Fertilizers and Nitrate Leaching

THOMAS M. ADDISCOTT

1 The Nitrate Problem

Nitrate is one of the facts of life. It is essential for the growth of many plant species, including most of those we eat, but it becomes a problem if it gets into water in which it is not wanted. It is perceived mainly as a chemical fertilizer used by farmers, but much of the nitrate found in soil is produced by the microbes that break down plant residues and other nitrogen-containing residues in the soil. There is no difference between nitrate from fertilizer and that produced by microbes, but, whatever its origin, this rather commonplace chemical entity has now become a major environmental problem and is also treated as a health hazard.

Concentrations of nitrate increased in natural waters for two decades before levelling off in the 1980s. Applications of nitrogen fertilizer followed a similar pattern, so many people drew the obvious conclusion that the increase in nitrate concentrations arose from the greater use of fertilizers. However, conclusions drawn from coincident changes need to be examined carefully. The fact that the birth-rate in Europe declined at the same time as the population of storks does not necessarily mean that storks bring babies! The stork and the birth-rate were probably both responding to the increased size and affluence of the human population and its increased and more intensive use of land for agriculture. Much the same can be said for nitrate concentrations and fertilizer use. It is not so much that more use of fertilizer has led to more nitrate in natural waters, as that increases in both the area of land used for arable agriculture and the intensity with which it is farmed have led to both greater concentrations of nitrate and greater use of nitrogen fertilizer. The fertilizer is part of the intensification package, and examining it as a cause of the nitrate problem without considering the rest of the package could lead to false conclusions. This point is particularly important because, as we shall see later, nitrogen fertilizers have both a direct and an indirect role in the nitrate problem.

Nitrate is seen as a threat to both public health and natural waters. Of these threats the latter is definitely the more immediate, but the health issue has attracted more public concern.

Nitrate as a Health Hazard

Nitrate is not a new problem. Excessive concentrations were recorded in many domestic wells in a survey conducted 100 years ago. What is new is the public concern about nitrate. This arises from two medical conditions that have been linked to nitrate: methaemoglobinaemia ('blue-baby syndrome') in infants, and stomach cancer in adults. Both are serious conditions, so we need to examine possible links carefully, but we need to note that these conditions are not caused by nitrate but by the nitrite to which it may be reduced. Nitrate itself is harmless and is most notable from a medical standpoint as a treatment for phosphatic kidney stones.

Methaernoglobinaernia. The 'blue-baby syndrome' can occur when an infant less than about one year old ingests too much nitrate. Microbes in the stomach convert the nitrate to nitrite and when this reaches the blood-stream it reacts with the haemoglobin, the molecule that transports oxygen around the body. Normal oxyhaemoglobin, which contains iron in the iron(II) state, becomes methaemoglobin in which the iron is in the iron(III) state, greatly lessening the capacity of the blood to carry oxygen and causing what might be described as chemical suffocation. Very young children are susceptible because foetal haemoglobin, which has a greater affinity for nitrite than normal haemoglobin, persists in the blood-stream for a while, and because their stomachs are not sufficiently acid to inhibit the microbes that convert nitrate to nitrite. Gastroenteritis greatly exacerbates the effects of the nitrite.

This condition is usually very rare. In the UK the last case was in 1972 and the last death in 1950, but in Hungary there were over 1300 cases between 1976 and 1982. One reason for this difference between the two countries may lie in the origin of the water. Practically all known cases were associated with water from wells, and the condition is known as 'well-water methaemoglobinaemia' in the USA. In 98% of these cases the wells were dug privately and may have been too close to disposal points for animal or human excreta, thereby increasing the risk of pollution not only by nitrate but also by E. coli and other organisms that cause gastroenteritis. The author is not aware of any case in which methaemoglobinaemia was caused by tap water from a mains supply such as that used by most households in the UK. That being said, there is no room for complacency about the condition. In the fatal case in 1950, the doctor reported that, 'There were diarrhoea and vomiting and the child's complexion was slate-blue'. In a similar but non-fatal case in the same year, 'Blood drawn from a vein was a deep chocolate-brown'.

Stomach Cancer. Of all the cancers, that of the stomach causes the second largest number of deaths. Only lung cancer kills more men, and only breast cancer kills more women. Stomach cancer is a painful and debilitating way to die, and the link to nitrate in water that has been suggested is a serious matter. There are good theoretical reasons for proposing such a link. Nitrite produced from nitrate could react in the stomach with a secondary amine coming from the breakdown of meat or other protein to produce an N-nitroso compound.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The N-nitroso compounds are carcinogenic, so the reaction could result in stomach cancer. This mechanism is essentially a hypothetical one, and tests were made to evaluate it. Three tests in particular suggested that the hypothesis was not correct, that is, that there is no clear link between stomach cancer and nitrate in water.

One test, made at the Radcliffe Infirmary in Oxford, identified two areas of the UK in which the incidence of stomach cancer was particularly high and two in which it was particularly low. People attending hospitals in these areas as visitors rather than patients were asked to provide samples of saliva. The hypothesis suggested that samples from the high-risk areas should contain more nitrite and nitrate than those from the low-risk areas, but this was not so. The samples from the low-risk populations had nitrate concentrations 50% greater than those from the high-risk areas.

Another test looked at...