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Ammonia (NH3), Ammonium Ion (NH4+)

Ammonia (NH3) is very soluble in water. The Henry's law constant for NH3 is 57.5 mol/(L-atm) at 25 C. NH3 is a weak base which combines with a proton (H+) to form an ammonium ion (NH4+).

The concentrations of NH3 and NH4+ at equilibrium are related by the following equation,

where Ka is a constant (5.5x10-10 at 25ºC) and square brackets indicate concentrations (mol/L).The value of Ka at other temperatures is given by the following equation,

where T is temperature in Kº (Wright et al., 1961). The following figure shows how NH3 and NH4+ concentrations vary with pH. In the pH range of most natural waters, NH4+ is the predominant form of inorganic N in the -3 oxidation state. NH3 is the species that is toxic to fish, not NH4+.

Figure 1. Percentage of NH4+ and NH3 as functions of pH.

As a weak base, NH3 gas (anhydrous ammonia fertilizer) injected into soil immediately raises the pH of the 5 cm of surrounding soil to 10. As the figure shows, at pH 10 most of the applied N fertilizer remains as NH3. Thus there are concerns that if the soil does not seal properly behind the applicator, or if the soil is too alkaline to convert NH3 to NH4+, or if drainage is too impeded to permit adequate incorporation, appreciable volatilization of NH3 - 20 to 40 percent - can occur. However, experiments have shown that properly applied anhydrous ammonia fertilizer has volatilization losses of 1 percent or less (e.g., Denmead et al., 1977). On average, gaseous losses due to denitrification plus NH3 volatilization from cropland add up to about 15 percent of annual nitrogen additions (Allison, 1955; Brady, 1974; Troeh and Thompson, 1993).

NH4+ is a major component of atmospheric aerosols. (see atmosphere)

NH4+ participates in cation exchange reactions in which dissolved ions replace ions bound by negatively charged sites on mineral particles, such as clays and natural organic matter.

Ion exchange may be an important reaction for NH4+ in soils, sediments, and aquifer systems. For example, the retardation coefficient of NH4+ (rNH4) in sandstone was estimated from ion exchange measurements to be 16<rNH4<80 (Drever, 1982). A pulse of NH4+ would move at a rate between 1/80th and 1/16th that of water in the sandstone aquifer.

Calculations of ion exchange equilibria can sometimes be used to qualitatively describe the ion exchange behavior of cations. Two useful empirical ion exchange parameters are the cation exchange capacity (CEC) and the selectivity coefficient. The CEC of a material (clay, soil, ...) is measured by saturating the material with one cation (e.g. NH4+), displacing the first cation with another cation (e.g., Mg2+), and measuring the concentration of displaced NH4+. Table 5 presents the CEC values of some clay minerals.

Table 5. Cation exchange capacities of
some clay minerals (meq/100g) (Drever, 1982)

   Montmorillonites 80-150
   Vermiculites 120-200
   Illites 10-40
   Kaolinite 1-10
   Chlorite <10

The selectivity coefficient can sometimes be used in the following equation to describe ion-exchange equilibria involving NH4+ and Na+.

XNH4 and XNa indicate the mole fractions of NH4+ and Na+ in the exchanger phase, KNH4Na is the selectivity coefficient, square brackets indicate aqueous concentrations, and n is an empirical coefficient with value close to 1 for singly charged ions and 0.7<n<0.9 for doubly charged ions. Similar equations apply for exchange of other ions having the same charge, including Na+/K+, NH4+/K+, and Ca2+/Mg2+. For ions of different charge the equation is more complicated. For example,

The following equation describes NH4+ sorption in some aquifer systems (Drever and McKee, 1980).

{NH4+}clay is the concentration of sorbed NH4+ (meq/kg), M is the total cation concentration (meq/kg), and the other symbols are as defined above.

Both the CEC and selectivity coefficient depend on the pH and solution composition. Thus, even for the same sample of the same material, neither parameter is a constant. Nevertheless, the ion exchange parameters allow the rationalization of cation behavior in many systems.

Nitric Acid (HNO3), Nitrate Ion (NO3-)

Nitric acid and nitrate salts are all very soluble in water. HNO3 is a strong acid and completely dissociates in water. HNO3 is volatile. NO3- absorbs solar UV radiation. It is a major source of hydroxyl radical in surface water and atmospheric water droplets.

Nitrous Acid (HNO2), Nitrite Ion (NO2-)

Nitrite (NO2-) is an intermediate product in many N transformations. It is produced by the oxidation of NH4+ in the first stage of nitrification (Nitrosomonas) and by reduction of NO3- in the first stage of denitrification. It is oxidized to nitrate by bacteria of the genus Nitrobacter in the second stage of nitrification and reduced to N2O, NO, or N2 in the second stage of denitrification. HNO2 is a weak monoprotic acid with pKa 5.2. The following figure shows how the relative concentrations of HNO2 and NO2- vary with pH. At the pH values of most natural water systems, the predominant form is NO2-.

Figure 1. Percentage of HNO2 and NO2- as functions of pH.

Nitric Oxide (NO)

NO is produced by combustion and by denitrification. Combustion produces both NO and NO2, collectively denoted NOx, but most of the N oxides produced by combustion is NO. Combustion produces NO both by a combination of atmospheric N2 and O2 and by oxidation of any organic N in the fuel. In the United States, transportation accounts for approximately 49% of anthropogenic NOx emissions. Combustion from stationary sources produces approximately 44%, including 26% from electric utilities and 18% from other industries (USEPA 1998).

NO is slightly soluble in water. The Henry's law constant for NO is 1.9x10-3 mol/atm. The typical atmospheric partial pressure of NO is 2x10-10 atm. NO reacts rapidly with O2 to form NO2. NO plays an important part in some biological processes. NO biochemistry is the subject of much current research.

Nitrogen Dioxide (NO2)

NO2 is produced by combustion, degradation of organic matter, and oxidation of NO. It is one of the few colored gases and gives a brownish tint to polluted air. It is visible at concentrations as low as 1 ppmv. NO2 is oxidized to HNO3 in the atmosphere. It also hydrolyzes and disproportionates to give HNO3 and HNO2.

NO2 is slightly soluble in water. The Henry's law constant for NO2 is 1.0x10-2 mol/atm. The typical partial pressure of NO2 in the atmosphere is 2x10-9 atm.

Nitrous Oxide (N2O)

N2O is the second most abundant N species in the atmosphere. Its partial pressure is

3x10-7 atm. It is relatively unreactive in the troposphere. The main sink for N2O is photochemical reactions in the stratosphere (see atmosphere). It is produced by denitrification. N2O is fairly soluble in water. Its Henry's law coefficient is 2.6x10-2 mol/atm.

Organic N

Organic-N includes all substances in which N is bonded to C. It occurs in both soluble and particulate forms. The largest fraction is made up of amino acids and peptides and is often called amino N. Particulate organic-N includes small organisms (algae, bacteria, ...), both living and dead, and fragments of organisms. Soluble organic N is from wastes excreted by organisms or from the degradation of particulate organic-N.

Organic-N concentrations in natural waters, soils, and sediments are operationally defined. In the Kjeldahl method, a sample of water, soil, or sediment is heated with H2SO4 and a catalyst and N from amino acids is converted to NH4+. Total Kjeldahl N (TKN) includes N from amino acids and any NH4+. Organic-N is calculated by subtracting NH4+ (determined separately) from TKN. In another method, a water sample is oxidized using various combinations of potassium persulfate, heat, and ultraviolet light and all N is converted to NO3-. Organic-N is calculated by subtracting NO3- (determined separately) from the NO3- in the oxidized sample. Concentrations of individual amino acids can be determined chromatographically.

Typically, most N in soils and surficial sediments occurs in organic form. The amount of organic-N in soils and sediments is influenced by climate - all else being equal, increasing with moisture and decreasing with temperature in the United States. It is also influenced by vegetation. In Illinois equivalent soils developed under prairie had twice the organic-N of soils developed under forest. It is also influenced by topography. Soils such as the more upland prairie of the Morrow plots are estimated to originally have had 0.3 percent by weight N whereas soils developed under prairie wetlands originally had 2.2 percent N. The amount of N is also influenced by the particle size of the soil and sediment (more accumulating in fine-grained material) and the amount of mineral nutrients (especially phosphorus) in the soil and sediment. It is also influenced by the age of the land surface - older surfaces generally being lower in C, N, and mineral nutrients. Agricultural practices have reduced the N content in the plow layer of cornbelt soils by about 40 percent, on average, from their estimated virgin condition.

Organic-N sometimes makes up a significant fraction of soluble and particulate N in natural waters. See Examples of Organic-N Measurements.

Urea (CO(NH2)2)

Urea is an organic N compound that is manufactured in large quantities. It is used as a fertilizer, including controlled-release fertilizers, and in many industrial processes. The U.S. production of urea was 9,330,000 tons in 1999 (Anon., 2000). Urea is the soluble form of N that is excreted by mammals. Therefore, large amounts of urea are added to the surface soils of feedlots and dairies. However, urea is rapidly hydrolyzed to NH3 and CO2, so it is not expected to make up a significant part of organic N in water bodies, soils, or sediments except to the degree that it solubilizes soil organic matter (SOM) and the ability of that solubilized SOM to pass through soils into surface and ground waters (Stevenson, 1982).

Uneven deposition of excretal N by grazing livestock can lead to spot application rates equivalent to 400 to 2000 kg N/ha (Watson et al., 2000). This can be compared to the average rate of N fertilization of Illinois cropland which is reported to be approximately 87 kg N/ha (David and Gentry, 2000). Corn is the major crop which receives the highest rate of N fertilization 100 to 200 kg N/ha (David et al., 1997).

Excrement from grazing livestock makes localized zones of soil become N saturated. Urine typically infiltrates into the soil where the urea is readily hydrolyzed to NH3 and nitrified to NO3-N. With this, N becomes more subject to leaching than volatilization. Experiments on the concentration of NO3-N draining through soil from cattle urine patches give concentrations ranging from 52 to 176 mg NO3-N/L, depending upon year and season (Stout et al., 1997). In contrast, it has long been reported that feces remaining on the surface lose about half of their N in 2 to 4 days due to NH3 volatilization (e.g., Clark, 1930; Lauer et al, 1976). Much N is also lost from feces as soluble organic-N. For example, lysimeter experiments show that between 100 and 600 mg soluble organic-N/L leach through soil from surficial land application of cow manure (Elrashidi et al., 1999).

There were appreciable amounts of herbivore excrement deposited on the landscape of the Prairie State prior to European settlement. For example, best estimates are that prior to European settlement there were 10-20 million buffalo east of the Mississippi River. Extensive analyses show that almost all of these buffalo were living in Illinois, southern Wisconsin, and the Ohio River Valley (Roe, 1951). In comparison, contemporary statistics on the number of cows and cattle for the same region plus Missouri and Iowa (east-central United States) has been stable for several decades at around 10 million head (White et al., 1981; Goolsby et al., 1999). Hence, the landscape in and around Illinois was more animal-rich prior to European settlement than it is now at the height of European agriculture.

References Cited:

Allison, F.E. 1955. The enigma of soil nitrogen balance sheets. Adv. Agron. 7:213-250.

Brady, N.C. 1974. The Nature and Properties of Soils. 8th Edition. Macmillan Publishing Co., Inc., New York.

Clark, N. 1930. Fertilizer value of manure greatly increased by immediate mixing with soil. Wisconsin Agricultural Experiment Station Bulletin 410, Madison. pp. 10-11.

David, M.B. and L.E. gentry. 2000. Anthropogenic inputs of nitrogen and phosphorus and riverine export for Illinois, USA. J. Environ. Qual. 29:494-508.

David, M.B., L.E. Gentry, D.A. Kovacic, and K.M. Smith. 1997. Nitrogen balance in and export from an agricultural watershed. J. Environ. Qual. 26:1038-1048.

Denmead, O.T., J.R. Simpson, and J.R. Freney. 1977. A direct field measurement of ammonia emission after injection of anhydrous ammonia. Soil Sci. Soc. Am. J. 41:1001-1004.

Drever, J. I. 1982. The Geochemistry of Natural Waters. Englewood Cliffs:Prentice-Hall.

Drever, J. I. and C. R. McKee. 1980. The push-pull test: a method of evaluating formation adsorption parameters for predicting the environmental effects of in situ coal gasification and uranium recovery. In Situ 4:41-43.

Elrashidi, M.A., V.C. Baligar, R.F. Korcak, N. Persaud, and K.D. Ritchey. 1999. Chemical composition of leachate of dairy manure mixed with fluidized bed combustion residue. J. Environ. Qual. 28:1243-1251.

Goolsby, D.A., W.A. Battaglin, G.B. Lawrence, R.S. Artz, B.T. Aulenbach, R.P. Hooper, and G.J. Stensland. 1999. Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin. National Assessment of Gulf Hypoxia, Topic 3 Report. (http://www.nos.noaa.gov/products/pubs_hypox.html).

Lauer, D.A., D.R. Bouldin, and S.D. Klausner. 1976. Ammonia volatilization from dairy manure spread on the soil surface. J. Environ. Qual. 5:134-141.

Morel, F. M. M. and J. G. Hering. 1993. Principles and Applications of Aquatic Chemistry. New York:Wiley.

Roe, F.G. 1951. The North American Buffalo. A Critical Study of the Species in Its Wild State. University of Toronto Press, Toronto, P.Q., Canada.

Stevenson, F.J. 1982. Origin and distribution of nitrogen in soil. In Stevenson, F. J., ed., Nitrogen in Agricultural Soils. Agronomy Monograph Number 22. American Society of Agronomy, Madison, WI. pp. 1-42.

Stevenson, F. J. 1972. Nitrogen Cycle. In Fairbridge, R. W., ed. The Encyclopedia of Geochemistry and Environmental Sciences. New York: Van Nostran Reinhold. 801-806.

Stout, W.T., S.A. Fales, L.D. Muller, R.R. Schnabel, W.E, Purdy, and G.F. Elwinger. 1997. Nitrate leaching from cattle urine and feces in Northeast USA. Soil Sci. Soc. Am. J. 61:1787-1794.

Troeh, F.R. and L.M. Thompson. 1993. Soils and Soil Fertility. Fifth Edition. Oxford University Press, New York.

U.S.E.P.A. 1998. National Air Quality and Emissions Trends Report, 1997. EPA 454-R-98-016.

Watson, C.J., C.Jordan, S.D. Lennox, R.V. Smith, and R.W.J. Steen. 2000. Inorganic nitrogen in drainage water from grazed grassland in Northern Ireland. J. Environ. Qual. 29:225-232.

White, R.K., L.B. Owens, R.W. VanKeuren, and W.M. Edwards. 1981. Nonpoint surface runoff from cattle pasture -- hydrology and nutrients. Livestock Waste: A Renewable Resource. Proc. 4th International Symposium on Livestock Wastes--1980. Amarillo, TX. American Society of Agricultural Engineers, St. Joseph, MI, pp. 293-296.

Wright, J. M., W. T. Lindsay, Jr., and T. R. Druga. 1961. The behavior of electrolytic solutions at elevated temperatures as derived from conductance measurements. USAEC Comm. R&D report WAPD-TM-204. 32 pp.

Suggested Reading

Aandahl, A.R.1948. The characterization of slope positions and their influence on the total nitrogen content of a few virgin soils of western Iowa. Soil Sci. Am. Proc. 13:449-454.

Brady, N.C. 1974. The Nature and Properties of Soils. 8th Edition. MacMillan Publishing Co., Inc, New York.

Bray, R.H. 1937. Chemical and physical changes in soil colloids with advancing development in Illinois soils. Soil Sci. 43:1-14.

Bushnell, T.M. 1944. The Story of Indiana soils: With descriptions of general soil regions and the key to Indiana soils. Purdue Univ. Agric. Exp. Sta. Spec Circ. June 1944, Lafayette, IN.

Cole, C.V. and R.D. Heil. 1981. Phosphorus effects on terrestrial nitrogen cycling. Terrestrial Nitrogen Cycles, Clark, F.E. and T. Rosswall (eds.) Ecol. Bull. (Stockholm) 33:363-374.

Conner, S.D. 1922. Nitrogen in relation to crop production in the middle west. J. Am. Soc. Agron. 14:179-182.

Cotton, F. A. and G. Wilkinson. 1972. Advanced Inorganic Chemistry, 3rd ed. New York:Wiley.

DeTurk, E.E. 1938. Changes in the soil of the Morrow plots which have accompanied long-continued cropping. Soil. Sci. Am. Proc. 3:83-85.

Fehrenbacher, J.B., J.J. Jansen, and K.R. Olson. 1986. Loess thickness and its effect on soils of Illinois. Ill. Agric. Exp. Sta. Bull. 782.

Fehrenbacher, J.B., J.D. Alexander, I.J. Jansen, R.G. Darmody, R.A. Pope, M.A. Flock, E.E. Voss, J.W. Scott, W.F. Andrews, and L.J. Bushue. 1984. Soils of Illinois. Ill. Agric. Exp. Sta. Bull. 778.

Greenwood, N. N. and A. Earnshaw. 1984. Chemistry of the Elements. Oxford:Pergamon.

Haynes, R.J. and R. Naidu. 1998. Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions. Nutrient Cycling in Agroecosystems 51:123-137.

Hopkins, C.G. 1910. Soil Fertility and Permanent Agriculture. Ginn and Company, Boston, MA.

Jenny, H. 1941. Factors of Soil Formation: A System of Quantitative Pedology. McGraw-Hill Book Company, Inc., New York.

Jones, R.L. and A.H. Beavers. 1966. Weathering in surface horizons of Illinois soils. Soil Sci. Soc. Am. Proc. 30:621-624.

Li. C., S. Frolking, and R. Harriss. 1994. Modeling carbon biogeochemistry in agricultural soils. Global Biogeochemical Cycles 8:237-254.

Mann, L.K. 1985. A regional comparison of carbon in cultivated and uncultivated alfisols and mollosols in the Central United States. Geoderma 36:241-253.

Matson, P.A., W.J. Parton, A.G. Power, and M.J. Swift. 1997. Agricultural intensification and ecosystems properties. Science 277:504-509.

Oades, J.M. 1988. The retention of organic matter in soils. Biogeochemistry 5:35-70.

Odell, R.T., S.W. Melsted, and W.W. Walker. 1984a. Changes in organic carbon and nitrogen of Morrow plot soils under different treatments, 1904-1973. Soil Sci. 137:160-171.

Parton, W.J., J.W.B. Stewart, and C.V. Cole. 1988. Dynamics of C, N, P and S in grassland soils: A model. Biogeochemistry 5:109-131.

Smith, G.D., W.H. Allaway, and F.F. Reicken. 1950. Prairie soils of the Upper Mississippi Valley. Adv. Agron. 2:157-205.

Snyder, H. 1905. Soils and Fertilizers. The Chemical Publishing Company, Easton, PA.

Stevenson, F.J. 1986. Cycles of Soil: Carbon, Nitrogen, Phosphorus, Micronutrients. Wiley Interscience, New York.

Walker, T.W. and A.F.R. Adams. 1959. Studies on soil organic matter: II. Influence of increased leaching at various stages of weathering on levels of carbon, nitrogen, sulfur, and organic and total phosphorus. Soil Sci. 87:1-10.

Walker, T.W., B.K. Thapa, and A.F.R.Adams. 1959. Studies on soil organic matter: 3. Accumulation of carbon, nitrogen, sulfur, and organic and total phosphorus in improved grassland soils. Soil Sci. 87:135-140.

Walker, T.W. and A.F.R. Adams. 1958. Studies on soil organic matter: I. Influence of phosphorus content of parent materials on accumulations of carbon, nitrogen, sulfur, and organic phosphorus in grassland soils. Soil Sci. 85:307-318.

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