Question

Explain/define/comment upon  the following item, making sure to define terms and addressing toxicity or other environmental aspects of the subject.

A recent article in Science indicates that soils – particularly those that are fertilized and have a low pH, are sources of nitrite (NO2-) that forms nitrous acid (HNO2). Explain how this nitrite is formed in soils, how else it may get there, and describe two potentially toxic consequences of having it in the air and/or in your tissues.

Answer 1

Nitrite is an intermediary compound formed during nitrification as well as denitrifiication. It occasionally accumulates in soils and drainage water. The nitrite can then undergo transformations to gaseous nitrogen compounds such as NO and NO2. Soil pH controls the abiotic nitrite decomposition to a large extent. Under acidic conditions(pH <5.5), nitrous acid spontaneously decomposes preferentially to NO and NO2. Nitrite also undergoes reactions with metallic cations (especially ferrous iron) and with organic matter. As a result of these reactions gaseous compounds such as NO, NO2, N2O and CH3ONO can be formed. Through reaction of nitrite with phenolic compounds nitroand nitrosocompounds can be formed, building up organic N. With normal agricultural practices on slightly acidic soils, the nitrite instability usually does not lead to economically important N losses from soils. However, the compounds formed through its degradation or interaction with other soil constituents are linked to environmental problems such as tropospheric ozone formation, acid rain, the greenhouse effect and the destruction of the stratospheric ozone.

Nitrite accumulation and toxicity

Nitrite occasionally accumulates in soils and drainage water, depending on soil characteristics and agricultural practices such as ammonium fertilization, nitrifying activity, soil or water pH, organic matter content, temperature and moisture content, and soil-fertilizer geometry . Alkaline conditions favour nitrite accumulation. Under mildly acidic conditions, nitrite is rapidly decomposed . Important sites for nitrite accumulation are urine spots , sites of animal waste application and urea application sites . These lead to high concentrations of NH +, and a rise in pH near the applied urea. Both the high pH and NH3 concentration inhibit nitrite oxidation and allow nitrite to accumulate . According to Warren the nitrite accumulation is linked to NH3 concentrations. As the fraction of NH3 increases with increasing pH, conditions stimulating alkaline conditions will induce nitrite accumulation. The activity of Nitrobacter spp. is adversely affected and nitrite accumulates because of slow conversion of nitrite to nitrate. Significant amounts of nitrite temporarily accumulate in the soils with pH 7.7-7.8 upon application of 100 mg urea-N kg-lof soil . Although the influence of pH on the nitrite accumulation is striking, other parameters such as urease activity, ferrous iron and organic matter might have influences. The sampling interval is critical to observe the nitrite accumulation, since it is possible to miss the nitrite peak by infrequent sampling. Nitrite accumulations from urea applications have been well documented

HOW CAN NITRATE AND NITRITE AFFECT HEALTH

Most people are not exposed to levels of nitrate and/or nitrite that would cause adverse health effects. Young infants (<6 months of age) appeared to be particularly sensitive to the effects of nitrite on hemoglobin after consuming formula prepared with drinking water that contained nitrate at levels higher than recommended limits; some of these infants died. The cause of methemoglobinemia (a change to hemoglobin that decreases the ability to transport oxygen to tissues) in many of these infants may have been gastroenteritis from bacteria or viruses in the drinking water or from other sources not related to nitrate. Some children and adults who ate food or drank fluids that contained unusually high levels of nitrite experienced decreases in blood pressure, increased heart rate, reduced ability of the blood to carry oxygen to tissues, headaches, abdominal cramps, vomiting, and even death.

WHAT HAPPENS TO NITRATE AND NITRITE WHEN THEY ENTER THE ENVIRONMENT?

Nitrate and nitrite ions naturally occur in the terrestrial (soil) and aquatic (water) environment as part of the earth’s nitrogen cycle and can therefore be found in both soil and water. In nature, nitrate and nitrite can also be found in igneous and volcanic rocks. Nitrate is formed naturally as an end product of vegetable and animal decomposition, making this a principal source for nitrate ion in both terrestrial (soil) and aquatic (water) environments. Nitrate and nitrite can also be released into the atmospheric (air), terrestrial (soil), and aquatic (water) environments at places where human-made materials such as fertilizers are produced or used. Human and animal wastes are important sources of ammonia, a compound containing nitrogen, which undergoes chemical reaction to produce nitrite and subsequently nitrate. In aerobic (containing oxygen) environments, ammonia is readily oxidized to nitrite by ammonia-oxidizing bacteria; nitrite is oxidized to nitrate by nitrite-oxidizing bacteria. This two-stage process is known as nitrification. Both human-made and natural sources of nitrogen may contribute to nitrate aerosols in the atmosphere, as well as nitrate and nitrite ions in terrestrial (soil) and aquatic (water) environments. Nitrate and nitrite have been detected in surface waters, drinking water (including public and private wells), and groundwater. Nitrate accounts for the majority of the total available nitrogen in surface waters. Contamination of waters is a result of agricultural runoff (use of chemical fertilizer or animal manure) and discharges from septic systems and municipal waste water treatment facilities. Nitrogen exists naturally in soils, typically bound to organic matter or mineral soil material such as rocks. Available forms of nitrogen, including nitrate and nitrite, are present in soils at a few kilograms (kg)/hectare. Nitrate and nitrite are a normal part of the human diet and can be found in vegetables, fruits, cured meats, fish, dairy products, beers, cereals, and cereal products. Some salts, such as sodium nitrite, are intentionally added to foods and beverages to preserve or cure them; inhibiting the formation of microorganisms that may cause disease such as botulism. Additionally, nitrites and nitrates may be present in some medicines as they can be employed in medicinal and therapeutic uses