enters terrestrial ecosystems

29. • Nitrogen-fixing bacteria assimilate N2
and transform it into NH3
– some N fixation occurs in free-living soil
bacteria
– most in roots of nitrogen-fixing plants
• Legumes – plants in the pea family
(Fabaceae)
– have root nodules inhabited by nitrogen-
fixing Rhizobium bacteria

30. The nitrogen fixing bacteria are found inlumps on the
roots called root nodules. The bacteria and the plant
have a symbiotic relationship: the bacteria benefits
by having food and shelter from the plant and the
plant benefits by having nitrates produced by the
bacteria.
Roots of a legume plant (peas, beans and clover).

31. Nitrogen fixation
b. Who does it?
• Carried out by bacteria
– Symbiotic N fixation (e.g., legumes, alder)
– Heterotrophic N fixation (rhizosphere and other carbon-rich
environments)
– Phototrophs (bluegreen algae)
• The characteristics of nitrogenase, the enzyme that catalyzes
the reduction of N2 to NH4
+, dictate much of the biology of
nitrogen fixation
– High-energy requirement (N triple bond)
• Requires abundant energy and P for ATP
– Inhibited by O2
– Requires cofactors (e.g., Mo, Fe, S)

32. Types of N-fixers
• There’s no such thing as a N-fixing
plant
• Symbiotic N-fixers
– High rates of fixation (5-20+ g-N m-2 y-1)
with plants supplying the C (and the plant
receiving N)
– Protection from O2 via leghemoglobin
(legumes)
– Microbial symbiont resides in root nodules
• Bacteria (Rhizobia) – Legumes (Lupinus,
Robinia)
• Actinomycetes (Frankia) - Alnus, Ceanothus
(woody non-legumes)
– N-fixation rates reduced in presence of high
N availability in the soil

33. Types of N fixers
• Associative N fixers
– Occur in rhizosphere of plants (non-nodulated);
moderate rates with C supply from plant root
turnover and exudates (1-5 g-N m-2 y-1)
– Reduced [O2] by rapid respiration from plant roots
– Azotobacter, Bacillus

34. Types of N fixers
• Free-living N fixers
– Heterotrophic bacteria that get organic C from environment and where N is
limiting (e.g., decaying logs)
– Rates low due to low C supply and lack of O2 protection (0.1-0.5 g-N m-2 y-1)
• Also, cyanobacteria (free-living photo-autotrophs); symbiotic lichens
(cyanobacteria with fungi offering physical protection)

35. – plants provide bacteria with carbohydrates
– bacteria reduce N2 to NH3, which can be
used by plants
• Nitrogen fixation is most important in
low-N environments, early in primary
succession

36. • Nitrification – oxidation of nitrogen by
bacteria
NH3  NO2
-  NO3
-
– energy-releasing reactions
– nitrates can be used by plants, but they
have to be reduced (requires energy)
• In low-oxygen settings (oceans, soils,
sediments), denitrification occurs
NO3
-  NO2
-  NO  N2O  N2
– nitrogen is lost from the systems

37. Ammonification – oxidation of carbon in
amino acids, freeing ammonia
– carried out by all organisms when recycling
proteins
– important in decomposition

39. A. Inputs
2. Nitrogen Deposition
• Wet deposition: dissolved in precipitation
• Dry deposition: dust or aerosols by
sedimentation (vertical) or impaction (horizontal)
• Cloud water: water droplets to plant surfaces
immersed in fog; only important in coastal and
mountainous areas

40. 3. Rock weathering as a source of N?
• Some sedimentary rocks contain substantial
amounts of N with high rates of N release (up to
2 g-N m-2 y-1); however, most rocks contain little
N.

41. B. Internal Cycling of Nitrogen
• In natural ecosystems, most N taken up by
plants becomes available through
decomposition of organic matter
– Over 90% of soil nitrogen is organically bound in
detritus in a form unavailable to organisms
– The soil microflora secrete extracellular enzymes
(exoenzymes) such as proteases, ribonucleases,
and chitinases to break down large polymers into
water-soluble units such as amino acids and
nucleotides that can be absorbed

42. 2. Nitrification
a. Why is Nitrification Important?
• Nitrate is more mobile than ammonium, so more
readily leached from soil
• Substrate for denitrification (N loss as a gas)
• Generates acidity if nitrate is lost from soil
• Loss of nitrate results in loss of base cations

43. 2.b. Controls on Nitrification
• NH4
+ + 2O2  NO3
- + 2H+ + H2O
– Two-step process conducted by chemoautotrophic
bacteria:
• First step conducted by Nitrosomonas (other Nitroso-),
NH4
+  NO2
- , ammonia mono-oxygenase, need O2
• Second step conducted by Nitrobacter, NO2
-  NO3
-
– Controls:
• NH4
+
• O2
• Slow growth of nitrifiers

44. Denitrification – where?
• Very important in wetlands, riparian areas.
• Spatially very patchy in well-drained soils.
http://www.wldelft.nl/cons/area/mse/ecom/im/wetland-1.jpg
http://en.wikipedia.org/wiki/Image:Riparian_zone_florida_everglades.

45. C. N outputs
2. Leaching
• Erosional losses
• Solution losses
– NO3
- >> DON >NH4
+
– Greatest when water flux is high and biological
demand for N is low (e.g., after snowmelt!)

46. Nitrogen in
the air
animal protein
dead plants & animals
urine & faeces
ammonia
nitrites
nitrates
plant made
protein
decomposition by bacteria & fungi
bacteria
(nitrifying bacteria)
nitrates absorbed
denitrifying
bacteria
root nodules
(containing nitrogen
fixing bacteria)
nitrogen fixing plant
eg pea, clover
bacteria

47. Nitrogen cycle

48. Nitrogen Cycle

49. Nitrogen Cycle (units = megatons, 106 tons)

50. • Sulfur - used in amino acids
• Most organisms acquire S as sulfates (SO4
2-)
and assimilate S by reduction
• Bacteria and atmospheric O2 oxidize organic
S in detritus, creating SO4
2-
• In anaerobic environments, bacteria use
sulfates as an energy source and produce S
or H2S
Sulfur cycle

51. Sulfur Cycle (units = megatons, 106 tons)

52. The Sulfur
Cycle
Hydrogen sulfide
(H2S)
+
Water (H2O)
Sulfur dioxide (SO2)
and
Sulfur trioxide (SO3)
Dimethl
(DMS) Industries
Sulfuric acid
(H2SO4)
Oceans
+
Ammonia (NH2)
+
Oxygen (O2)
Ammonium sulfate
[(NH4)2SO4]
Animals
Plants
Sulfate salts
(SO4
2-)
Hydrogen sulfide
(H2S)
Decaying
organisms
Sulfur
(S)
Fog and precipitation
(rain, snow)
Aerobic conditions
in soil and water
Anaerobic
conditions in
soil and water
Volcanoes
and
hot springs
Atmosphere

53. Decomposition and nutrient cycling
rates
• The rates at which nutrients cycle is
strongly affected by the rates at which
decomposers work. In the tropics, warmer
temperatures and abundant moisture
cause organic material to decompose 2-3
times faster than it does in temperate
regions.

54. Decomposition and nutrient cycling
rates
• High rate of decomposition means little
organic material accumulates as leaf litter.
• In tropical forest about 75% of nutrients
are in woody trunks of trees and only
about 10% in soil. In temperate forest
about 50% of nutrients are in the soil
because decomposition is slower.

55. Decomposition and nutrient cycling
rates
• In aquatic ecosystems decomposition in
anaerobic sediments can be very slow (50
years or more).
• As a result sediments are often a nutrient
sink and only when there is upwelling are
marine ecosystems highly productive.

56. Human effects on nutrient
cycles
• Agriculture and nutrient cycling. Soils differ in
the amount of nutrients stored in organic matter
that they contain.
• Soils with large stores (e.g. prairie soils) can be
used for agriculture for many years before
requiring fertilization. In tropical forest soils,
however, there are few stored nutrients and the
soil quickly becomes exhausted.

57. Human effects on nutrient
cycles
• Nitrogen is main nutrient removed through
agriculture (when the biomass is removed
from the field).
• The removed nitrogen needs to be
replaced and industrially produced
fertilizers are used.

59. Human effects on nutrient
cycles
• Recent studies suggest that human
activities (fertilization and increased
planting of legumes) have approximately
doubled the supply of nitrogen available to
plants.
• A major problem with intensive farming is
that fertilizer runoff.

60. Human effects on nutrient
cycles
• Fertilizers that are applied in amounts
greater than plants can use or that are
applied when plants are not in the fields
leach into groundwater or run off into
streams and rivers.

61. Human effects on nutrient
cycles
• The heavy supply of nutrients causes
blooms of algae and cyanobacteria as well
as explosive growth of water weeds.

62. Human effects on nutrient
cycles
• Because respiration by plants depletes the
oxygen levels at night this process of
eutrophication can cause fish kills.
• Eutrophication of Lake Erie, for example,
wiped out commercially important
populations of fish including lake trout,
blue pike and whitefish in the 1960’s.

63. Acid rain
• Burning wood, coal and other fossil fuels
releases oxides of sulfur and nitrogen that
react with water in the atmosphere to form
sulphuric and nitric acid.
• These acids fall to Earth as acid
precipitation (rain, snow, sleet), which has
a pH of less than 5.6.

64. Acid rain
• Pollutants produced by power plants travel large
distances on the prevailing winds before falling
to the Earth.
• As a result, the areas harmed by acid rain are
usually far from the pollution’s source.
• Acid rain in Eastern U.S. caused by power
plants in midwest.

65. Acid rain
• In terrestrial ecosystems the acid rain can
leach nutrients from the soil and stunt
plant growth.
• Freshwater lakes very vulnerable to
effects of acid rain especially where
bedrock is granite. Such lakes lack a
buffering capacity because bicarbonate
levels are low.

66. Acid rain
• Many fish are intolerant of low pH levels
(e.g. at <pH 5.4 newly hatched trout die).
• Thus, acid rain has had major effects on
fish communities.

67. Acid rain
• Environmental regulation and new technology
have reduced sulfur dioxide emissions over the
past 30 years in the U.S. and they fell 31%
between 1993 and 2002.
• Water chemistry in eastern U.S. is improving,
but will require another 10-20 years to recover
even if emissions continue to decrease.

68. Environmental toxins
• Huge variety of toxic chemicals are
produced and move through food webs.
• Some are excreted, but other accumulate
in fat and become more concentrated in
upper levels of the food chain (a process
called Biomagnification).

69. Environmental toxins
• Chlorinated hydrocarbons (which include
pesticides such as DDT and industrial
chemicals such as PCBs) are well known
to biomagnify.
• PCB levels in herring gulls in Great Lakes
are 5,000 times greater than in
phytoplankton

70. 54.23

71. Environmental toxins
• At higher levels chlorinated hydrocarbons
can be toxic or severely affect hormone
levels.
• Story of DDT is well known. Widely
sprayed after WWII to kill mosquitoes and
agricultural pests.

72. Environmental toxins
• DDT accumulated in tissues of predatory birds
and interfered with deposition of calcium in
eggshells so eggs were brittle and could not be
incubated.
• DDT was banned in U.S. in 1971 after public
outcry inspired by Rachel Carson’s Silent
Spring. Still used elsewhere on the globe.

73. Increases in atmospheric CO2
levels
• Since the industrial revolution carbon dioxide
levels in atmosphere have increased due to
burning of fossil fuels and burning of forests.
• Since 1958 CO2 levels have increased 17%. By
2075, at current rates of increase, CO2 levels will
be double what they were in the mid 1800’s.

74. Increases in atmospheric CO2
levels
• Consequences of increased CO2 levels
include:
– effects on growth of plants
– changes in plant distributions
– effects on global climate

75. Increases in atmospheric CO2
levels
• Most plants grow better with higher levels of
CO2.
• However, the growth of one group of plants
called C3 plants is more limited by low
atmospheric CO2 levels.
• Under hot dry conditions, when they must limit
water loss by closing the air exchange pores in
their leaves (stomata), the CO2 levels in their
leaves falls so much that photosynthesis almost
shuts down.

76. Increases in atmospheric CO2
levels
• If global CO2 levels increase, these plants may
spread into areas where they have not
previously occurred displacing the other group
(C4 plants), which are more efficient at
photosynthesis under low CO2 levels.
• Major agricultural crops include both C3 plants
(rice, wheat and soybeans) and C4 plants (corn)
so changes in CO2 levels may affect which
plants farmers choose to plant.

77. Increases in atmospheric CO2
levels
• Effects of increased CO2 levels on forests
are being explored in a large-scale
experiment at the Duke University Forest.
• In the long-term work high levels of CO2
are being pumped into the air over forest
plots. In the experimental plots CO2 levels
are increased to 1.5X current CO2 levels.

78. Increases in atmospheric CO2
levels
• Comparison of tree growth in experimental
and control plots has shown higher rates
of photosynthesis in experimental plots,
higher soil respiration, and that pine seeds
are heavier in experimental plots.

79. Increases in atmospheric CO2
levels
• Concerns have been raised about effects
of increased CO2 levels on global climate.
• CO2 and water vapor in atmosphere trap
some heat radiated from Earth and reflect
it back. This greenhouse effect keeps the
planet warm.

80. Increases in atmospheric CO2
levels
• There is widespread concern that
increased CO2 levels is causing global
warming.
• Temperature data show a great deal of
variation and there is debate about how
much warming has occurred and how fast
it it occurring. However, there is a general
consensus that warming has occurred.

81. Increases in atmospheric CO2
levels
• An increase of one or two degrees Celsius
in global average temperature could have
major consequences.
• For example, melting of the polar icecaps
could raise sea levels dramatically.

82. Increases in atmospheric CO2
levels
• Models of climate change are complex
and include a lot of assumptions, but
global climate changes might be dramatic.
• For example a shift in the direction of the
Gulf Stream caused by changes in sea
temperatures would cause the western
European climate to become much colder.

83. Increases in atmospheric CO2
levels
• Many models also predict an increase in the
frequency and strength of hurricanes as warmer
sea temperatures provide energy that feeds
these storms.
• Increases in atmospheric CO2 levels may also
increase the acidity of oceans as some CO2 is
converted to acids such as carbonic acid.
Increased acidity may have drastic effects on
coral reefs, which may dissolve.

84. Increases in atmospheric CO2
levels
• Efforts to reduce CO2 emissions are
underway, but there is considerable
political wrangling because of economic
concerns.
• Whether the current trends will be
reversed is thus unclear.

85. Depletion of ozone levels
• A protective layer of ozone (O3) shields the
earth from harmful levels of UV radiation.
• However, the ozone layer has been
thinning since the mid 1970’s especially
over the southern hemisphere.

87. Depletion of ozone levels
• Cause of the depletion appears to be
accumulation of chlorfluocarbons (CFCs,
which are used as refrigerants and aerosol
propellants).
• When CFC breakdown products rise into
the stratosphere, chlorine contained in
them breaks down ozone and produces
oxygen.

88. Depletion of ozone levels
• Cold temperatures over Antarctic facilitate these
atmospheric reactions. Ozone hole over
Australia has resulted in higher incidences of
skin cancer.
• CFCs have now been widely banned and the
rate of ozone depletion has slowed, but the
chlorine already in the atmosphere will continue
to exert an effect for at least a century.