Organisms are instrumental in the cycling of elements through ecosystems. Elements such as C, N, P and S take part in global cycles . These elements are taken up by organisms from their environment in form of organic and/or inorganic compounds , and are eventually released back into the environment in an altered form. The turnover rates can vary from months to years (atmoshere), hundred to thousand years (ocean) and millons of years (continents)

• Carbon Cycle
  The global carbon cycle includes the earth's atmosphere, oceans, vegetation, soils and fossil fuels.
  • Carbon reservoirs:
    

    Carbon reservoir	Amount in Billions of Metric Tons
    Atmosphere	578 (as of 1700) - 766 (as of 1999)
    Soil Organic Matter	1300 to 1500
    Ocean (dissolved carbon)	38,000 to 40,000
    Marine Sediments and Sedimentary Rocks	50,000,000 to 100,000,000
    Terrestrial Plants	550 to 600
    Fossil Fuel Deposits	5000 - 10000

    Carbon Cycle , Fig. 56.13

  • Increased CO2 in the atmosphere
    The concentration of CO2 in the atmosphere has increased by about 30% since the start of the industrial revolution ( middle of the 19th century). The increase in atmospheric CO2 concentrations comes primarily from fossil fuels combustion (coal, oil, and natural gas) and from changes in land use (clearing of forests) CO2 emissions , Fig. 56.4

    The increasing atmospheric CO2 concentrations are caused by human activities. Until the industrialization in the 19 th century atmospheric CO2 levels were stable, that means the amount of carbon removed by carbon fixation is in equilibrium with the amount of carbon released on Earth The calculation below shows a discrepancy between additional CO2 emission by human activity and increase in atmospheric CO2 levels. , carbon either has to accumulate on land or in the atmosphere.

    CO2	Amount in Billion Metric Tons
    Atmospheric increase	+ 3.3±0.2
    Emissions from Fossil fuels	+ 5.5±0.5
    Emissions from changes in land use	+ 1.6±0.7
    Oceanic uptake	- 2.0±0.8
    Missing carbon sink	- 1.8±1.2

    source: Woods Hole Research Center

  • Effects of Increased CO2 levels:
    Fig. 56.15

    Global warming
    Sun radiation passes through the atmosphere, warming the earth’s surface. The Earth surface (land and water) release the heat (infrared radiation). Water vapor, CO2, CH4 as well as chlorofluorocarbons absorbs infrared radiation, resulting in increased temperatures. The contineously increasing atmospheric CO2 concentrations trap more heat and cause the greenhouse effect . Will the increase in carbon dioxide lead to global warming?
    Some evidence:
      * Gradual increase of ocean and land temperatures.
      *Receding glaciers, large ice sheets breking off Antarctic ice cover
      * Woody shrubs growing in northern Alaska tundra, which 50 years ago were barren
      * Many species of birds and butterflies are moving north and breeding earlier in the spring
    

• Nitrogen cycle , Fig. 26.15 , Fig. 56.16
  Within in most ecosystems nitrogen is primarily stored in living and dead organic matter. Organic nitrogen ( amino acids, DNA bases) is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition. Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter from ammonium salts to nitrates or nitrites. (nitrification). These forms of N are soluble and can be carried through groundwater to rivers and into the oceans. Processes such a denitrification (in the ocean as well as in anaerobic soils )return N back into the atmosphere

  Atmospheric nitrogen must be fixed by micro-organisms before it is available for use by plants and the animal. Human activities have at least doubled the amount of nitrogen available for uptake by plants ( more than nitrogen supplied by natural processes ):
  

  • nitrogen fertilizers, nitrates leaching into groundwater
  • nitrogen oxides released through combustion.
  • Livestock release a large amounts of ammonia from their wastes.
  
  The huge increase in nitrogen loading of the environment has had a number of consequences.
  • large rise in the nitrogen levels of drinking water supplies
  • Acidification of forests, soils and surface waters by N-oxide emissions
  • unwanted plant and algal growth in many freshwater habitats and coastal areas ( resulting in oxygen deprivation for other species )
  • reduction of plant diversity

• Sulfur cycle
  Compared to CO2 and N2 naturally occuring sulfur is "already fixed". Sulfur occurs in nature as sulfate, e.g. gypsum (CaSO4), (FeS2) and elemental sulfur. The sulfate concentration in the ocean is 2.4 g/L . In anearobic waters, sedimentary organisms generate hydrogen sulfide. The atmospher contains SO2 gas ( rleased by volcanic activity)
  . Sulfate is reduced to organic sulfur compounds (R-SH) by plants, fungi and bacteria. Sulfate reducing bacteria generate hydrogen sulfide (H2S) from sulfate and photosynthetic green and purple sulfur bacteria as well as some chemolithotrophs oxidize hydrogen sulfide to elemental sulfur. Organic sulfur compounds such a dimethyl sulfide are released into the ocean by algae .
  Fig 56.17

• Phosphorus cycle
  Soucre of P : from rocks, minerals through weathering (P forms insoluble phosphates with Ca, Al). Organisms need P for : ATP, RNA, DNA, phospholipids). P is returned to dissolved phosphates by excretion and phosphatizing bacteria . Turn over rates: several thousands of years. P- pollution: phosphates in detergents (fresh water algal blooms)