Nitrogen Information Sheet

Background

Nitrogen gas (N2) makes up 79% of the atmosphere; however, this form is of no use to plants and animals. In order to use this source of nutrients, nitrogen must be converted to other forms called ammonia (NH3) and nitrates (NO3-). The process by which this is completed is called the nitrogen cycle.

In the first step of the nitrogen cycle, called nitrogen fixation, bacteria, blue green algae, and lightning convert nitrogen to ammonia (NH3). Some plants can use ammonia directly as a nitrogen source, some cannot. So bacteria can convert the ammonia first into nitrites (NO2-) then into nitrates (NO3-). Plant roots pick up nitrates to use for protein, DNA, and RNA synthesis. Herbivore animals obtain the desired amount of nitrates by eating plants, while carnivores obtain nitrates by eating the herbivores. Nitrogen can be reused when it is returned to the soil through animal urine, feces, carcass decay, and plant decay. Bacteria break down the nitrogenous compounds in organic matter (dead plants and animals or waste products) into ammonia, which then can be oxidized (combined with oxygen) by other bacteria to form nitrites and nitrates again. There are even bacteria that convert nitrates from dead plants and animals directly back to nitrogen gas (N2) in a process called denitrification. The whole cycle can start again.

Nitrate levels in nature can sometimes limit plant and animal growth. In freshwater ecosystems, the limiting nutrient for growth is usually phosphorus, not nitrates. See the phosphorus information sheet to learn more about that nutrient. Our test kits measure for the amount of nitrates (NO3-) or the specific amount of nitrogen (N) within those nitrates.

Human Impact

The nitrogen cycle can supply all the necessary nitrates to a locally adapted, undisturbed ecosystem. However, humans can alter the nitrogen cycle by disrupting the ecosystem or adding excessive nitrogen to the system. Repeated planting and harvesting of crops can deplete nutrients. For instance, if a nutrient rich wheat field is harvested and that wheat is shipped to the other side of the country, we have removed the opportunity for some of those nitrates to return to that field. A completely closed nitrogen cycle in this case would include leaving all wheat plant remains on the field, having the farmer eat the wheat, return the resulting human manure to the field, and eventually having the farmer die and decompose on the field. That would keep the nitrogen in the same location. Society doesn't work that way. Instead, crops are sent everywhere on earth. People eat them and then human waste is sent to a wastewater treatment plant. There it is treated to remove bacteria and the remaining nutrient rich sludge is usually sent to a landfill instead of back to farmers' fields. In some cases, the sewage treatment systems fail or are overloaded (often in rainstorms) and sewage (full of nutrients) is dumped into a stream heading downstream far from the farmer's field.

To replace all the missing nitrates from the cycle, farmers must add fertilizers to their fields. If more is added than the plants need, extra nitrates and ammonia can be washed into waterways by rain. Nitrates can also wash off over-fertilized lawns or pastures where animals waste has accumulated. Any practices that promote soil erosion (poor tilling practices on farms, removal of riparian zones, deforestation, and construction) can contribute nitrates to a waterway as nutrient rich soil washes into the water. Extra nitrates in our waterways from over-fertilization, soil erosion, and sewage inputs can disrupt the stream ecosystem.

Even though phosphorus is usually the growth-limiting nutrient for plants, extra nitrogen to a stream can cause excessive growth by photosynthetic aquatic life such as phytoplankton (algae, some protists, and cyanobacteria), and macrophytes (flowering, leafy plants and mosses). Algal blooms (excessive growth) can create a soupy green stream. Aquatic weeds can clog waterways making boating and swimming difficult. Most importantly, these plants cannot live forever. Eventually they die and are decomposed by bacteria, a process that pulls oxygen out of the water. This process is called eutrophication. Lower oxygen levels can stress fish and aquatic insects, possibly even causing pollution sensitive creatures to leave or die. See the dissolved oxygen information sheet to learn more about the importance of oxygen in a stream.

Excretions of aquatic organisms can be very rich in ammonia, but unless a stream is overpopulated with organisms, the levels of ammonia in a stream will not rise. Some lakes and ponds have large ammonia loads added from the excrement of large goose and duck populations, whose natural predators may have been eliminated by man.

In some areas with high nitrate contents in drinking water, cases of infant methemoglobinemia or "blue baby syndrome" have occurred. The nitrates interfere with the blood's (red blood cell's) ability to carry oxygen. Cattle also can have a similar response when drinking nitrate contaminated water. Fish can suffer from a condition called "brown blood disease".

Water Quality Criteria

The Environmental Protection Agency limits the amount of nitrates to 10 mg/L for domestic water supplies for health reasons. Although there is no set criteria for nitrate concentrations in fresh waters, to protect ecological habitats a maximum of 5 mg/L of nitrates has been recommended.