Ammonia release, from soils

One primary determinant for atmospheric, hence fogwater, acidity is the relative abundance of ammonia (2). In an ammonia source region such as the SJV, there are factors which can suppress or accelerate NH release. Conditions under which fogwater acidity was low were related to factors favoring ammonia release from sources, such as higher soil moisture and temperatures (6). 

For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

Waughman [14] has described a microdiffusion method for the determination of ammonium and nitrate in soils. Nitrate in the sample solution is reduced to ammonia by titanous sulfate and the ammonia is then released from the solution and diffused and absorbed onto a nylon square impregnated with dilute sulfuric acid. The nylon is then put into a solution which colours... 

HMSO (UK) [17] have published a method for the determination of ammonia, nitrate and nitrite in potassium chloride extracts of soil extracts. An aliquot of the extract is made alkaline and the ammonia released, originating from ammonium ions, is determined either with an ammonia-selective probe or, after removal by distillation, by titration (Crompton TR, private communication). 

Ammonia may be released to soils by natural or S5mthetic fertilizer application animal (including livestock) excrement degradation decay of organic material from dead plants and animals and the natural fixation of atmospheric nitrogen. 

The application of fertilizer to soil, as ammonia, ammonium compounds, or ammonia precursors (such as urea), is a well documented source of ammonia release to the atmosphere (ApSimon et al. 1987 Beyrouty et al. 1988 Buijsman et al. 1987 Kucey 1988 Olivier et al. 1998 Reynolds and Wolf 1988). The rate of ammonia emission from ground sources, such as freshly fertilized fields and cattle feedlots, is dependent on variables such as the pH, temperature, soil characteristics, rainfall, method of application, wind speed, etc. (Bouwmeester and Vlek 1981 Brunke et al. 1988 Denmead et al. 1982 Hoff et al. 1981 Kucey 1988 Nason et al. 1988 Reynolds and Wolf 1988). Ammonia can volatilize from sewage sludge that has been spread on the surface of the soil (Beauchamp et al. 1978 Ryan and Keeney 1975) as well as from poultry litter (Brinson et al. 1994). In the latter case, composted poultry litter released far less volatile NH3 to the atmosphere (0-0.24% of applied) than did fresh poultry litter (17-23%) (Brinson et al. 1994). In contrast, the crops themselves are often minor sources of atmospheric NH3. Harper and Sharpe (1995) demonstrated almost no net atmospheric NH3 flux in com crops, due to their relatively similar emission and uptake rates of NH3 over the growing season. 

Ammonia can enter the soil by direct application of fertilizers. Of the total U.S. production of anhydrous ammonia, 30% is applied directly to the soil under pressure (Kramer 2000). Approximately 80% of the U.S. production of ammonia is applied to soil in fertilizer formulations designed to release ammoniacal nitrogen. Application of natural fertilizers obtained from livestock excreta will also result in the release of ammonia to the soil (Asman et al. 1998 Beauchamp et al. 1982 Hoff et al. 1981 Olivier et al. 1998). High levels of ammonia in soils can result from the decomposition of animal wastes on cattle feedlots or other confinement areas. Ammonia in soil can also arise from the decay of organic material from plants and animals, etc. (Dawson 1977 Dawson and Farmer 1984). Microbial fixation of nitrogen from the atmosphere is a natural and continual source of ammonia in soil (Galbally 1985 National Science Foundation 1999). 

Tables 9-8 summarizes the budget of N20 in the troposphere. In addition to emanations from soils and seawater, N20 is released from polluted rivers and estuaries, as a product in the burning of agricultural wastes and biomass, and in the combustion of fossil fuels. Yet another source process of uncertain magnitude is the reaction NH2+N02— N20 + H20 associated with the oxidation of ammonia by OH radicals.

Ammonium may be discharged directly into the sub-surface, or can be generated within the soil by ammonification. Its transport and fate involves a combination of the processes of adsorption, cation exchange, incorporation into biomass and release to the atmosphere in a gaseous form (Canter et al, 1987). Adsorption is possibly the main method of ammonium removal, and is particularly important under anaerobic conditions. With increasing pH, ammonium reacts to produce ammonia gas and can therefore be more easily released from the soil. 

Anhydrous ammonia, a fertilizer. At normal temperatures, anhydrous ammonia (ammonia gas containing no water) can be held as a liquid under tank pressure. On release from pressure, the ammonia returns to the gaseous state and is injected into the soil by an ammonia knife." In slightly acid soil, the ammonia is immediately converted to the water-soluble ammonium ion and enters the natural nitrogen pathways of the soil. 

The release point was instrumented to measure the instantaneous rate of ammonia discharge from the 6-in. diameter release line. The spill region was instmmented with three thermocouples placed at the soil surface at distances downwind along the centerline at 3, 5,... 

Ammonia, on the other hand, is one of the important reduced gases of biological origin like CH4 and H2S. It is released to the atmosphere in large amounts from soils [45] and oceans [46]. The NH2 radical is the first species in the photooxidation of ammonia. Subsequent reactions of NH2 radicals present a source or sink for nitrogen oxides. The oxidation can proceed via... 

Trees and soils of forests act as sources of NH3 and oxides of nitrogen. Ammonia is formed in the soil by several types of bacteria and fungi. The volatilization of ammonia and its subsequent release to the atmosphere are dependent on temperature and the pH of the soil. Fertilizers are used as a tool in forest management. The volatilization of applied fertilizers may become a source of ammonia to the atmosphere, especially from the use of urea. 

The released ammonia forms a pool of refrigerated liquid which evaporates by heat transfer from the soil. A constant mass value was assumed for the evaporation rate and a heavier-than-air gas dispersion model was used. 

Enhanced nutrient cycling in both the rhizosphere and bulk soil may depend on the bacterial grazing by protozoa or nematodes with release of inorganic N. Nematodes appear to be the primary consumers of bacteria in the rhizosphere, whereas protozoa are equally prevalent in rhizosphere and bulk soil (41,97). Estimated C-to-N ratios of bacterial-feeding nematodes range from 5 1 to 10 1 (98,99) and are generally higher than those of their bacterial food source thus the excess N is excreted as ammonia (100,101) by nematodes. The estimated... 

Ammonium (NH4 ) may be added as a fertilizer or released during organic matter decomposition. It is the chief species present immediately after the injection of anhydrous ammonia into soil as a fertilizer. It is labile in soil in that, under aerobic, moist, moderate temperature conditions, it is rapidly oxidized to nitrite and nitrate. Ammonia may also be volatilized from the soil solution, particularly under basic soil conditions. 

This is the basis for a common method for the determination of ammonia in soil.1 Soil is suspended in water and placed in a Kjeldahl flask. The suspension is made basic by the addition of a strong (5-50%) sodium hydroxide solution, and the flask is immediately attached to a steam distillation setup. Steam distillation of the suspension carries the released ammonia to an Erlenmeyer flask, catching the distillate in a standardized acid solution that is subsequently back titrated via acid-base titration. The amount of ammonia in soil can be calculated from the end point of the titration. This procedure is similar to a standard Kjeldahl determination and can be carried out using the same equipment, although no digestion is needed. 

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