Introduction
Nitrate is a polyatomic ion with the molecular formula NO3- and a molecular mass of 62.0049 g/mol. Nitrates also describe the organic functional group RONO2 the nitrate ion. The net charge of the whole ion is 1-. Almost all inorganic nitrate salts are soluble in water at standard temperature and pressure. A common example of an inorganic nitrate salt is potassium nitrate (saltpeter). A rich source of inorganic nitrate in the human body comes from diets rich in leafy green foods, such as spinach. Nitrate salts are found naturally on earth as large deposits, particularly of Nitratine, a major source of sodium nitrate. Nitrites are produced by a number of species of nitrifying bacteria. Nitrates are mainly produced for use as fertilizers in agriculture because of their high solubility and biodegradability. The main nitrates are ammonium, sodium, potassium, and calcium salts. Several million kilograms are produced annually for this purpose.
Leaching of Nitrates
Nitrates are a primary form of leached Nitrogen. Ammonium is another major form of Inorganic nitrogen but it cannot go deep in soil. Nitrates form when microorganisms break down fertilizers, decaying plants, manures or other organic residues. Usually plants take up these nitrates, but sometimes rain or irrigation water can leach them into groundwater. Although nitrate occurs naturally in some groundwater, in most cases higher levels are thought to result from human activities (Dasrafi gandia et al., 2003). Common sources of nitrate include fertilizers and manure, animal feed lots, municipal wastewater and sludge, septic systems, and N-fixation from atmosphere by legumes, bacteria and lightning.
In some cases where dense hardpans are present, nitrate leaching will not progress beyond the depth of the hardpan. Once nitrates get into the groundwater, the greatest concerns are for infants less than one year old and for young or pregnant animals. High levels of nitrates can be toxic to newborns, causing anoxia, or internal suffocation. Seek alternative water sources if nitrate levels exceed the health standard of 10 ppm nitrate-N. Do not boil water to eliminate nitrates.
Accumulation of Nitrates in Leafy Vegetables (Spinach)
The anthropogenic activities aimed at enhancing food production may facilitate accumulation of undesirable substances in plants and affect the quality of soil and water resources adversely. Excessive amounts of nitrogenous fertilizers are applied to crops, considering that it is a reasonable insurance against yield losses and their economic consequences. However, when input of nitrogen exceeds the demand, plants are no longer able to absorb it, and nitrogen then builds up in the soil, mostly as nitrates (Nosengo, 2003). There is a conflicting evidence regarding the potential long-term health risks associated with nitrate levels encountered in the human diet. The factors responsible for nitrate accumulation in plants are mainly nutritional, environmental and physiological. Nitrogen fertilization and light intensity have been identified as the major factors that influence the nitrate content in vegetables.
Measurement of Nitrates in Vegetables (Spinach)
This is done by Spectrophotometer UV/Visible for this we do, 10g from 200 homogenized vegetables weighted precisely, added hot deionized water and add 5ml tetra borate and heated on water bath for 15 minutes.
Toxicity and Impacts on Environment
Nitrate toxicosis can occur through enterohepatic metabolism of nitrate to nitrite being an intermediate. Nitrites oxidize the iron atoms in hemoglobin from ferrous iron (2+) to ferric iron (3+), rendering it unable to carry oxygen (Kim-shapiro et al., 2005). This process can lead to generalized lack of oxygen in organ tissue and a dangerous condition called methemoglobinemia. Although nitrite converts to ammonia, if there is more nitrite than can be converted, the animal slowly suffers from a lack of oxygen. Humans are subject to nitrate toxicity, with infants being especially vulnerable to methemoglobinemia due to nitrate metabolizing triglycerides present at higher concentrations than at other stages of development. Methemoglobinemia in infants is known as blue baby syndrome. Infants have high intestinal PH. There is chance of converting nitrates to nitrites. Although nitrates in drinking water were once thought to be a contributing factor, there are now significant scientific doubts as to whether there is a causal link. Ruminant livestock with multiple stomachs like cows and sheep tend to experience a similar illness to Blue Baby. Nitrate is converted to nitrite in the rumen or first stomach of livestock, which depletes the blood of oxygen. If too much hemoglobin changes to methemoglobin, the animal will begin to show signs of a lack of oxygen: low tolerance to exercise, lack of coordination, labored breathing, blue coloring of mucous membranes, rapid heartbeat, abdominal pain and vomiting, blood discolored chocolate-brown, and abortions”. Thus far, no symptom or chemical diagnosis specifically points to nitrate poisoning. Additionally, the economy can be negatively impacted as demonstrated by the following examples. A new well could be installed or an existing well could be drilled deeper, but both have high costs. There are no guarantees that these actions will improve water quality. A nitrate removal system could be installed on a private or municipal water supply, which has varying costs. The installation of a nitrate removal system on Plover and Whitings municipal water supply cost nearly $3 million dollars.
Practices to avoid nitrates leaching
Identify fields and areas sensitive to nitrogen in areas where nutrient applications are planned. For instance, sandy or gravelly soils, and soils with shallow water tables are more susceptible to nitrogen leaching. Match nitrogen applications with crop requirements. Use the spring or pre-side dress soil nitrogen test where available (e.g., for corn and barley). In your Nutrient Management Plan, account for nitrogen contributions from green manure crops and any previous crop rotations. (Jivinen, P. Knekt, P.L. Pentill, 1996) In your Nutrient Management Plan, account for nitrogen from any manure or biosolid application. Apply most of the nitrogen just before the time of maximum crop uptake (e.g., sidedress corn). Split applications of nitrogen through techniques such as fertigation. Practise crop rotations to make efficient use of nitrogen and maintain healthy soils. Establish cover crops as needed to “tie up” any excess nitrogen at the end of the season.
The writer is from
University of Agriculture
Faisalabad, Pakistan.