Nitrogen in the Environment: Denitrification
Reviewed by David Brune
Department of Agricultural Engineering
Scott C. Killpack and Daryl Buchholz
Department of Agronomy
Soil microorganisms need oxygen for fuel. When the soil is very wet, water fills in the spaces between soil particles. This leaves very little room for oxygen. Some soil microorganisms can get the oxygen they need from the oxygen portion of the nitrite (NO2-) and nitrate (NO3-) forms of nitrogen. When this happens, nitrogen (N2) and nitrous oxide (N2O) gas are formed. These gases return to the atmosphere, and there is a net cycle in the soil. This is called denitrification
Two main factors influence denitrification:
- The oxygen supply in the soil.
- The soil microorganisms.
Anything that changes these two factors will change how much nitrogen is lost and how fast this happens. These factors include the amount of organic matter, soil water content, soil oxygen supply, soil temperature, soil nitrate levels and soil pH.
A small amount of denitrification can be found taking place in soils all the time. Denitrification becomes significant when the soil is waterlogged for 36 hours or more. The longer the soil is waterlogged, the greater the potential loss of nitrogen from the soil system.
The conditions that cause denitrification can have positive as well as negative implications on water quality. The positive implication is that denitrification converts nitrates (NO-) to nitrogen gas, resulting in a net loss of nitrate from the soil system. The negative aspect of denitrification is that it takes place in soils that are waterlogged. In this situation, water will move downward in the soil. Because nitrate can move easily with water, nitrate can move below the root zone of plants and potentially on down into groundwater.
Whether nitrates continue to move downward, and into groundwater, depends on underlying soil and/or bedrock conditions, as well as depth to groundwater. If depth to groundwater is shallow and the underlying soil is sandy, nitrates will likely enter groundwater. However, if depth to groundwater is deep and the underlying soil is heavy clay, groundwater contamination from nitrates is not likely. In some soils, dense hardpans may be present, in which case nitrate movement will not progress beyond the depth of the hardpan. In these kind of soils, nitrate is more likely to undergo denitrification than move into groundwater.
Once nitrates get into the groundwater, the greatest concerns are for infants less than one year old and for young or pregnant animals. High levels of nitrates can be toxic to newborns, causing anoxia, or internal suffocation. Seek alternative water sources if nitrate levels exceed the health standard of 10 ppm nitrate-N. Do not boil water to eliminate nitrates. It increases nitrate levels rather than decreases them. The most common symptom of nitrate poisoning in babies is a bluish color to the skin, particularly around the baby's eyes and mouth. These symptoms of nitrate toxicity are commonly referred to as the "blue-baby" syndrome.