Illinois Fertilizer Conference Proceedings January 27-29, 1992 Home 1992 Index Search

Factors Affecting the Efficient Use of Nitrogen Fertilizers

R.L. Mulvaney¹

Introduction

The supply of nitrogen (N) limits the growth and productivity of non-leguminous crops more often than the supply of any other mineral nutrient. The development, availability, and use of synthetic N fertilizers have played a major role in the dramatic increases in crop yields (especially corn) that have occurred in the U.S. since World War II. Domestic consumption of N fertilizers has increased tremendously during this period, and nowhere has the increase been greater than in Illinois, which leads the nation in fertilizer use (Hargett et al., 1988). In Illinois, as in other parts of the Corn Belt, the cost of N fertilizer used in corn production usually exceeds the combined cost of other fertilizers and limestone. According to a survey by Brunoehler (1987), N fertilizer accounts for about 15% of the cost of corn production for a typical corn-soybean rotation.

When fertilizer N is applied to soil, the N changes from one form to another, or is transformed. Most of the transformations are brought about by microorganisms in the soil. The nature and extent of the transformations ultimately determine the fate of the fertilizer N, its availability to the crop, and its pollution potential. Among the possible transformations that can occur are:

1. Fixation of ammonium (NH₄⁺) by clay.
   
   
2. Volatilization of NH₄⁺, with gaseous loss of ammonia (NH₃).
   
   
3. Nitrification - the conversion of NH₄⁺ to nitrite (NO₂^-) and then nitrate (NO₃^- ).
   
   
4. Immobilization - the uptake of NH₄⁺ or N03 by soil microorganisms.
   
   
5. Mineralization - the release of NH₄⁺ from organic forms by microorganisms.
   
   
6. Denitrification - the conversion of NO₃^- or NO₂^- to gaseous forms of N, either dinitrogen (N₂) or nitrous oxide (N₂O).

The N transformations that occur after application of N fertilizer to soil depend upon many factors, including weather conditions (i.e., temperature and precipitation), soil properties (e.g., organic matter and clay content), and the type of fertilizer applied. A great deal of information has been collected about N transformations in soil, but only one or two fertilizers have been used in most studies, which precludes meaningful comparisons among different fertilizers.

The purpose of this paper is to summarize current information about the transformations of N fertilizers in soil, with particular emphasis on the differences that may exist between fertilizers.

Ammonium Fixation

The number of possible N transformations is greater for NH₄⁺ or NH₄⁺-producing (e.g., urea) fertilizers, than for fertilizer in which all of the N is in the form of NO₃^-. Depending upon the soil's content of clay and organic matter, some of the NH₄⁺ from ammoniacal fertilizers is retained, or adsorbed, onto the soil colloids. This NH₄⁺ is referred to as exchangeable NH₄⁺ because it is subject to replacement by other cations in the soil solution. It is protected against leaching, yet is readily available for uptake by plants.

In addition to exchangeable NH₄⁺, some of the NH₄⁺ added as fertilizer becomes trapped, or fixed, in the interlayer spaces of certain clay minerals. Fixed NH₄⁺ is held more tightly than exchangeable NH₄⁺ and is not readily available to plants. The amount of NH₄⁺ fixed by a given soil depends upon many factors, including the NH₄⁺ concentration and pH (Nommik and Vahtras, 1982). Due to the high localized NH₄⁺ concentration, fixation is likely to be greatest when an ammoniacal fertilizer is applied in a band, as in the case of anhydrous NH₃, or in the microsite surrounding a fertilizer granule. Fixation has been found to decrease markedly under acidic conditions (Nommik, 1965), which suggests that very little fixation may occur when N is applied in the form of mono- or diammonium phosphate (MAP or DAP). However, experimental support for this view is lacking, as is evidence from a direct comparison that NH₄⁺ fixation is greatest with anhydrous NH₃ or in the immediate vicinity of a dissolving fertilizer granule.

Ammonia Volatilization

Ammonia volatilization refers to the process by which gaseous NH₃ escapes from the soil into the atmosphere. It can occur whenever free NH₃ is present near the soil surface. The NH₃ is derived from NH₄⁺ in the soil solution via the equilibrium,

NH₄⁺ ↔ NH₃(solution) + H⁺

Volatile loss of NH₃ results from the reaction,

NH₃(solution) ↔ NH₃(air)

The magnitude of loss depends upon the NH₄⁺ concentration, pH, soil type, temperature, and mositure content (Hargrove, 1988). Extensive loss of NH₃ can occur when anhydrous NH₃ is applied under conditions where the applicator slit does not seal properly, but losses can also be serious with other NH₄⁺ or NH₄⁺-producing fertilizers, particularly if they are applied to a neutral or calcareous soil or to a soil having a low cation-exchange capacity. In such cases, losses will be greater with urea than with an acidic material such as ammonium sulfate [(NH₄)₂SO₄] or DAP (Bayrakli, 1990).

Nitrification

In most agricultural soils, NH₄⁺ from fertilizer is readily converted to N03- by the process of nitrification. The change occurs in two steps: NH₄⁺ is first converted to NO₂^-, and the NO₂^- is then converted to NO₃^- Nitrification is carried out largely by a rather select group of soil microorganisms and, for that reason, the process is quite sensitive to environmental conditions. Nitrification is favored by a high pH (the optimum is around 8.5), although it does not occur under the extreme conditions of high pH and high NH₄⁺/NH₃ concentrations that exist at the center of an anhydrous NH₃ band (Norman et al., 1987a). Early work by Eno and Blue (1957) with three sandy soils from Florida indicated that nitrification of NH₄⁺ derived from urea was much more rapid than nitrification of NH₄⁺ from (NH₄)₂SO₄; the difference was attributed to the rise in pH associated with hydrolysis of urea by soil urease. More recent studies have given mixed results, with urea being nitrified more rapidly than (NH₄)₂SO₄ in most cases (Martikainen, 1985; Vilsmeier and Amberger, 1980; de Boer et al., 1989), but not in all (Wickramasinghe et al., 1985). If pH is the major factor affecting the rate at which different ammoniacal fertilizers undergo nitrification, then:

• NO₃^- should be formed more rapidly from fertilizers that give an alkaline reaction, such as urea or UAN, than from those that are acid-forming, such as MAP or DAP; and
  
  
• the differences should be greater in magnitude for acidic or neutral soils than for calcareous soils such as Harpster.

Immobilization-Mineralization

Immobilization of N results from uptake of NH₄⁺ and/or NO₃^- by microorganisms in the soil. The N taken up is incorporated into proteins, nucleic acids, and other organic N constituents of microbial cells and cell walls; as such, it becomes part of the biomass. As the microbes die and decay, some of the biomass N is released as NH₄⁺ through the process of mineralization; the remainder undergoes conversion to more stable organic N compounds, ultimately becoming a part of soil organic matter. The stabilized organic compounds are not readily available to plants; therefore, the net result of immobilization-mineralization is a decrease in the availability of the N added to soil as fertilizer, and also the partial conversion of this N to a form that is not subject to loss from the soil, except by erosion. The microorganisms responsible for immobilization utilize NH₄⁺ in preference to NO₃^- which accounts for reports that immobilization-mineralization is more extensive with ammoniacal fertilizers than with NO₃^- fertilizers (e.g., Wickramasinghe et al., 1985). But it is also important to recognize that, like other biological N transformations, immobilization-mineralization is affected by environmental conditions, and this can lead to differences among ammoniacal fertilizers in the rate and extent of incorporation of NH₄⁺-N into organic forms due to their effects on soil pH. For example, recent work by Norman et al. (1987a) indicates that, following injection of anhydrous NH₃, immobilization does not occur in the center of the injection band until there has been moderation of the extreme pH and NH₄⁺/NH₃ concentrations, which may persist for several weeks. To assess the availability and pollution potential of different N fertilizers, information must be obtained on their relative rates of immobilization-mineralization, and also on microsite effects associated with fertilizer granules.

Denitrification

When soils are saturated with water, gaseous loss of N can occur as a result of bacterial denitrification. This process involves the conversion of NO₃^- and NO₂^- to the gases, N₂ and N₂O, which escape from the soil. Ammonium-N is not subject to denitrification (it must first be converted to NO₂^- or NO₃^-), but there is evidence that N₂O can be produced during the conversion of NH₄⁺ to NO₃^- by nitrifying microorganisms present in soils (Bremner and Blackmer, 1978). Field studies in Iowa (Breitenbeck et al., 1980; Breitenbeck and Bremner, 1986) indicate that emission of N₂O is greater for ammoniacal fertilizers than for NO₃^- fertilizers and that emission is much greater with anhydrous NH₃ than with other N fertilizers (Bremner et al., 1981; Breitenbeck and Bremner, 1986). To some extent, this work challenges the usual assumption that denitrification is the major process by which gaseous loss of N occurs from soils of the Corn Belt. However, further research is clearly merited in view of:

• the extensive spatial and temporal variability in the rate of emission of N20 from field soils (Blackmer et al., 1982), which makes it extremely difficult to detect treatment effects;
  
  
• the fact that no measurements of N2 emission were made in the Iowa work (emission of N2 is difficult to measure because it is the major consitituent of air); and
  
  
• evidence that application of liquid anhydrous NH3 leads to some solubilization of soil organic matter (Norman et al., 1987b), which may account for the increased emissions of N20 observed in the Iowa work with this fertilizer (the solubilized organic matter is a potential energy source for denitrifying bacteria).
  

Footnotes and References

¹Richard L. Mulvaney is Associate Professor of Soil Fertility, Department of Agronomy, University of Illinois at Urbana-Champaign.

Bayrakli, F. 1990. Ammonia volatilization losses from different fertilizers and effect of several urease inhibitors, CaCl2 and phosphogypsum on losses from urea. Fertilizer Research, 23:147-150.

Blackmer, A. M., S. G. Robbins, and J. M. Bremner. 1982. Diurnal variability in rate of emission of nitrous oxide from soils. Soil Science Society of America Journal, 46:937942.

Breitenbeck, G. A., A. M. Blackmer, and J. M. Bremner. 1980. Effects of different nitrogen fertilizers on emission of nitrous oxide from soil. Geophysical Research Letters, 7:8588.

Breitenbeck, G. A., and J. M. Bremner. 1986. Effects of various nitrogen fertilizers on emission of nitrous oxide from soils. Biology and Fertility of Soils, 2:195-199.

Bremner, J. M., and A. M. Blackmer. 1978. Nitrous oxide: emission from soils during nitrification of fertilizer nitrogen. Science, 199:295-296.

Bremner, J. M., G. A. Breitenbeck, and A. M. Blackmer. 1981. Effect of anhydrous ammonia fertilization on emission of nitrous oxide from soils. Journal of Environmental Quality, 10:77-80.

Brunoehler, R. 1987. Paring expenses in '88. Agri Finance, 29(10):18-19.

de Boer, W., H. Duyts, and H. J. Laanbroek. 1989. Urea stimulated autotrophic nitrification in suspensions of fertilized, acid heath soil. Soil Biology & Biochemistry, 21:349-354.

Eno, C. F., and W. G. Blue. 1957. The comparative rate of nitrification of anhydrous ammonia, urea, and ammonium sulfate in sandy soils. Soil Science Society of America Proceedings, 21:392-396.

Hargett, N. L., J. T. Berry, and S. L. McKinney. 1988. Commercial fertilizers. National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, AL.

Hargrove, W. L. 1988. Evaluation of ammonia volatilization in the field. Journal of Production Agriculture, 1:104-111.

Martikainen, P. J. 1985. Nitrification in forest soil of different pH as affected by urea, ammonium sulphate and potassium sulphate. Soil Biology & Biochemistry, 17:363-367.

Nommik, H. 1965. Ammonium fixation and other reactions involving a nonenzymatic immobilization of mineral nitrogen in soil. In: Soil Nitrogen (W. V. Bartholomew and F. E. Clark, ed.) Agronomy Monograph 10. American Society of Agronomy, Madison, Wisconsin. pp. 198-258.

Nommik, H., and K. Vahtras. 1982. Retention and fixation of ammonium and ammonia in soil. In: Nitrogen in Agricultural Soils (F. J. Stevenson et al., ed.) Agronomy Monograph 22. American Society of Agronomy, Madison, Wisconsin. pp. 123-171.

Norman, R. J., L. T. Kurtz, and F. J. Stevenson. 1987a. Distribution and recovery of nitrogen-15-labeled liquid anhydrous ammonia among various soil fractions. Soil Science Society of America Journal, 51:235-241.

Norman, R. J., L. T. Kurtz, and F. J. Stevenson. 1987b. Solubilization of soil organic matter by liquid anhydrous ammonia. Soil Science Society of America Journal, 51:809-812.

Wickramasinghe, K. N., G. A. Rodgers, and D. S. Jenkinson. 1985. Transformations of nitrogen fertilizers in soil. Soil Biology & Biochemistry, 17:625-630.

Vilsmeier, K., and A. Amberger. 1980. Umsetzung von Cyanamid, Harnstoff and Ammonsulfat in Abhangigkeit von Temperatur and Bodenfeuchtigkeit. Zeitschrift fur Pflanzenerntthr and Bodenkunde, 143:47-54.

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