"New/February 1991/10M.""Water Quality."" Focus area : nutrients and bacterial 
wastes.""Nitrogen in the environment.""Published by University Extension. University of Missouri-Columbia.

Buchholz, Daryl

Killpack, Scott

University of Missouri: MOspace

Focus Are.a NutrientsWater Bacterial WastesQuality	 and PUBLISHED BY UNIVERSITY EXTENSION, UNIVERSITY OF MISSOURI-COLUMBIA - Nitrogen in the Environment-Nitrogen Cycle N Scott Killpack and Daryl Buchholz, Extension Agronomy itrogen is important to all life. Nitrogen in the atmosphere or in the soil can go through many complex chemical and biological changes, be combined into living and non­living material and return back to the soil or air in a continuing cycle. This is called the nitrogen cycle. A B~sic Look at the Nitrogen Cycle Plants need nitrogen to grow, develop and produce seed. The main source of nitrogen in soils is from organic matter. Soils in Missouri commonly contain one to four percent organic matter. Organic matter largely arises from plant and animal residues. The nitrogen in organic matter is largely in organic forms which plants can not use. Bacteria found in soils, convert organic forms of nitrogen to inorganic forms which the plant can use. Nitrogen is taken up by plant roots and combined into organic substances in the plant, such as enzymes, proteins and chlorophyll. Chlorophyll gives the plant its green color. When the plant dies, it decays and becomes part of the organic matter pool in the soil. The basic nitrogen cycle is illustrated in Figure 1. It shows nitrogen changing from organic matter in the soil, to bacte­ria, to plants and back to organic matter. Nitrogen Cycle Nitrate--Bacteria convert Ammonium to Plant-Usable Nitrate Ammonium-Bacteria convert Organic Nitrogen into Plant-Usable Ammonium Plants take up Nitrates and Ammonium Figure 1: Oversimplifying, the nitrogen cycle works this way: 1.	 It begins when plant and animal wastes decompose, adding nitrogen to the soil. 2.	 Bacteria in the soil convert those forms of nitrogen into forms plants can use. 3.	 Then plants actually use the nitrogen in the soil to grow. 4.	 People and animals eat the plants; then animal and plant residues return nitrogen to the soil again, completing the cycle. $0.25	 WQ252 Printed with Soy Ink on Recycled Paper Archive vrson -- See extension.missori.eduAtmospheric Nitrogen (N2)Nitrogen Cycle near the soil ......Denitrification­ :~~~~:Bacteria convert Nitrate back to At­mospheric Nitrogen Nitrate-Bacteria convert Ammonium to Plant-Usable Nitrate Leaching-Leaching moves Nitrates Factories produce Nitrogen fertilizers. Municipal Waste Manage­ment Fac;:ili­ties produce Nitrogen­containing sludge. Legume Plants convert inorganic Atmospheric Nitrogen into plant-usable form Volatilization -Urea fertilizers and manure on or surface turn to ammonia gas and escape back into the atmosphere Rain storms deposit inorganic, Atmospheric Nitrogen, directly into the soil. Plants take up Nitrates and Ammonium below the root zone and out of the cycle. Figure 2: A More Comprehensive Look at the Nitrogen Cycle Other Ways Nitrogen Enters the Cycle is through Inorganic Ntirogen from the Atmosphere and Factories. 1.	 Rain storms contribute atmospheric nitrogen through rain drops that reach the soil. 2.	 Legumes, such as soybeans, alfalfa and clovers, are plants that can convert atmospheric nitrogen into plant-usable nitrogen. See WQ277. . 3.	 Factories that produce nitrogen fertilizers add nitrogen to the soil when farmers and gardeners "feed" their crops. 4. Nitrogen in sewage sludge from municipal waste plants can be used to fertilize farm fields. The concern with -these forms is that the incremental amount of nitrates they add to the nitrogen cycle may threaten groundwater. Ways. Nitrogen is Lost to th~ Cycle For the most part, the n~trogen cycle is soil based.- Nitrogen is lost from the cycle in four ways: 1.	 Denitrifcation ---- Bacteria change nitrate in the soil to atmospheric nitrogen, which joins the atmosphere. See WQ255. 2.	 Volatilization turns urea fertilizers and manures on the soil surface into gases that also join the atmosphere. See WQ257. Together, these first two processes account for most of the nitrogen lost to the cycle - a concern for soil fertility. 3.	 Runoff carries the nitrogen in fertilizers and manures and the nitrogen in the soil into our rivers and streams - a concern for water quality.­4.	 Leaching carries nitrates so deep into the soil, plants can no longer use them, producing a dual concern ­for lost fertility and for water, quality, as nitrates enter the groundwater and the wells that provide our drinking water. See WQ262 For more Information on the process, see W0260 - dMlnerahzaflon,lt W0254 - Nltnflcatlon and W0259 "Nitrogen in the Plant." WQ252 .More About the Nitrogen Cycle The largest single source of nitrogen is the atmosphere. It is made up of 78 percent of this colorless, odorless, nontoxic gas. However, plants are unable to use nitrogen as it exists in the atmosphere. Nitrogen from the air (N2) enters the nitrogen cycle through several unique types of microorganisms that can convert N2 gas to inorganic forms usable by plants. Some of these microorganisms live in the soil, while others live in nodules of roots of certain plants. Nitrogen also can enter the cycle form other sources besides the air, manures and decaying plant materials. Nitrogen also can enter the cycle from the application of commercial nitro­gen fertilizers. Nitrogen can be lost from the cycle. It can be lost to the atmosphere, removed by harvesting crops or lost to surface water or groundwater. However lost, nitrogen can enter the cycle again through one of the processes discussed above or through other processes. These additional pathways of gains and losses to the nitrogen cycle are illustrated in Figure 2. Impact to Water Quality· Nitrogen becomes a concern to water quality when nitrogen in the soil is converted to the nitrate (N03-) f<?rm. It is a concern because nitrate is very mobile and easily moves with water in the soil. The concern of nitrates and water quality i~ .generally directed at ground­water. However, nitrates can also enter surface waters such as ponds, streams and rivers. The presence of nitrates in the soil are largely the result of natural biological processes associated with the decomposition of plant residues and organic matter. Nitrates can also come from rainfall, animal manures, and nitrogen fertilizers. Whether nitrates actually enter groundwater depends on underlying soil and/or bedrock conditions, as well as the depth to groundwater. If depth to groundwater is shallow and the underlying soil is.sandy, the potential for nitrates to enter groundwater is relatively high. However, if depth to groundwater is deep and the underlying soil is heavy clay, groundwa­ter contamination from nitrates is not likely-. Once nitrates get into groundwater, the greatest concerns are for infants, less than one year old, and for young and 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-nitrogen. 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. WQ252 The initial draft was written by Karen DeFelice, former associate extension agronomist; Nyle Wollenhaupt, former state extension agronomist; and Daryl Buchholz, state extension agronomist. This material is based upon work supported by' the United States Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9203. • Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 3D, 1914~\l University in cooperation with the United States Department of Agriculture. Gail L. Imig, Director, ~Extension Cooperative Extension Service, University of Missouri and Lincoln University, Columbia, UNIVERSITY OF MISSOURI Missouri 65211.• An equal opportunity institution. COLUMBIA WQ252 New/February 1991/1OM 

Nitrogen Cycle (1991)

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