Nutrient cycles
Joanna George-Johnson, Contributor

Shanique Coleman speaks during The Gleaner's students forum on agriculture at William Knibb High School, in Trelawny, on February 7. - Claudine Housen/Staff Photographer

Like energy, matter can neither be created nor destroyed. However, it is no doubt that you have studied that matter is constantly being used up by the organisms in ecosystems. For example, plants take up water and minerals from the soil. You may then ask how is it that these resources are not completely removed from the ecosystem. Mechanisms exist to ensure that these materials are returned from the organic environment (e.g., the plant) to the inorganic environment (e.g., the air). These mechanisms are known as nutrient cycles. This week, we take a look at the carbon and nitrogen cycles.

Carbon in Living Systems

Carbon is a component of all organic substances, and as all organisms are made up of organic molecules, this element is required for maintaining life.

For example:

• Amino acids - the building blocks of protein contain carbon.
  
  
• Carbohydrates - the chief sources of energy to living things are made mostly of carbon.
  
  
• Lipids, hormones and DNA are substances which all contain carbon.

The Carbon Cycle

• Carbon is always present in the air in the molecule carbon dioxide.
  
  
• Plants take in this carbon dioxide by the process of gaseous exchange, and this decreases the CO2 concentration in the air.
  
  
• The CO2 absorbed is used in photosynthesis. This is the process by which CO2 is combined with water to produce glucose (an organic compound). Carbon is, therefore, made available to the organic environment by this process.
  
  
• Plants produce other organic substances using the glucose (e.g. starch) and are, therefore, food sources for animals.
  
  
• Animals eat these plants, and produce energy from the glucose, starch and other organic substances present, by the process of respiration. Note: Plants also carry out respiration.
  
  
• The process of respiration produces CO2 as a waste product. The dead bodies and wastes of plants and animals also decompose, releasing CO2.
  
  
• Alternatively, some plants and animals die and become fossils over millions of years. Under heat and pressure they become hydrocarbons, which are used as fuels (e.g., crude oil).
  
  
• The fuels are mined and burnt, producing CO2 as a waste product of their combustion.
  
  
• Respiration, decomposition and combustion of fossil fuels all release CO2, thus returning CO2 to the air (i.e. the inorganic environment).

Nitrogen

Nitrogen is a component of amino acids - the building blocks of protein. However, nitrogen in the air is chemically unreactive, and must be converted into a form, which can be used by living systems.

The Nitrogen Cycle

For ease of study, I will divide the nitrogen cycle into three groups.

Processes which cause decrease in atmospheric nitrogen

• Nitrogen gas is converted to nitrates by the process of nitrogen fixation. This is carried out by nitrogen fixing bacteria (e.g. rhizobium) present in the roots of leguminous plants and the soil.
  
  
• Nitrogen fixation may also be achieved by the action of lightning on nitrogen gas, which supplies energy for the reaction. The nitrates are then dissolved in rain water and are absorbed into the soil.

Intermediate Processes

• The nitrates are subsequently absorbed by plants which produce proteins from these nitrates.
  
  
• Some plants are eaten by animals and the nitrogen in the plant proteins becomes a part of the animal proteins.
  
  
• Death and decay of plants and animals, as well as putrefaction (rotting) of their nitrogenous wastes produces ammonium compounds. Urea (NH2CONH2), a solute dissolved in urine, is an example of a nitrogenous waste. The proteins in the plants' and animals' dead bodies are also converted to ammonia. This involves the action of decay/putrefying bacteria.
  
  
• By the process of nitrification, the ammonia is converted first to nitrites which are then converted to nitrates. Nitrifying bacteria cause this process.

Processes which cause increase in atmospheric nitrogen

• By the process of denitrification, nitrates in the soil are converted to atmospheric nitrogen. Bacteria, namely denitrifying bacteria catalyze this process.

In summary:

• Matter is being constantly cycled within ecosystems
  
  
• Plants are often the link between the organic and inorganic environment. In other words, they convert inorganic materials (e.g. nitrates) into organic materials (e.g. protein).
  
  
• Bacteria also play a major role in the cycling of nutrients in ecosystems.
  
  
• The carbon and nitrogen cycles ensure that the carbon and nitrogen removed from the atmosphere is returned.
  
  
• In this way, the concentration of nitrogen and carbon dioxide gases in the air are kept at an approximately constant concentration.

Questions:

• Outline the role of bacteria in the cycling of nitrogen. [8 marks]
  
  
• The balance of carbon within ecosystems is self-regulatory. However, human influences upset this balance.

i) Do you consider this statement to be true?
ii) Give one reason for your answer. [2 marks]