What Happens to Nitrogen in Soils?
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E-59 6-01 What Happens to Nitrogen in Soils? T. L. Provin and L.R. Hossner* fficient crop production requires an adequate Three important methods for changing nitrogen gas + supply of all essential plant nutrients. However, (N2) to ammonium (NH4 ) are: the use of commercial nitrogen (N) fertilizers to E I Free-living N -fixing bacteria increase production, maintain profits and provide low- 2 cost food and fiber is a necessity of modern agricul- I N2-fixing bacteria in nodules on the roots of ture. In general, crops need nitrogen in the greatest leguminous plants, and quantity of all plant nutrients. I Nitrogen fertilizer production factories. The environmental effect of nitrogen fertilizers has been a long-term issue. Concern over nitrogen pollu- Another important method of N2 conversion is tion of rivers, lakes and groundwater has caused agri- through lightning. When lightning flashes, the nitrogen - cultural producers to become increasingly aware of gas in super-heated air is converted to nitrate (NO3 ) - their potential contribution to the total pollution prob- and nitrite (NO2 ). Lightning may account for 1 to 50 lem. pounds of plant-available nitrogen per acre per year. To effectively use nitrogen and to limit its adverse Although nitrogen enters the soil in several chemi- impact on the environment, producers need to develop cal forms, it eventually converts to the inorganic - - an awareness of the chemistry of nitrogen and how it nitrate (NO3 ) ion. Figure 1 shows that NO3 can be is added to and removed from the soil. used by plants, be converted back to nitrogen gas or Commercial fertilizers used by agricultural produc- be leached downward with soil water. ers are a significant source of nitrogen addition to Commercial fertilizers, plant residues, animal soils. Nitrogen is continuously recycled through plant manures and sewage are the most common sources of and animal waste residues and soil organic matter. nitrogen addition to soils. Rates of application vary Nitrogen is removed from the soil by crops, gaseous widely. Single application rates may be as high as 150 loss, runoff, erosion and leaching. The magnitude and mechanism responsible for nitrogen losses depend upon the chemical and physical properties of a given soil. Figure 1 is a schematic representation of the pos- sible gains and losses of soil nitrogen. Chemistry of Nitrogen Nitrogen accounts for 79 percent of the air we breathe. The surface 6 inches of a fertile prairie soil may contain 2 to 3 tons of nitrogen per acre. The air above this same acre will contain about 35,000 tons of inert nitrogen gas (N2). Most of the nitrogen found in soil originated as N2 gas and nearly all the nitrogen in the atmosphere is N2 gas. This inert nitrogen cannot be used by the plant until it is changed to ammonium + - (NH4 ) or nitrate (NO3 ) forms. *Extension Soil Chemist and Crop Sciences Professor; The Texas Figure 1. The nitrogen cycle is a complex system A&M University System. involving the air, soil and plant. pounds of nitrogen equivalent per acre for crops such growth and profitable yields or desired aesthetics. The as coastal bermudagrass. However, such high applica- actual requirements for a given crop varies according tion rates should be limited to soils with a low poten- to the production potential and are influenced greatly tial for erosion and runoff. by climatic factors. Nitrogen in organic materials (plant residues, ani- Because most soils are low in plant-available nitro- mal manures, sewage, soil organic matter) is present as gen, the nitrogen requirements are often supplied as part of proteins, amino acids and other plant and commercial nitrogen fertilizer. Nitrogen requirements microbial materials. It becomes available to plants above 150 pounds per acre generally are divided into only after the compound is decomposed by soil two or more applications. However, only plant nitro- microorganisms. This is called “mineralization” (Fig. gen in the harvested crop actually leaves the field. The 2). The first step of mineralization is “ammonification.” remainder of plant nitrogen is returned to the soil as + The ammonium (NH4 ) derived from ammonification plant residue and reenters the cycle as organic nitro- - is then converted to nitrate-nitrogen (NO3 -N) by “nitri- gen as illustrated in Figure 1. fying” bacteria in the soil through the process called Gaseous loss of nitrogen takes place by denitrifica- “nitrification.” tion or ammonia volatilization. Denitrification is a - Figure 2. The mineralization process. process through which nitrate nitrogen (NO3 -N) is converted to gaseous nitrogen oxide (N O) or elemen- Mineralization = Ammonification + Nitrification 2 R* – NH – R* tal nitrogen (N2). This involved the action of anaerobic bacteria (those which do not require free oxygen) and R* – NH NH + NO- NO - 2 4 2 3 commonly occurs in wet or water-logged soils. R* = N – R ammonium nitrite nitrate - - Ammonium Nitrification NO3 NO2 N2O or N2 Organic N to Inorganic N Nitrate Nitrite Nitrous oxides or Gas Mineralization Nitrate to nitrite to nitrogen gases R = Complex organic molecule, organic radical Since this is an anaerobic process, gaseous losses The locations of the ammonification and nitrifica- from a normal (aerobic) soil are small. However, when tion reactions in the nitrogen cycle are shown in soils stay very wet or saturated for long periods, a + Figure 1. The positively charged ammonium (NH4 ) large portion of the nitrate can be lost. ion produced by ammonification or added to the soil Ammonia gas can evolve from nitrogen compounds in fertilizers is attracted to negatively charged clay par- such as urea on the surface of a soil. Urea is present ticles in the soil. However, in most non-arid soils the in animal manure and may be purchased in pure form + NH4 ion is rapidly transformed to nitrate nitrogen as a fertilizer (45-0-0). - (NO3 N) Growing plants absorb most of their nitrogen - NH2 in the form of nitrate (NO3 ). CO + 2H2O H2CO3 + 2NH3 Common sources of inorganic nitrogen include NH2 + + ammonia (NH3), ammonium (NH4 ), amine (NH2 ) urea water carbonic ammonia - and nitrate (NO3 ). Most fertilizer materials contain or acid (volatilization) + - will form NH4 which is converted rapidly to NO3 once in the soil. Other ammonium-containing fertilizer compounds, including ammonium sulfate (21-0-0) and, to a lesser Removal of Nitrogen from Soil extent, ammonium nitrate (33-0-0) and ammonium phosphate, have been shown to produce free ammonia Nitrogen is removed from soils by four major in the presence of calcium carbonate. This condition processes: exists in some high pH soils (pH>7.3). I Plant uptake Runoff and erosion losses may include nitrate - + I Gaseous loss (NO3 ), ammonium (NH4 ), and organic nitrogen. The negatively charged NO - ion remains in the soil water I Runoff and erosion 3 and is not held by soil particles. If water containing I - + Leaching dissolved NO3 or NH4 runs off the surface, these Plant uptake refers to nitrogen absorption by roots. ions move with it. However, when nitrogen fertilizers Cotton, corn, tomatoes and turf grasses require 60 to are applied to dry soils, and rain or irrigation water is 300 pounds of nitrogen per acre to produce good applied, the first water dissolves the fertilizer and car- ries it into the soil. Rainfall does not generally cause where grass is the predominant crop, minimum leach- surface losses of fertilizer nitrogen, unless very inten- ing losses of nitrogen generally are expected from sive rainfall occurs shortly after application. nitrogen fertilization statewide. Ammonium held by clay particles can be carried While studies have shown limited problems with - into surface water supplies by soil erosion. In fact, soil nitrate (NO3 ) movements, improper applications of erosion moves more nitrogen than does rainfall in commercial and organic nitrogen fertilizers can result - moving dissolved nitrogen compounds. When erosion- in NO3 runoff into surface waters and leaching into al soils are deposited in rivers and lakes, microbial groundwater. activity will slowly convert nitrogen compounds into soluble forms. Preventing Nitrogen Loss Leaching losses involve the movement of water The best way to prevent losses of nitrogen from downward through a soil below the root zone. This agricultural lands is through good soil and water man- - loss most frequently occurs with nitrate (NO3 ) in agement practices. The first step in reducing potential areas of high rainfall, under excessive irrigation and nitrogen losses is to have the soil tested. A properly with coarse-textured (sandy) soils. Losses of nitrogen obtained soil sample will provide an estimate of through leaching reduce the amount of nitrogen avail- - nitrate-nitrogen (NO3 –N) present in the soil, and can able to crops and may potentially contaminate shallow be used as a guide for applying the appropriate water wells and aquifers. amount of nitrogen fertilizer for the crop being grown. The rates of nitrogen used and the time of applica- Proper fertilization and control of surface runoff tion should be related to soil conditions and crop and erosion offer the best methods for preventing requirements to minimize leaching losses. Numerous nitrogen from getting into streams and lakes. Leaching research studies show that because of plant uptake, lit- losses can be prevented by dividing the nitrogen - tle nitrate nitrogen (NO3 -N) leaches from soils on requirement into several applications where coarse- which a crop is actively growing. Since the sandy soils textured soils and high rainfall are common. most subject to leaching are located in East Texas, Manure also can be evaluated and scored based variation exists. However, abrupt changes in appearance on its consistency, which may indicate ration imbal- of feces can indicate changes in ration composition and ances and signal potential problems. Table 4 lists fecal alert managers to potential problems. consistency scores and descriptions as well as example situations when certain fecal consistencies may occur.