Biogeochemical Cycles

1
Biogeochemical Cycles

• All matter cycles...it is neither created nor
  destroyed...
• The Earth is essentially a closed system with
  respect to matter
• by matter we mean elements (carbon, nitrogen,
  oxygen) or molecules (water)
• Biogeochemical cycles
• the movement (or cycling) of matter through a
  system
• Among the Earths Geosystem are five major
  INTERCONNECTED spheres
• Atmosphere
• the vast gaseous envelope of air that surrounds
  the Earth
• Hydrosphere
• the waters of the Earth
• Lithosphere
• solid inorganic portion of the Earth (composed of
  rocks, minerals and elements).
• Biosphere
• all living things, plant and animal.
• Anthrosphere
• The human environment, Humans and all parts and
  processes of the Earth that they influence

2
The Geosystem
3
BioGeochemical Cycles

• The Cycling Elements
• Material and energy, but we can focus on several
  important components
• Macronutrients required by organisms in
  relatively large amounts
• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Phosphorous
• Sulfur
• other macronutrients
• Potassium
• Calcium
• Iron
• Magnesium
• Micronutrients required in very small amounts,
  (but still necessary)
• Boron (green plants)
• Copper (some enzymes)
• Molybdenum (nitrogen-fixing bacteria)

4
Biogeochemical Cycles

• Geological cycles include
• tectonic cycle
• rock cycle
• hydrologic cycle
• biogeochemical cycles
• Carbon
• Phosphorus
• Nitrogen
• Sulfur

5
Concept of Residence Time

• Average length of time a substance spends in a
  reservoir at steady state.
• Residence Time (time) reservoir size at steady
  state (mass) / inflow rate or outflow rate
  (mass/time)
• Example Water in oceans with respect to runoff
• Voceans / Rateriver Runoff
• 1.23 x 109 km3 / 0.026 x 106 km3/yr
• 47,300 yr
• Elements with long residence times occur in
  constant ratios with each other (e.g. ocean
  salts)
• Less easily affected by changes in rates or
  processes

6
Hydrologic Cycle

• The Earths reservoirs of water

7
Hydrologic Cycle Fluxes

• Precipitation
• total 496,000 km3/yr
• land 111,000 km3/yr
• ocean 385,000 km3/yr
• Evaporation Plant Transpiration
  (Evapotranspiration)
• total 496,000 km3/yr
• land 71,000 km3/yr
• ocean 425,000 km3/yr
• Surface runoff
• 26,000 km3/yr
• Groundwater flow
• 14,000 km3/yr
• Human Use
• 3000 km3/yr

8
Hydrologic Cycle Residence Times

• Atmosphere
• RT 12,700 km3 (relative to sum of in fluxes)
  496,000 km3/yr 0.03 yr or 9 days
• in fluxes equal out fluxes, the RT is the same
  relative to the sum of the out fluxes
• as anything that is removed from the atmosphere
  by rain or snow will also have an RT in the
  atmosphere nearly equal to this
• Ocean
• RT 1,230,000,000 km3 (relative to evap) 425,000
  km3/yr 2,900 years
• applies to the whole ocean (which can be
  separated into the surface and deep water) and
  does not incorporate circulation
• Streams and rivers
• RT 1,200 km3 with respect to outflow 26,000
  km3/yr .05 yr or 17 days
• an average, gives an idea time water spends in
  rivers and streams before it flows into the ocean
  
• Ground water
• RT 4,000,000 km3 with respect to outflow 12,000
  km3/yr 330 years
• an average... oldest ground waters can be 10,000
  to 40,000 years old
• ground waters are generally old compared with
  human lifetimes (we tend to view them as
  "eternal")
• ground waters have large sizes and long residence
  times... hard to pollute, but once polluted, hard
  to clean up

9
Carbon Cycle

• Life depends on carbon compounds
• Atmosphere
• CO
• Greenhouse Gases - climate
• carbon dioxide (CO2)
• methane (CH4)
• Water acidity control
• HCO3-
• CO32-
• Lithosphere
• Carbonate rocks
• Fossil fuels

10
Carbon Cycle Fluxes

• Flux to the atmosphere
• Plant respiration soil respiration fossil
  fuel burning deforestation ocean exsolving
  weathering...
• 6060621030.6 231.6 bmt/yr
• Flux from the atmosphere
• Plant photosynthesis ocean dissolving...
• 120 107 227 bmt/yr
• difference is buildup of carbon dioxide in the
  atmosphere of about 4 bmt/yr

11
Carbon Cycle

• Anthropogenic flux (fossil fuel burning and
  deforestation) to atmosphere 8 bmt/yr
• but atmospheric increase is only 4 bmt/yr
• Question Where does the missing 4 bmt/yr go?
• Two possibilities Photosynthesis vs. Ocean
  uptake
• Important to know this because the residence
  times are so different
• Carbon gt plants recycles quickly ( lt70 yr ) to
  atmosphere
• Carbon gt ocean recycles slowly ( gt300 yr ) to
  atmosphere

12
Carbon Cycle Residence Times

• Residence times (years) (all relative to sum of
  out fluxes)
• Land plants 5
• atmosphere 3
• the RT of carbon in the air (mostly carbon
  dioxide , but some methane ) is long enough that
  the air is well mixed (atmosphere mixes in about
  1 year)
• soils 25
• the average RT... some parts cycle very slowly
  (1,000's of years), some parts very rapidly (a
  few weeks to months... leaves, for example)
• Fossil fuels 650
• this is a combination of
• recoverable
• unrecoverable (both physically and economically)
• oceans 350
• Average of the surface water (short RT, few
  months to years) and deep water (long RT, 200 to
  400 years)... average is weighted towards deep
  water, as this is most of the water
• reflects the circulation of the ocean ( deep
  water formation )
• carbonates 150,000,000 

13
Short Term Carbon Cycle

• Dominated by 2 processes
• Photosynthesis
• Inorganic carbon converted to organic carbon
• CO2 H2O Energy ? CH2O (biomass/carbohydrates)
  O2 gas
• CH2O is a simple carbohydrate, most have N and P
• Respiration/degradation
• Organic carbon converts back to inorganic CO2
• Aerobic respiration
• CH2O (biomass/carbohydrates) O2 gas ? CO2 H2O
  Energy
• Accelerated by enzymes
• hydrolysis by hydrolase enzymes
• Reduction by reductase enzymes
• Aerobic decomposition
• Same reaction as above
• Accomplished by soil bacteria and fungi

14
Short Term Carbon Cycle

• Anaerobic decomposition (produce 80 Atm. CH4)
• Methanogenic Bacteria convert CO2 to CH4
• CO2 8H 8e- bacteria ? CH4 2 H2O
• Where the CO2 comes from organic carbon
• CH2O H2O bacteria ? CO2 4H 4e-
• Combine reactions
• 2CH2O bacteria ? CO2 CH4
• CH4 is quickly oxidized to CO2 by
• CH4 O2 ? CO2 2H2O
• Marine Organic Carbon cycle
• Producers
• Phytoplankon
• Coccolithophores
• Diatoms
• Zoopalnkton
• Formainifera
• radiolarians

15
Long Term Carbon Cycle

• Fluxes are small, Reservoirs are large
• Response time is slow
• Carbon Burial in Sediment
• Isolated from oxidation
• no decomposition, respiration
• Formation of fossil fuels
• Terrestrial Plants ? peat ? coal
• Marine Organic matter ? hydrocarbons
• Organic-rich shales and limestones
• Weathering of Organic carbon
• OC O2 ? CO2 H2O
• Accelerated by Humans
• Extraction and Burning of fossil fuels

16
Carbon Cycle

• Mauna Loa Observatory Trends
• Short Term
• Years to decades
• Not steady state, varies seasonally
• Photosynthesis (spring and summer)
• Respiration/degredation (winter)
• Long Term
• CO2 rising due to
• Fossil fuel combustion
• deforestation

17
Carbon Cycle

• Summary
• Carbon Cycle operates on both long and short
  terms
• Involves all of the Geosphere
• We will investigate aspects of the carbon cycle
  in more detail throughout the course.

18
Nitrogen Cycle

• Definitions
• Limiting Nutrient - Amount of an element
  necessary for plant life is in short supply
• Nitrogen Fixation - Chemical conversion of N2 to
  more reactive forms, e.g. NH3 (ammonia) or NO3 -
  (nitrate)
• Denitrification - Chemical conversion from
  nitrate (NO3 -) back to N2

19
Nitrogen Cycle

• Pathways and Reactions
• N-fixation
• plants and humans
• N2 to organic-N
• mineralization
• Organic-N to NH4
• by bacteria and fungi
• nitrification
• NH4 to NO3
• by bacteria
• producing NO and N2O

• denitrification
• NO3- to N2
• by bacteria
• producing N2O
• photosynthesis
• NO3- NH4
• uptake by plants
• organic-N

20
Nitrogen Cycle

• Reservoirs (in millions of metric tons )
• Atmosphere 4,000,000,000
• this form can't be used by plants (N2)
• Land Plants 3500
• Soils 9500
• Oceans 23,000,000
• Sediments and Rocks 200,000,000,000
• largest pool of nitrogen, but minor part of the
  cycle. -

21
Nitrogen Cycle

• Fluxes (in millions of metric tons/year )
• Atmospheric
• Biological Fixation 140
• Land Denitrification 130
• Oceanic Fixation 50
• Oceanic Denitrification 110
• Industrial Fixation 100
• Industrial fixation is used to make fertilizers
  to provide usable nitrogen for crops.
• Fossil Fuel Burning 20
• Biomass Burning 10
• Lightning 20
• Other
• Decay 1200
• Growth 1200
• Most flux is in land plants to/from soils plants
  recycle nitrogen since it's a limiting nutrient
• Land-to-Ocean 48
• (Rivers 36)
• (Dust 6)
• (NOx 6)

22
Nitrogen Cycle

• Specialized bacteria and lightning are the only
  natural ways that nitrogen is fixed.
• Relationship to Agriculture
• Early civilizations had to rely on natural
  regeneration of fixed nitrogen
• Annual floods bring fresh sediments (e.g., Nile
  Valley)
• Slash/burn agriculture once the soil nutrients
  are depleted, move on to a new place
• Crop rotation
• certain crops (e.g. soybeans) are good at fixing
  nitrogen,
• others (e.g. corn) use it up
• plant on alternate years

23
Nitrogen Cycle

• Residence Times
• Major Reserviors
• Atmosphere 14 million yrs.
• Land plants 3 yrs.
• Oceans 20,000 yrs.
• Soils 9 yrs.
• Atmospheric pollutants
• NO x 4 days
• N2O 120 yrs.
• Reservoirs where N2 is the dominant form of
  nitrogen ( atmosphere, ocean ) have long
  residence times.
• Reservoirs where fixed nitrogen is dominant (
  soils, plants ) have short residence times.
• N2 is very stable, but fixed nitrogen compounds
  are very reactive (that's why plants can utilize
  them)
• e.g. a common fertilizer is ammonium nitrate ,
  which is also an explosive!
• N2O , a strong greenhouse gas, doesn't go away
  quickly!

24
Atmosphere N2 and N2O Traces of NO, NO2, HNO3,
NH4NO3
Fixation of molecular N2 as amino nitrogen
Fixation of N2 as NH3
Evolution of N2, N2O, NH3 by bacteria
Emission of NO and NO2
Biosphere Biologically-bound nitrogen Such as
amino (NH2) nitrogen In protiens
Anthrosphere NH3, HNO3, NO2, Inorganic
nitrates Organic nitrogen compounds
Dissolved NH4 NH3-.
Mined Nitrates
Fertilizer, NO3-
Hydrosphere Geosphere Dissolved NO3- and
NH4 Organically-bound N in dead Biomass and
fossil fuels
Fertilizers other N pollutants
NH4, NH3- from decomposition
25
Sulfur Cycle Sources and Fluxes
Air over Continents
Air over Oceans
22
8 (44)
8
57
(534)
0.35
17
9
66
(26 40)
67
6-9
2.5
44
Biomass Burning
Volcanic S
Fossil Fuel SO2
Biogenic H2S DMS
Dry Deposition SO2 SO42-
Sea Salt
Biogenic DMS
Dry Deposition SO2
Rainfall Dry Deposition SO42-
Rainfall SO42-
Modified from Berner and Berner, 1996. Fluxes in
Tg S/tr. is sea salt component DMS dimethyl
sulfide (CH3)2S
26
Sulfur Cycle
27
Sulfur in Atmosphere

• Atmospheric reservoir small
• Lifetime in the atmosphere short
• Atmospheric chemistry complex
• 2 SO2 O2 ?2 SO3
• OH SO2 light? HO SO2 water? SO42-
• Typical lifetimes in urban atmosphere Half-life
  (hr)
• H2S 53
• CS2 1.8 x 105
• COS 0.3 x 105
• CH3SH 3 to 13
• DMS 31

28
Sulfur Cycle

• Typical Anthropogenic Emission Factors
• Coal combustion 19 g/kg coal
• Nat. gas combustion 6.4 mg/m3 gas
• Fuel oil 20 g/L oil
• Gasoline engine 1.1 g/L gasol.
• Diesel engine 5 g/L diesel
• Copper smelting 625 g/kg ore
• Cu2S O2 ?2Cu SO2
• Lead smelting (prim.) 330 g/kg ore
• Lead smelting (sec.) 75 g/kg metal
• Zinc smelting 265 g/kg ore

29
Sulfur Cycle Reaction History
Atmospheric sulfur SO2, H2S, H2SO4 (aq), CS2,
(CH3)2S
Interchange of Atmospheric S species with all
other spheres
Inorganic SO42- Soluble and insoluble forms
Sulfide oxidation
S Oxidation
Sulfate reduction
Assimilation by organisms
Elemental Sulfur, S
H2S oxidation
Biological Sulfur Incl. SH groups
Sulfides such as H2S metal sulfides, such as
FeS
Decomposition
Biodegradation
Microbial metabolism
Microbially produced Organic S in small
molecules Largely as SH and R-S-R groups
Xenobiotic sulfur as in insecticides
30
Phosphorus Cycle

• Phosphorus is a necessary, limiting nutrient
• No gas phase involved
• Phosphate runoff causes eutrophication
• increasing the nutrients in a body of water
• Dissolved phases
• Organic
• HPO42-
• H2PO4-
• Polyphosphates
• Inorganic
• PO43-

31
Phosphorus Cycle

• Reservoirs (in millions of metric tons )
• Earth's Crust 20,000,000,000 ( recoverable
  20,000)
• Note that most of the phosphorus is in rocks that
  are unrecoverable.
• Ocean 100,000
• Freshwater 100
• Land Plants 3000
• Soils 100,000 --

32
Phosphorus Cycle

• Fluxes (in millions of metric tons/yr )
• Mining 50 (humans)
• Humans have greatly accelerated P transfer from
  rocks to plants and soils
• Fertilization 50 (humans)
• Weathering 10
• Runoff 20
• Burial 13
• Decay 200
• Growth 200
• Most P in plants cycles between living and dead
  plants
• Other fluxes
• Ocean to land by sea spray 0.03
• Ocean to land by guano 0.01
• Industrial wastes 2
• Phosphorous has no stable gas phase, so addition
  of P to land is slow
• Phosphorous is a strongly limiting nutrient
  because it cannot be transferred from the ocean
  to plants very effectively --.

33
Summary Thoughts

• Humans clearly disrupt many, if not all
  biogeochemical cycles...and in the process
  threaten many ecosystems.
• In the absence of humans, are the biogeochemical
  cycles stable? Probably not...
• Advent of O2-rich atmosphere
• Catastrophic events (e.g. K-T impact)
• Periods of tectonic (volcanic) increased activity
  
• Glacial-intergalcial events
• Change is a part of natural biogeochemical cycles