Nitrogen (N) is essential for plant growth. It ranks behind only carbon, hydrogen, and oxygen in total quantity needed and is the mineral element most demanded by plants. Because N is mobile within the plant, deficiency symptoms are expressed on older leaves. These leaves are generally uniform pale green or yellow. When N is limiting, crop growth is slow and yields are reduced.
Too much available N may lower yields and lessen crop quality. If soil N supply is greater than crop demand, excessive nitrate (NO3-) may enter ground or surface water. Nitrogen in the vegetation of field crops is approximately 2-3 percent of the dry matter weight. The quantity needed is more than Mississippi soils have the capacity to provide during a growing season, thus supplemental N is usually required for economical crop production. Nitrogen behavior is complex, but must be understood so growers may manage N for maximum profitability and for minimum environmental impact.
Sources of nitrogen in the environment:
Nitrogen is in the atmosphere, soils, and biological material of the terrestrial environment. Interchanges between these pools are due to climatic conditions, plant growth, soil biological activity, and management. The largest quantity of N is in the atmosphere which is about 78 percent N2 gas. Nitrogen is this form is unavailable to most plants directly. About 5 pounds of N per acre is deposited with precipitation each year although there are about 37,000 tons of N directly above each acre. The small quantity that falls is due to lightning action on nitrogen oxide compounds. Even though the amount added by direct deposition is small, most N used by crops originated in the atmosphere, and was retrieved by industrial or biological processes.
The quantity of N in soils is intimately associated with organic matter levels. Legumes such as soybeans and alfalfa convert atmospheric N2 to plant available forms via a symbiotic biological process involving Rhizobium bacteria and the plant roots. This fixed N may either return to the soil to ultimately become part of soil organic matter and serve as a N source to subsequent crops, or be removed in harvested plant materials.
Small quantities of soil N are provided by residue from plants that do not fix atmospheric N. Organic matter in Mississippi soils typically ranges from 0.5 to 2.0 percent by weight of the upper six inches. Typically organic matter is approximately 5 percent N, so total N in the topsoil ranges from 500 lb/acre to 2000 lb/acre. However, only a very small portion of the total N is available to plants within a growing season. Organic matter is replenished by returning crop residues to the soil or introducing other organic sources such as manures or animal bedding.
Almost all N in commercially available fertilizers is derived by combining atmospheric N2 with H2 to form ammonia (NH3), which may be used as fertilizer (anhydrous ammonia), or as a starting point in the manufacture of other nitrogen fertilizers. Anhydrous ammonia is an efficient source of fertilizer N, but because it is under high pressure, it requires specialized handling and stringent safety precautions. Because of theses requirements for anhydrous ammonia, other N products have increased in popularity.
Animal manures are important sources of N in the environment. The quantity of N in the manure depends on the animal species, their age and diet, and bedding materials. Manure begins contributing to plant nutrition and soil organic matter when added to agricultural soils, but not all the N within manure is immediately available to plants.
Forms of nitrogen in soils:
Nitrogen is in organic and inorganic forms in soils. Over 90 percent of soil N is associated with soil organic matter. Nitrogen is in compounds identifiable as part of the original organic material such as proteins, amino acids, or amino sugars, or in very complex unidentified substances in advanced stages of decomposition. These uncharacterized substances resist further microbial degradation and account for the very slow availability of soil N.
Plants may use either ammonium (NH4+), or nitrate (NO3-) which behave quite differently in soils. Positively charged NH4+ is attracted to negatively charged sites on soil particles as are other cations. It is available to plants, but the electrostatic attraction protects it from leaching. Conversely, negatively charged NO3- does not react with the predominately negatively charged soil particles, so it remains in the soil solution and moves with soil water. Therefore NO3- may leach out of the root zone when rainfall is excessive, or accumulate at the soil surface when conditions are dry.
Nitrogen transformations in soils:
Nitrogen conversions depend on soil moisture conditions, soil acidity, temperature, and microbial activity. Because the Mississippi climate is warm and humid, microbial transformations occur throughout the year. This constant breakdown results in lower organic matter levels in Mississippi soils than in cooler, drier climates.
Ammonium is absorbed on the cation exchange complex or taken up by plants without transformation, but most likely it is converted to NH4+ soon after its formation or addition as fertilizer. This nitrification is a two step process involving two different groups of soil bacteria. First Nitrosomas bacteria produce nitrite (NO2-). Nitrobacter species then convert NO2- to NO3- soon after its formation. The carbon used by these bacteria is derived solely from atmospheric CO2.
a) 2NH4+ + 3O2 = 2NO2- + 2H2O + 4H+ + energy
b) 2NO2- + O2 = 2NO3- + energy
Two things to note: 1) NH4+ has a short residence time in soils before conversion to the more mobile NO3- form; and 2) hydrogen ions are produced which lower the soil pH. This conversion may be slowed by commercially available nitrification inhibitors which maintain N in the NH4+ form longer, and thus may lessen loss of N as NO3- by leaching.
Mineralization is the process of converting organic N to plant available inorganic forms. It is a gradual breaking down of large molecules to smaller molecules by a succession of soil microorganisms. After these microbes complete their relatively brief life cycle, they are decomposed by other microbes. Energy for this process is obtained from carbon in the material being used, so introduction of fresh plant materials stimulates breakdown activity.
Immobilization is the process of incorporating inorganic into organic form by microbes or plants. Because it is largely dependent on microbes, the availability of carbon and other nutrients determine the rate of immobilization. When residues with high carbon:N ratios are being decomposed, all readily available N within the soil system may be tied up by the microbes and therefore unavailable for plant uptake. This effect eventually fades because, without external N, the microbial population dies off and decomposes, releasing N which is available to plants. The risk of immobilization is avoided by mixing plant residues into the soil well before the next cropping cycle.
Loss of nitrogen from soils:
Even though soil N may be unavailable to plants through immobilization, it has not disappeared from the soil. Nitrogen can be permanently removed from soil by erosion, leaching, denitrification, or volatilization.
Organic matter, which contains the majority of native N, is concentrated in the plow layer of soils, and is thus susceptible to loss by erosion. Loss by erosion is reduced by soil conservation practices.
Leaching losses occur when NO3- remains in the soil water and moves away from root uptake areas with downward water movement. Nitrogen lost in this fashion is a contaminant if it reaches ground or surface waters. Leaching is more likely to occur when rainfall exceeds crop use. Prudent management options that lessen the probability of NO3- leaching include using recommended rates of fertilizer N for the crop being grown, using realistic yield goals in cropping decisions, applying N as close as possible to when the crop will need it, using winter cover crops to scavenge N left over from the previous summer and giving credit for all possible N inputs such as legumes or manure application.
A major mechanism of N loss in Mississippi is denitrification in waterlogged soils. As water content of soils increase, the amount of air in the soils decrease. Bacteria can utilize the oxygen from nitrite and nitrate proliferate, but the process results in the production of N gases, N2 and N2O. These gases are released to the atmosphere, removing the N from the soil.
Conditions necessary for denitrification are waterlogged soils, carbon sources (from organic matter or plant residues) for use by the microbes, and N as either NO3- or NO2-. The rate is greatly accelerated by higher temperatures.
Volatilization is the loss of ammonia gas (NH3) to the atmosphere from anhydrous ammonia, urea, or N solution fertilizer sources. Losses to volatilization are minimalized through management of fertilizer applications. Anhydrous ammonia applicators should have good sealing of the soil after fertilization. The first step in urea N conversion to NH4+ is the production of NH3. Escape losses from urea are more likely with warm temperatures (over 50º F) and on high pH soils (greater than 7).
Ideally, urea should be incorporated into the soil or applied prior to a rainfall. Urea volatilization loss is increased when applied to vegetative cover, or plant residue, so care should be exercised using urea as a topdressing N source. Urea should never be placed in contact with seed because of toxic effects of NH3 on seedlings. The same precautions apply to N solutions as they contain urea.
Using nitrogen efficiently:
Fertility management of phosphorus and potassium should always be based on a soil testing program. The frequent interchanges of N between forms limit the value of N soil testing as a predictive tool in warm, humid Mississippi. Supplemental N fertilization is usually necessary for economic production of nonlegume crops, but heavy N applications cannot substitute for poor management. Research-based economic and environmentally friendly recommendations are available for all crops produced in Mississippi.
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