Activity 9: Background Information

Nitrogen Cycle
Even if nitrogen constitutes 78% of the atmosphere in the form of nitrogen gas (N₂), it is not accessible to living things that need nitrogen for protein or deoxyribonucleic acid (DNA). N₂ or atmospheric nitrogen is made available only through lightning and soil-bacteria action. Lightning allows N2 to react with oxygen-forming nitrates (NO₃–) that enter the soil where they are absorbed by the plants’ roots. Plants transform the nitrates into amino acids to form protein. When animals eat the plants, the plant proteins are broken down into amino acids and then formed into animal proteins.

Another way to access N₂ involves the soil bacteria (nitrogen-fixing bacteria) found in root nodules of legumes like peas, beans, and clover. These bacteria change N₂ into nitrates. Excess nitrates produced go into the soil for other plants to use.

Oxygen-using decomposers or bacteria (aerobic bacteria) change the nitrogen-containing chemicals in dead bodies and waste into ammonia (NH₃). This ammonia is transformed into nitrites (NO₂) by other bacteria and still others convert the nitrites back into nitrates, which are then absorbed by the plants’ roots.

Bacteria that do not need oxygen (anaerobic bacteria) do the opposite. They change the nitrates into nitrites and back into N₂. This process called denitrification ensures the return of nitrogen into the atmosphere in the form of nitrogen gas, making the cycle complete.

Phosphorous Cycle
Unlike the other cycles, phosphorous has no atmospheric counterpart. It is needed in DNA, cell molecules, and in bones and teeth. It cycles in two ways–short term and long term.

The short but faster term involves decomposers that release the phosphates as they break down waste and dead tissue. The phosphates are absorbed by the plants through their roots and are then cycled through the food webs.

The long term involves millions of years where weathering of rocks and leaching result in phosphates reaching the ocean. Here they may be absorbed by aquatic organisms and is cycled through the food webs. With the death of these aquatic organisms, their bones and shells settle on the ocean floor and eventually become rocks. These rocks may become exposed by uplifting where they are then subjected to weathering and the cycle continues.

This activity focuses on the observational skills of the students as they experimentally investigate the effect on the growth of algae by a lawn fertilizer and a detergent. The students are encouraged to figure out how and why these two chemicals that are known to be “good” can actually be “bad” where other organisms are concerned.