Chapter 23: Biogeochemical Cycles

Many chemical reactions take place in abiotic components of the ecosystem


The biogeochemical cycle


Two types of biogeochemical cycles:
  • Gaseous biogeochemical cycles
  • Sedimentary biogeochemical cycles
    • Rock phase (weathering)
    • Salt solution phase



All biogeochemical cycles have a common structure
Internal cycling






Outputs Represent a Loss of Nutrients from the Ecosystem

The output (export) of nutrients depends on the cycle
Organic matter can be carried out of an ecosystem
Nutrients are released slowly from organic matter as it is decomposed
Farming and Logging
Fire converts a portion of the standing biomass and soil organic matter to ash






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





The Carbon Cycle

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


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


Billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions in billions of tons of carbon per year. White numbers indicate stored carbon.




Carbon Cycling Varies Daily and Seasonally


Barrow, Alaska



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!



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
Dead organic matter (in soil): 1500 Gt
Living matter: 560 Gt
Atmosphere: 750 Gt

The global carbon cycle (flux values in gigatons)





The surface water acts as the site of main exchange of carbon dioxide between atmosphere and ocean



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)**** Global Warming!




The Nitrogen Cycle

Nitrogen is available to plants in two forms
Ammonium (NH4+)
Nitrate (NO3)

The nitrogen cycle in terrestrial and aquatic ecosystems



  • The nitrogen cycle consists of four processes:
    • fixation
    • mineralization or ammonization
    • nitrification
    • denitrification


The blue boxes represent stores of nitrogen, the
green writing is for processes that occur to move the
nitrogen from one place to another and the red
writing are all the bacteria involved.





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


*Lichens: composite organism consisting of a fungus and a photosynthetic partner growing together in a symbiotic relationship.

The photosynthetic partner is usually either green algae (commonly Trebouxia) or cyanobacterium (commonly Nostoc).



Ammonification occurs when ammonium (NH4+) is converted to NH3 as a waste product of microbial activity


Nitrification is the stepwise conversion of NH4+ to NO2 (by genus Nitrosomonas) and then conversion of NO2 to NO3 (by genus Nitrobacter)


Denitrification is the chemical reduction of NO3 to N2O and N2 (by Pseudomonas) which are then returned to the atmosphere

The importance of bacteria in the cycle is immediately
recognized as being a key element in the cycle, providing
different forms of nitrogen compounds assimilable by
higher organisms



The nitrogen pool
Atmosphere: 3.91021 g
Biomass: 3.51015 g
Soils: 951015 to 1401015 g


Nitrogen loss
Terrestrial and aquatic denitrification: 200 1012 g/yr
Nitrogen input
Freshwater drainage: 36 1012 g/yr
Precipitation: 30 1012 g/yr
Biological fixation: 15 1012 g/yr




  • Global nitrogen overload: fixed nitrogen (ammonia and nitrogen oxides) is beginning to overwhelm a wide range of ecosystems:
    • forests
    • lakes
    • rivers
    • coastal areas







The Phosphorus Cycle






  • On land, phosphorus gradually becomes less available to plants over very long periods of time due to surface runoff.


In freshwater and marine ecosystems, the phosphorus cycle moves through three states
Particulate organic phosphorus (PP)
Dissolved organic phosphates (PO)
Inorganic phosphates (Pi)



  • 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 Net Primary Production)










The Sulfur Cycle



Atmospheric sulfur sources (as H2S, hydrogen sulfide):


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












File:Acid rain woods1.JPG

Effect of acid rain on a forest in the Czech Republic











The Oxygen Cycle





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





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


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