Ammonification is a crucial step in the nitrogen cycle, breaking down nitrogen-containing compounds in dead organic matter to form ammonia, which is then converted into nitrate. Loss of nitrogen from soil can occur, but can be replaced through nitrate fertilizers or decaying organic matter. Excessive nitrogen can lead to environmental problems, such as eutrophication.
Ammonification is an important step in the nitrogen cycle, a natural cycle that 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 held in soil or water in the form of ammonium ion. Other groups of microorganisms then convert this into nitrate, which can be taken up by plants, maintaining the cycle. Ammonification is therefore essential for all plant and animal life on the planet. In agriculture and horticulture, adding compost and manure to soil provides an additional source of nitrogen for ammoniification.
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 directly absorbed in its elemental form by most organisms. Some 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 into nitrite and then nitrate. 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 the animals that eat them, die, and through animal waste, but most 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, of breaking down proteins, amino acids and other nitrogen-containing compounds in dead and waste organic matter to form ammonia. Proteins are first broken down into amino acids, which are compounds containing an amino group (NH2) by bacteria using enzymes known as proteases. Amino acids and other compounds with amino groups, such as nucleic acids and urea, are then decomposed by microorganisms known as ammoniating bacteria, releasing ammonia (NH3). This dissolves in water and usually forms ammonium ions (NH4+), combining with hydrogen ions (H+), which are abundant in most soils. This ammonium is oxidized to nitrites and nitrates by nitrifying bacteria, in the same way as nitrogen which has been “fixed” by the atmosphere.
Loss of nitrogen from the soil
Although, under ideal circumstances, nitrogen is recycled back into 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 recycled again as described above. In alkaline soils, however, H+ ions are not available and ammonia, which is a gas, can escape, resulting in loss of nitrogen. A more common reason for nitrogen loss is the leaching of nitrates, which are very soluble, from water. If either of these processes occurs at a faster rate than fixation, an overall loss of the element can occur, resulting in poor plant growth.
Replacement of lost nitrogen
There are two main ways that lost nitrogen can be replaced on farmland and in gardens. Nitrate fertilizers provide the element in a soluble, directly usable form and are commonly used on farms. Decaying organic matter, such as compost and manure, supplies the element through the ammoniification of nitrogenous compounds by microorganisms. Fertilizers can produce quicker results, but sometimes nitrates can be washed out before they are absorbed by plants. Compost and manure have the advantage that usable nitrogen is released more slowly, but spreading manure on farmland can cause odor problems for neighboring residents.
Problems resulting from excess nitrogen
Agricultural practices can lead to excessive amounts of available nitrogen being released into the environment. For example, nitrates washed from farmland 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 your drinking water, they can cause health problems. These problems are usually due to overuse of nitrate fertilizers, but can also result from ammoniation and subsequent nitrification of manure. Using excessive amounts of organic waste, such as manure, can also lead to a buildup of ammonia to levels toxic to plants, when ammoniification produces more of this compound than the nitrifying bacteria can handle.
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