In this article we will discuss about:- 1. Introduction to Nitrogen Cycle 2. Forms of Nitrogen 3. Sources 4. Biochemical Conversion Processes.
Introduction to Nitrogen Cycle:
Nitrogen, in a number of different forms, can play an important role in several environmental problems. Like many chemical compounds, nitrogen is cycled in nature from one form to another, with losses in one area being replaced by additions from another. To understand how and when nitrogen can become a problem, we need to examine how it occurs and moves through the biosphere.
We shall examine the sources of nitrogen and the pathways it can take through an ecosystem. Because most of the conversions of nitrogen from one form to another (which enables it to take different pathways) are performed by organisms, it is referred to as a biochemical cycle. In fact, it is the most complex of the biochemical cycles.
Bacterial oxidation and reduction of nitrogen-containing compounds is coupled with photosynthetic assimilation and utilization by algae and larger plants. Because most of these changes are the result of biological activity, it is a slow cycle, slower than the phosphorous cycle, on the order of 100 years.
Nitrogen is very important as a nutrient for both plants and animals. It is a major component of amino acids, which form proteins. Insufficient amounts of protein in a diet are a major cause of malnutrition. Nitrogen in the form of proteins, including enzymes and hormones, accounts for about 20% of our body weight. In addition, nitrogen is an essential component of genetic material—that is, RNA and DNA.
In general, humans do not really receive the proper types of protein throughout the world, and so we have protein shortages. In the poorer nations of Africa, young children’s distended stomachs are caused by protein swelling in their stomachs as water builds up in between cells. To alleviate protein deficiencies in food, we may “fortify” them. Soybean is important in this respect and has become a major agricultural product in the United States. It is primarily used in animal feed.
The main reservoir of nitrogen is the atmosphere, which contains about 79% nitrogen. Unfortunately, when it exists as nitrogen oxides and dioxides (NOx), it forms a principal component of photochemical smog. This yellow-brown haze is a mix of NOx, light and hydrocarbons. It forms a corrosive, toxic, and potentially carcinogenic gas, mostly from the industrial complex and from electric power production. Many efforts are under way to reduce these sources.
Nitrogen can exist in a number of different forms. The reason for this is the high number of oxidation states it has, varying from +5 to –3. Nitrates are +5 and nitrites are +3. Ammonia, amino acids, and proteins are –3.
The importance of nitrates and nitrites as plant nutrients and sometimes as limiting elements. We also know that they are a major product of the biological oxidation processes employed in sewage treatment. These nutrients also can exist in groundwater where they can pose environmental and health problems. The most important aspect of nitrates, however, is their fertilizing effect in aquatic ecosystems.
Ammonia (NH3), and the ammonium ion (NH4), exist as by-products of biological activity, specifically resulting from the breakdown or decay of plant and animal proteins and amino acids. It is usually in excess and must be eliminated. In mammals it occurs as uric acid, C5H4N4O3, in urine.
The liquid waste of aquatic animals is urea, NH2CONH2. Fish also manage the problem through the elimination of ammonia across the membranes of their gills. Ammonia also is created as a by-product of both aerobic and anaerobic decomposition. In waters that are about neutral, most ammonia is in the form of the ammonium ion.
Sources of Nitrogen:
There are a variety of natural sources that can supply forms of nitrogen to an aquatic system. Precipitation usually accounts for less than 0.2 mg/l total nitrogen. Compounds of nitrogen transported by rainfall events include dissolved nitrogen, dilute nitric acid, ammonium, nitrate, and dissolved and particulate organic compounds. Nitrogen brought into aquatic systems by rainfall is a very important nutrient source for oligotrophic lakes. Volcanic activity supplies a source of nitrogen in its dust fall.
Depending on the erosive characteristic of the soil, non-urban runoff can also provide up to 0.2 mg/l of nitrogen. This is mostly from decaying vegetation in the soil. Good soils have organic humus that has natural nitrogen in complex molecules from decayed woody fibers, animal waste, and other decaying substances.
Inorganic nitrogen accounts for less than 1% of the total nitrogen in soil. Fixation through biological activity in soil generates volatile nitrogen compounds from the decomposition process. Lightning performs meteorological fixation, changing gaseous nitrogen to nitrates and nitric acid type compounds. Lastly, forest fires can supply nitrogen to the atmosphere as organic matter containing nitrogen is burned.
Whether through pollution of the air or land or directly into the water, anthropocentric activities generate the majority of the excess nitrogen in aquatic systems. The effluents from domestic or municipal wastewater treatment plants are a major and direct source of nitrogen into receiving water bodies.
In raw wastewater, 60% of the nitrogen present is in the form of ammonia, 40% is organic, and less than 1% exists as nitrites and nitrates. In the effluent of secondarily treated wastewater, almost all the nitrogen has been converted to nitrites and nitrates, with a typical discharge of 15 to 50 mg/l.
Runoff from feedlots and other areas where large numbers of domesticated animals are kept contains much nitrogen. In rural areas this can be a significant problem that not only results in surface water contamination but also contamination of shallow aquifers.
Much of the inorganic nitrogen pollution is in the form of nitrates and especially ammonium, which results from the hydrolysis of urea. Organic nitrogen from animal waste can be as high as 600 mg/l. On eastern Long Island, duck farms have caused major problems.
Agricultural lands are fertilized and irrigated. Runoff carrying nitrogen, mostly in the form of nitrates, can be significant. Some estimates indicate that less than 75% of applied fertilizers are taken up by plants. The rest can leach down and pollute subsurface waters. In Illinois, a study showed that at least 55-60% of the nitrates in the surface waters entering Lake Decatur were from synthetic fertilizers.
Storm sewer systems in urban environments are also a major source of nitrogen pollution. Because these systems do not have treatment technology, the system concentrates the pollution through collecting the runoff. In New York City, pollution from storm water management has become such a large problem that system treatment technologies are being planned for the storm waters.
In rural and suburban environments, septic tanks and similar small treatment systems discharge nitrogen contamination directly to the subsurface soils. (The grass does grow greener over the septic tank!)
Certain industries are noted as developing waste streams where high nitrogen concentrations are common. These industries in general must provide pretreatment before discharging into a POTW, complete treatment in a “package plant,” or even a full-scale plant. However, even with treatment, these wastewater discharges are still a major source of nitrogen.
Some of the industries where nitrogen is a particular problem include:
(i) Meat processing,
(ii) Milk processing,
(iii) Petroleum refining,
(iv) Ice plants, and
(v) Fertilizer production plants.
Lastly, the atmosphere must be considered a source of nitrogen pollution in the urban environment. Between vehicles, power plants, industrial incinerators, and manufacturing facilities, a heavy burden of nitrogen pollution is added to the atmosphere. When it rains, this may be transferred to aquatic systems directly or through runoff.
Biochemical Conversion Processes:
Assimilation is the metabolic utilization or conversion of some forms of nitrogen into organically bound molecules. A good example is protein synthesis in organisms. Plants can use nitrates or ammonia with CO2 in the presence of light to make protein for them to build more plant tissue.
Nitrogen fixation is the utilization of atmospheric gaseous nitrogen into principally nitrates and nitrites but also organically bound nitrogen. Fixation accounts for most of the natural transformation of nitrogen that isn’t bound in soil or leached to ground waters or surface waters. Fixation is generally thought to be more important in fresh water than it is in saltwater. There are three types of fixation.
Atmospheric fixation by lightning converts N2 to nitrates and nitric acids. Industrial fixation is equal to about half of the fixation that takes place naturally. The process is employed principally in the manufacture of fertilizers and explosives.
Biological fixation of nitrogen is the most important process from the perspective of water pollution biology. The process includes the conversion, by bacteria and blue-green algae, principally into nitrates, but also into ammonia and organic compounds. The predominant nitrogen fixers are several types of free-living bacteria in soil and water.
Leguminous plants such as alfalfa, clover, and soybeans have root nodules that accommodate nitrogen-fixing bacteria (Rhizobium). In soil, anaerobic species such as Clostridium convert nitrogen to ammonia. The aerobic and epiphytic Azobacter also does this. Other species of bacteria convert nitrogen to nitrates.
Several species of blue-green algae can perform fixation—species such as Anabaena, Nostoc and Trichodesmium among others. They produce the nitrates, increasing the fertility of the waters in which they live. This can augment the process of eutrophication.
In this biological conversion, ammonia compounds are converted by nitrifying bacteria to nitrites and nitrates. Nitrosomonas oxidizes ammonia to nitrite. Nitrobacter then oxidizes the nitrite to nitrate. Because oxygen is used up and carbon dioxide produced, there is also a lowering of the pH.
Denitrifying bacteria free or reduce nitrates to gaseous nitrogen and some nitrous oxides, which go back into the atmospheric reservoir. This process generally takes place in an anaerobic environment by facultative heterotrophic bacteria. The oxygen bound in nitrites and nitrates is the source of oxygen for respiration. Some of the more well-known species that perform denitrification include Pseudomonas, Micrococcus, Achromobacter, and Bacillus.
In this process, organically bound nitrogen is converted to ammonia and ammonia compounds. The decomposition of organic matter and waste products by bacteria and fungi is a good example. Urea is converted to ammonium carbonate through hydrolysis.
The organically bound nitrogen of proteins and amino acids is eventually converted to nitrates through decomposition. The production of ammonia is a step in this process. It should be noted that both aerobic and anaerobic decomposition will produce ammonia.