Ammonification is an important stage in the nitrogen cycle, a natural cycle which makes the Earth's supply of this essential element available to living organisms. It is carried out by a variety of microorganisms found in soil and water, which break down proteins and amino acids in dead plant and animal matter, and feces, releasing ammonia, which is usually retained in soil or water in the form of the ammonium ion. Other groups of microorganisms then convert this into nitrate, which can be absorbed by plants, maintaining the cycle. Ammonification is therefore essential to all plant and animal life on the planet. In agriculture and horticulture, the addition of compost and manure to soil provides an extra source of nitrogen for ammonification.
The Nitrogen Cycle
Nitrogen is essential for all life forms as it is required for amino acids, proteins and DNA; however, although it is abundant in the atmosphere, it cannot be absorbed directly in its elemental form by most organisms. Certain types of soil bacteria are able to trap atmospheric nitrogen — a process known as nitrogen fixation — and combine it with hydrogen to produce ammonia, which is then oxidized by nitrifying bacteria to nitrites and then to nitrates. This form of the element can be absorbed by plants and converted into amino acids, which are linked together to form proteins. These compounds are returned to the soil when plants, or animals that eat them, die, and through animal waste, but the majority of organisms cannot absorb and process them: they must first be broken down into a suitable form.
Ammonification is the process — carried out by a variety of microorganisms — that breaks down proteins, amino acids, and other nitrogen-containing compounds in dead and waste organic matter to form ammonia. Proteins are first split up into amino acids, which are compounds containing an amine (NH2) group by bacteria using enzymes known as proteases. The amino acids, and other compounds with amine groups, such as nucleic acids and urea, are then decomposed by microorganisms known as ammonifying bacteria, releasing ammonia (NH3). This dissolves in water, and usually forms ammonium (NH4+) ions, by combining with hydrogen (H+) ions, which are abundant in most soils. This ammonium is oxidized to nitrites and nitrates by nitrifying bacteria, in the same way as nitrogen that has been “fixed” from the atmosphere.
Loss of Nitrogen from the Soil
Although, under ideal circumstances, nitrogen is recycled in the soil, it can sometimes be lost. The ammonia released by the ammonification process is usually converted into NH4+, which tends to remain in the soil, where it is cycled round again as described above. In alkaline soils, however, H+ ions are not available, and ammonia, which is a gas, can leak away, resulting in a loss of nitrogen. A more common reason for nitrogen loss is the leaching away of nitrates, which are very soluble, by water. If either of these processes is occurring at a greater rate than fixation, there can be an overall loss of the element, resulting in poor plant growth.
Replacing Lost Nitrogen
There are two main ways in which lost nitrogen can be replaced on agricultural land and in gardens. Nitrate fertilizers provide the element in a soluble, directly useable, form, and are commonly employed on farms. Decaying organic matter, such as compost and manure, provide the element through ammonification of nitrogen compounds by microorganisms. Fertilizers may produce quicker results, but nitrates can sometimes be washed away before they are taken up by plants. Compost and manure have the advantage that useable nitrogen is released more slowly, but the spreading of manure on agricultural land can cause problems with odor for nearby residents.
Problems Resulting from Excess Nitrogen
Agricultural practices may result in excessive amounts of available nitrogen entering the environment. For example, nitrates washed from agricultural land into streams and lakes can lead to excessive growth of algae and other microorganisms, a phenomenon known as eutrophication. If these compounds find their way into drinking water, they may cause health problems. These problems are usually due to the overuse of nitrate fertilizers, but they can also result from the ammonification and subsequent nitrification of manure. Using excessive amounts of organic waste, such as manure, can also lead to a build-up of ammonia to levels that are toxic to plants, when ammonification produces more of this compound than nitrifying bacteria can cope with.