"It was great! I am excited to use this information with my own classroom in the future."
"Keep doing this workshop! This is by far the most useful workshop I've ever been to."
-Workshop Participants

Nitrogen Cycle - Background and Discussion

Background
Discussion Questions

Background Information:

Nitrogen is all around us and is found in a variety of forms throughout the global environment. The nitrogen cycle demonstrates the many different paths nitrogen may follow around our earth and the different reservoirs in which nitrogen is stored.

Although nitrogen is an important component of proteins for both plants and animals, most plants cannot use the nitrogen gas (N₂)directly. The process of converting nitrogen to a “biologically available” form - in other words, converting nitrogen gas to a form that plants can use - is called nitrogen fixation. Only specialized bacteria in soil and certain types of algae in water can fix nitrogen. Lightning strikes also result in some nitrogen fixation.

Human activities have a huge impact on global nitrogen cycles. The amount of biologically available nitrogen generated by human activities now far exceeds nitrogen fixed by bacteria, algae and lightning. Humans produce synthetic fertilizers, burn fossil fuels, grow legumes (which fix nitrogen) as a crop, and engage in various land clearing, burning and wetland draining activities, which all release nitrogen in forms that plants use. See the table below for more details on the amount of fixed nitrogen humans produce.

Global Sources of Biologically Available (Fixed) Nitrogen
Anthropogenic (Human) Sources	Annual Release of Fixed Nitrogen (teragrams)*
Fertilizer	80 tg
Legumes and other plants grown as crops	40 tg
Fossil fuels (coal plants and automobiles)	20 tg
Biomass burning	40 tg
Wetland draining	10 tg
Land clearing	20 tg
Total from human sources	210 tg

Natural Sources
Soil bacteria, algae, lightning	10 tg
Source: Vitousek, Peter M., et al. (1997). Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications7(3), 737-750.

* 1 tg (teragram is equal to 1 million metric tons)

Discussion Questions:

1. Discuss the following terms with your students. Be sure they understand how these relate to the nitrogen cycle.

Microorganisms are extremely important in converting nitrogen from one form to another.
Nitrification is the transformation of ammonia to nitrite and finally to nitrate. This usually happens by microorganisms in conditions with plenty of oxygen.
Denitrification, occurs by a different group of microorganisms, is the transformation of nitrate to nitrite and finally to nitrogen gas.
Nitrogen fixation is the conversion of nitrogen gas to nitrate or ammonia, and occurs when there’s little or no free oxygen in the environment (e.g., in the sediments at the bottom of a lake).

2. What form of nitrogen is measured in the water test?

This test measures the nitrate (NO₃) + nitrite (NO₂) concentration in the water, but we generally refer to the results as nitrate only, because nitrite concentrations are usually extremely low in surface waters. Nitrate is one of the two common forms of inorganic nitrogen found in water, and is readily used by plants. The other form, ammonia, is less common in unpolluted surface water.

The nitrate test has two steps. The first step is to shake the water sample with a small amount of ground cadmium. This “strips away” one oxygen atom from the nitrate molecule, converting everything to nitrite. The second step is a color test designed to analyze for nitrite concentrations. The intensity of the pink color is proportional to the amount of nitrite.

3. What are the impacts of the large amounts of biologically available nitrogen released by human activities?

Nitrogen in such abundance has many impacts.

• Over-fertilization of lakes and estuaries is called eutrophication. Runoff of synthetic fertilizers, runoff from feed lots, runoff from drained wetlands, burned areas, and atmospheric deposition all contribute to this problem. The nitrogen stimulates excessive plant growth. The plants eventually die and decompose, which can use up all the dissolved oxygen in the water. This can lead to fish kills in lakes and has produced an area the size of Massachusetts on the floor of the Gulf of Mexico that is a “dead zone”. This means that no oxygen is left so nothing else can live there.

• Burning of fossil fuels creates nitric and nitrous oxide as a byproduct in the atmosphere. This falls to earth as nitric acid, a strong acid that can cause acidification of lakes, especially in areas where soils are not well buffered. Heavy nitrogen deposition on soils can acidify the soils, which damages terrestrial ecosystems. They are also a source of fertilizer.

• Combustion engines also convert nitrogen gas to nitrous oxide, which is a greenhouse gas (traps heat and contributes to global warming).

• Nitrogen can be directly toxic to humans and animals as well. Drinking water nitrate concentrations above 10 ppm (mg/l) cause blue baby syndrome and ammonia at much lower concentrations can be toxic to fish.