Ron Hester is an emeritus professor of chemistry at the University of York. In addition to his research work on a wide range of applications of vibrational spectroscopy, he has been actively involved in environmental chemistry and was a founder member of the Royal Society of Chemistry’s Environment Group. His current activities are mainly as an editor and as an external examiner and assessor on courses, individual promotions, and departmental/subject area evaluations both in the UK and abroad.

Roy Harrison OBE is Queen Elizabeth II Birmingham Centenary Professor of Environmental Health at the University of Birmingham. In 2004 he was appointed OBE for services to environmental science. Professor Harrison’s research interests lie in the field of environment and human health. His main specialism is in air pollution, from emissions through atmospheric chemical and physical transformations to exposure and effects on human health. Much of this work is designed to inform the development of policy.

Agricultural Chemicals and the Environment

Issues in Environmental Science and Technology

By R.E. Hester, R.M. Harrison

The Royal Society of Chemistry

Copyright © 1996 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-220-3

Contents

Fertilizers and Nitrate Leaching Thomas M. Addiscott, 1,
Eutrophication of Natural Waters and Toxic Algal Blooms Alastair J. D. Ferguson, Mick J. Pearson and Colin S. Reynolds, 27,
Impact of Agricultural Pesticides on Water Quality Kathryn R. Eke, Alan D. Barnden and David J. Tester, 43,
Agricultural Nitrogen and Emissions to the Atmosphere David Fowler, Mark A. Sutton, Utte Skiba and Ken J. Hargreaves, 57,
Drugs and Dietary Additives, Their Use in Animal Production and Potential Environmental Consequences Thomas Acamovic and Colin S. Stewart, 85,
Detection, Analysis and Risk Assessment of Cyanobacterial Toxins Steven G. Bell and Geoffrey A. Codd, 109,
Subject Index, 123,

CHAPTER 1

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 nitrate concentrations in 229 urban areas in the UK between 1969 and 1973 and at deaths from stomach cancer in the same areas at the same time. The hypothesis suggested that there should be a positive relationship between them. The results showed a negative one.

The third test is particularly interesting in the context of a paper about nitrogen fertilizer because it involved a plant making it. If you work in, or even visit, a plant producing ammonium nitrate or another fertilizer, you can taste fertilizer within minutes of entering, as the fine dust in the air dissolves in your saliva. If anyone is going to get stomach cancer as a result of exposure to nitrate, it is workers in nitrate fertilizer plants, but an epidemiological survey showed that their mortality due to stomach cancer did not differ from that among workers in comparable jobs (Table 1). Twelve deaths occurred from stomach cancer during the 35 years of the study, while 12.06 deaths would have been expected from the mortality rate among the other workers.

Although there are a few caveats that need to be noted about these tests, the results suggest strongly that there is no real link between stomach cancer and nitrate in water. Further support for this conclusion comes from the fact that, while nitrate concentrations have been increasing in water during the past 30 years, the incidence of stomach cancer has been declining, and about ten years ago the absence of any link was accepted officially.

Is there a Safe Limit for Nitrate? The two previous sections show that any safe limit for nitrate needs to be related to methaemoglobinaemia rather than stomach cancer. ‘Blue-baby’ cases in the USA were associated with nitrate concentrations ranging from 283 to 1200 g m-3. The only fatal case in the UK arose from a nitrate concentration of 200 g m-3. The non-fatal case in the same report involved bacterially polluted water with 95 g m-3 of nitrate. There is no obvious safe level, but in evidence to the House of Lords three august bodies, The Medical Research Council, The Institute of Biology, and the Institute of Cancer Research, stated that the majority of cases have occurred when the water contained more than 100 g m-3 of nitrate. The limit of 50 g m-3 imposed by the European Commission thus has a safety factor of two. This safety factor is a mixed blessing. To some it represents prudence, but to farmers and water suppliers it is a problem. A limit of 100 g m-3 for water draining from agricultural land would be quite easy to achieve, but 50 g m-3 is far more difficult, particularly in the drier areas of the UK and when there is a substantial input of nitrogen to the soil from the atmosphere.

Nitrate as an Environmental Problem

It is not only land plants that need nitrogen for growth. Plants growing in water respond to extra nitrogen like crop plants, but their extra growth is not welcome. Increased nitrate concentrations in rivers and lakes may encourage reeds to grow to excess, narrowing waterways and possibly overloading and damaging banks. Underwater plants also proliferate, so that anglers lose tackle, the propellers of boats get fouled and conduits for water supply become clogged and machinery damaged.

Large water plants, though a nuisance, are not the main environmental problem. Algae are very small single-celled plants that grow on a variety of surfaces, including that of water. They are not noticeable on water until they grow to excess and form the ‘blooms’ — possibly better described as scum — that are now seen to a worrying extent on our rivers and lakes. These blooms look messy when they grow, but they become a far bigger problem when they die. The bacteria that decompose them use oxygen when they do so and thus deprive fish and other desirable organisms, which may die. The whole ecological balance of the river or lake may change, usually to the detriment of species that we would like to see there. This process, known as ‘eutrophication’, is limited by phosphate rather than nitrate in fresh water, and it is discussed in detail elsewhere in this issue. Algae can also grow to excess in the sea, but these marine blooms seem to be stimulated more by nitrate than by phosphate. Algal blooms took on a new significance a few years ago when it was discovered that some species were toxic to humans and dogs, a problem that is again discussed in more detail elsewhere in this issue. The indirect effects of nitrate in water proved in this instance to be a far more tangible health hazard than the direct effects.

2 The Contribution of Fertilizer to the Nitrate Problem

At the root of the nitrate problem lies a very simple relationship

availability = vulnerability

Any nitrogen in the soil that is available to crops is likely to be present as nitrate itself, or as ammonium, which microbes in the soil soon convert to nitrate. Nitrate is completely soluble in water in the presence of all cations likely to be in the soil solution and it is not adsorbed. It is thus vulnerable to being washed out of the soil by percolating rainfall or irrigation. The surest way of avoiding such nitrate losses is to ensure that as little nitrate as possible is in the soil at any time. When crops are growing fast they take up nitrogen quickly, as much as 5 kg ha-1 d-1 sometimes, so they need a generous supply of nitrate in the soil. Once they cease to grow and to take up nitrate, however, we need to make sure that there is as little nitrate as possible left in the soil so that it is not vulnerable to leaching. Any nitrate present is there at the wrong time and we can see that what we have is not so much a nitrate problem as a problem of untimely nitrate. Most untimely nitrate happens because either the soil microbes or the farmer have put it in the soil when the crop cannot use it. However, there is another scenario that occurs when the farmer applies the fertilizer at a perfectly sensible time, but substantial amounts of rain fall before the crop has had a chance to use the nitrogen supplied. To assess the contribution of nitrogen fertilizer to the nitrate problem, we need to think about how it might become untimely nitrate. In the first few months after application the four most likely fates for the fertilizer are:

1. It may be taken up by the crop, as intended.

2. It may become incorporated in the soil’s organic matter, where it will remain unless it is remobilized by the bacteria and other organisms in the soil.

3. It may be leached out of the soil.

4. It may be denitrified. This happens when microbes hungry for oxygen utilize

the oxygen atoms of the nitrate ion so that NO-3 becomes dinitrogen (N2) or nitrous oxide (N2O), both of which are gases. Denitrification lessens the nitrate problem but N2 contributes to the ‘greenhouse effect’. Ammonium in the fertilizer may be volatilized as ammonia.

Fertilizer nitrogen is usually applied in the spring, but to assess its overall contribution to the nitrate problem we need to consider what happens not only in the period between its application and the harvest of the crop but also after harvest.

Birds and animals are ‘tagged’ so that their wanderings and ultimate fate can be followed. To do the same for nitrogen fertilizer, we use the heavy isotope of nitrogen, 15N, as a ‘tag’ or ‘label’. This is a safe isotope to use because it is not radioactive. The 15N of the label is taken up by the crop, incorporated in the soil’s organic matter or leached or denitrified in almost exactly the same way as the 14N which makes up 99.6% of the nitrogen in the rest of the fertilizer and in the soil. The ratio of 15N to 14N in the fertilizer is known, so when the crop and the soil are analysed and the 15N to 14N ratios are determined (usually by mass spectrometry), the amounts of nitrogen of fertilizer origin that are in the crop or in the organic matter, ammonium, or nitrate in the soil can be determined.

The fact that 15N is not radioactive means that it can be used safely in experiments in the field, but it also means that much patient work is needed to obtain results. The approach is demanding in terms of time, equipment, and skilled manpower, but it has made a great contribution to the understanding of the nitrate problem. The results that are outlined here are from experiments made by staff at Rothamsted, but key contributions have also come from Scotland and France. The majority of the Rothamsted experiments involved winter wheat, but oilseed rape, potatoes, beans, and sugar beet were also grown. The soil is a factor in nitrate leaching, and three types were used, the flinty, silty clay loam at Rothamsted, a sandy loam at Woburn in Bedfordshire and a heavy sandy clay at Saxmundham in Suffolk.

Winter Wheat — the Baseline

Winter wheat formed the backbone of the 15N programme for several reasons. One is simply that it is the most widely grown crop in England and Wales. Winter cereals occupy about 60% of the arable land in England. Winter wheat is also representative of those crops which are sown in the autumn and have a well-established root system by the time they receive applications of nitrogen fertilizer in the spring. It is a notably parsimonious crop — only sugar beet allows less nitrate to escape its roots — so it provides a useful baseline against which to compare other crops.

Between Fertilizer Application and Harvest. The results from Rothamsted showed that 50–80% of the labelled fertilizer nitrogen was recovered in the crop, while a further 10–25% was in the soil when the crop was harvested. The key finding was that almost all the labelled nitrogen in the soil was in organic forms (Figure 1). Some will have been in dead roots and some in living matter, such as microbes and small soil animals that will have fed on the roots and on exudates produced by the roots while they were still alive. The proportion of the labelled nitrogen left vulnerable to leaching as ammonium or nitrate was only 1–2%, and nearly all the nitrate found in the soil at harvest was not labelled and therefore did not come from the fertilizer.

The best overall recovery of labelled fertilizer in crop and soil was 99 % and the least satisfactory 65%. Thus 1–35% of the labelled nitrogen, on average 15.7%, was ‘missing, presumed lost’. These losses occurred between the time of the fertilizer application in spring and the time the crop was harvested; but why and how did they occur?

The reason why the losses occurred is simply ‘because it rained’. The more rain that fell in the period after application, the more labelled nitrogen was lost. The critical period seemed to be the first three weeks after application; this was the period that showed the closest relationship between the losses and the rainfall (Figure 2).

(Continues…)Excerpted from Agricultural Chemicals and the Environment by R.E. Hester, R.M. Harrison. Copyright © 1996 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.