Nutrient Cycles

Nutrient Cycles

1. The Carbon Cycle

   • All organisms need carbon to make essential compounds
   • The carbon cycle is how carbon moves through living organisms and the environment
   • 1. The nutrients are taken up by the producers as simple inorganic molecules
     • 2. The molecule is incorporated into more complex molecules within the producer
       • 3. When the producer is eaten, the nutrient passes into consumers
         • 4. It then passes through the food chain
           • 5. When the organism dies, its more complex molecules are broken down back into simple molecules by saprobiotic organisms.
             • Saprobiotic organisms secrete enzymes which break down the carbon compounds in dead organic material. The decomposers then absorb the products of digestion for use in respiration
   • Fluctuations in CO2 Concentration
     • The amount of respiration and photosynthesis going on varies on a daily and a yearly basis, so the amount of atmospheric CO2 changes

2. The Greenhouse Effect and Global Warming

   • When solar radiation reaches the earth, some is reflected back into space, some is absorbed by the atmosphere and some reaches the earth.
     • The radiation that reaches the earth is absorbed, and reemitted back into space.
       • However, some of this radiation is absorbed by clouds and greenhouse gases that will reflect the radiation back to earth. This causes a heating effect known as the greenhouse effect
   • Greenhouse Gases
     • CO2
       • Responsible for approx. 50 – 70% of global warming
         • Remains in the atmosphere for >100 years and the increase in CO2 concentration is mainly due to human activity (burning fossil fuels)
     • Methane
       • Remains in the atmosphere for ~ 10 years Produced when microorganisms breakdown the organic molecules of which other organisms are made (decomposers/intestinal dwellers)
   • The earth has always shown periodic fluctuation in temperature so we cannot say for certain that human activity is to blame
     • What we can say however is that the atmospheric levels of carbon dioxide have increased since the start of the industrial revolution and that these seem to be linked with increasing temperature
   • Consequences of Global Warming
     • Will affect the niches available in a community and so the distribution of species will alter
     • Melting of Polar Ice Caps
     • A rise in sea level
     • Higher temperatures and less rainfall in some areas
     • Greater rainfall and intense storms
     • The life cycles and population of insect pests would alter as they adapt to the changed conditions so tropical diseases would spread
     • Fill reservoirs as greater rainfall in some areas
     • Increase the rate of photosynthesis

3. The Nitrogen Cycle

   • All living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogen containing compounds
   • Stages
     • Ammonification
       • It is the production of ammonia from organic ammonium containing compounds
       • Saprobiotic organisms, feed on these compounds releasing ammonia which then forms ammonia ions in the soil
         • Ammonia containing compounds include urea, proteins, nucleic acid and vitamins found in faeces and dead organisms
     • Nitrification
       • It is the conversion of ammonium ions to nitrate ions
         • This is an oxidation reaction and so releases energy. It is carried out by free living soil microorganisms called nitrifying bacteria and these require oxygen
         • The conversion occurs in two stages
           • Oxidation of ammonia ions to nitrite ions (NO2-)
           • Oxidation of nitrite ions to nitrate ions (NO3-)
     • Nitrogen Fixation
       • It is the process by which nitrogen gas is converted into nitrogen containing compounds
         • It is carried out industrially, naturally when lightening passes through the atmosphere and by microorganisms
           • There are two types of these organisms
             • Free-living nitrogen fixing bacteria
               • These bacteria reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen rich compounds are released from them when then die and decay
             • Mutualistic nitrogen-fixing bacteria
               • These bacteria live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria
     • Denitrification
       • When soil becomes waterlogged and therefore short of oxygen there is an increase in the number of anaerobic denitrifying bacteria
         • These convert soil nitrates into gaseous nitrogen
       • This reduces the availability of nitrogen containing compounds for plants and so for an area of land to be productive there must be oxygen in the soil to prevent the build up of denitrifying bacteria

4. Use of Natural and Artificial Fertilisers

   • In agricultural systems, the crop is removed and so the nitrate is not returned and has to be replaced
     • This is done by the addition of natural or artificial fertilisers
       • Natural (organic) fertilisers
         • Which consist of the dead and decaying remains of plants and animals as well as animal wastes such as manure and bone meat
       • Artificial (inorganic) fertilisers
         • Which are mined from rocks and deposits and then converted into different forms and blended to give the appropriate balance of minerals for a particular crop.
           • Compounds containing nitrogen, phosphorus and potassium are almost always present
   • How fertilisers increase productivity
     • Nitrogen is an essential component of proteins and DNA.
     • Both mineral and nitrogen are needed for plant growth. Where nitrates are readily available, plants are likely to develop earlier, grow taller and have a greater leaf surface area
       • Which means they have a higher rate of photosynthesis and greater productivity

5. Environmental Consequences of using Nitrogen Fertilisers

   • Reduces species diversity because nitrogen-rich soils favour the growth of rapidly growing species and these out-compete many other species.
   • Leaching
     • Rain water can dissolve soluble nitrates and carry them deeper into the soil beyond the reach of plant roots.
     • The nitrates may then be able to find there way to water courses and into water that is used for human consumption.
     • High levels of nitrates in water can cause inefficient transport of oxygen to the brain in babies.
   • Eutrophication
     • Eutrophication is the process by which nutrients build up in bodies of water
     • In most lakes and rivers there is naturally very little nitrate and so nitrate is the limiting factor for plant and agal growth
       • As the nitrate concentration increases as a result of leaching, it ceases to be a limiting factor for the growth of plants and algae so both grow exponentially
         • As algae grow at the surface, the upper layers of water become densely populated with algae. This is called an algal bloom
           • The dense surface layer of algae absorbs light and prevent it from penterating to lower depths
             • Light then becomes the limiting factor for the growth of plants and algae at lower depths and so they eventually die
               • The lack of dead plants and algae is no longer a limiting factor for the growth of saprobiotic organisms and so these too grow exponentially, using the dead organism as food.
                 • The saprobiotic bacteria use up the oxygen in the water and release the nitrates from the decaying organims
                   • Oxygen is a limiting factor for aerobic organisms such as fish and so they eventually die.
                     • Without any aerobically respiring organisms, anaerobically respiring organisms no long have to compete and so they begin to reproduce exponentially.
                       • Anaerobic organisms further breakdown other dead material thus producing more nitrates as well as some toxic wastes such as, hydrogen sulphide which makes the water putrid.

Images and files in the map