agriculture group symposium keeping nitrate in its place

J Sci Food Ayric 1990,53, 133-141

Agriculture Group Symposium Keeping Nitrate in Its Place

The following are summaries of papers presented at a joint meeting of the Agriculture and the Water and Environment Groups of the Society of Chemical Industry held on 21 November 1989 at the Society of Chemical Industry, 14-15 Belgrave Square, London S W l X 8PS . The papers published here have not been refereed and do not necessarily reflect the views of the Editorial Board of the Journal of the Science of Food and Agriculture.

Nitrate and the Law-the Present and the Future

Trevor Adams

McKenna & Co, Inveresk House, Aldwych, London WC2, UK

In order to understand the framework governing nitrate and other pollutants, it is essential to have some comprehension of the general law. The main division of English law is into criminal and civil law, typical problems coming under the latter head being those concerning breach of contract or negligence. The standard of proof required differs in civil and in criminal law, in that in civil law one is concerned with the ‘balance of probabilities’ which can be described as a 51 % to 49% situation, whereas in criminal law the standard is ‘beyond reasonable doubt’ which is higher than the ‘balance of probabilities’ and is impossible to quantify though one could suggest figures in the range 70-90%. In order to get around this high standard of ‘beyond reasonable doubt’, Parliament has decided to dispense with the so-called mental element for some offences, typically driving offences and pollution offences, so that the offender does not have to be, for example, intentional, reckless or negligent as to the course of his action in order for him to be liable. These are so- called offences of strict liability and are obviously a very harsh form of justice.

Water quality has been regulated since 1 September 1989 by the Water Act 1989 and the Regulations there under. The principal water-pollution offence is now contained in Section 107 of the Water Act 1989, and one limb of this offence is construed as an offence of ‘strict liability’ where no intention, recklessness or

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J Sci Food Agric (53) (199O)-Q 1990 SCI. Printed in Great Britain

134 Agriculture Group Symposium

negligence is required to make the offender liable. This is done by interpreting the word ‘cause’ in a very wide sense to include, for example, the ‘normal’ operations of a factory. Exemptions under Section 108 to the offences under Section 107 include having a consent or a licence for the discharge in question.

There are two principal items of regulations directly relevant to nitrate, the first being the Surface Waters (Classification) Regulations 1989 which prescribe the system for classifying the quality of inland waters according to their suitability for abstraction by water companies for supply, after treatment, as drinking water. Also, the Water Supply (Water Quality) Regulations 1989 as amended implement those parts of EEC Directive 80/778, the so-called Drinking Water Directive, relating to the quality of water intended for human consumption. Under both sets of Regulations the limit for nitrate is 50 mg litre- ’.

The British Government has proposed to control the input of nitrate into drinking water from agricultural sources in the proposed Nitrate Sensitive Areas Scheme whereby improved agricultural practices would be implemented by means that range from voluntary compliance, through civil ‘breach of contract’ provisions to the criminal law. However, it is clear that, especially with regard to criminal sanctions, there would be great difficulties of proof. The United Kingdom Environmental Law Association has suggested the use of a soil test value, with certain caveats or, for chemical fertiliser only, a documentation scheme whereby records are kept of fertiliser delivered to the farm and so the quantity applied to the land can be calculated by the difference between the quantity delivered and the quantity remaining in the farmer’s store.

There is also a proposed directive from the EEC to control nitrate pollution from ‘diffuse sources’, which aspect largely overlaps with the Nitrate Sensitive Areas Scheme, and also to control eutrophication of water. There would be a four-year implementation period after the directive is finalised. The provisions of the directive include designating ’vulnerable areas’ as defined in the Directive, fixing the amount of manure which can be spread on the farm by specifying a stocking density which varies with the type of animal, ie dairy cow, sheep etc. Among the difficulties with the proposal is to take into account the type of soil on a specific farm, the nitrate- producing capacity of the manure and the number of animals in the total area.

Overall, the control on nitrate pollution is likely to increase both under the Nitrate Sensitive Areas Scheme and the EEC Directive, though it is as yet unclear what the final form of either will be and, indeed, how effective they will be in practice.

Managing Nitrate in Arable Farming

David S Powlson

AFRC Institute of Arable Crops Research, Rotharnsted Experimental Station, Harpenden, Herts AL5 UQ, UK

Some loss of nitrate from arable soil is inevitable. Whenever free nitrate is present, there is a risk of denitrification occurring if soil conditions become suitable.

Keeping nitrate in its place 135

Leaching will occur in virtually all years because production of nitrate from soil organic matter is not well synchronised with crop uptake. Some nitrate is produced during late summer and autumn, when crop uptake is small, and will be exposed to leaching during winter. The quantity of nitrate in a soil in autumn is affected by a variety of factors. Some, such as soil organic matter content, long-term cropping history and climate, are outside the farmer's control. Others can be manipulated: these include choice of cropping sequence, method of disposal of crop residues, cultivation, use of organic manures, use of cover crops and time of sowing of next crop.

Where winter wheat has been grown, there is abundant evidence that the amount of inorganic fertiliser applied in the previous spring has little effect on residual nitrate unless excessive applications are given. With some other crops this is not so, and a decrease in fertiliser N application would lead to a significant decrease in residual nitrate. With spring beans (grown without N fertiliser) residual inorganic N in soil at harvest was 30 kg ha-' greater than with winter wheat (given 225 kg fertiliser N ha-') because uptake of mineralised soil N was less.

The use of organic manures and the ploughing of leguminous leys to provide N for arable crops are both practices that can lead to increased leaching as nitrate production is not necessarily matched to crop uptake in either quantity or time.

The indirect effects of agricultural practices on nitrate production and leaching risk also require attention as these will influence the effectiveness of the various options intended to decrease nitrate leaching. For example, the continued application of 144 kg N ha- as inorganic fertiliser to wheat in the Broadbalk experiment for almost 150 years has increased soil N mineralisation rate by 70%. This is presumably because the larger crop returns larger quantities of organic N to soil in roots, stubble and exudates. Another example is the incorporation of cereal straw. This may cause some short-term immobilisation of inorganic N and thus decrease leaching, but in the long term the additional organic N retained in soil each year leads to greater mineralisation.

Pattern of Autumn Mineral (NH,+NO,) Nitrogen in Silt Loam Soils with Free- Draining Homogeneous Parent Materials

W S Wilson

University of Essex, Wivenhoe Park, Colchester C 0 4 3SQ, UK

Prediction of autumn soil mineral nitrogen levels in the United Kingdom is still fraught with uncertainty due to variable weather patterns, diverse soil conditions and different soil management practices. Consequently, the amount of data for use by modellers and advisers is limited. However, current demands to reduce nitrate in ground and surface waters and the drive for more economic use of nitrogenous fertilisers prompt the need for further investigations.


The results reported are derived from a set of experiments, started in 1985-86, to study the effect of soil and fertiliser nitrogen on cereal growth at 20 sites, incorporating five soil types, throughout Essex. The previous crops at the sites were, in equal numbers, winter wheat and various break crops. The report is confined to two of the sites, one in west Essex (A) featuring oilseed rape, winter wheat, winter wheat and winter barley (malting) and the other in mid Essex (B) featuring peas, winter wheat, winter barley (feeding) and winter barley (malting), respectively, from 1984 to 1988.

The experimental layout comprised a 120 m x 84 m site with three 84 m x 40 m randomised blocks with seven 40 m x 12 m macroplots per block in the first year. In the subsequent years the macroplots were divided into seven 12 m x 4.25 m microplots. Seven fertiliser treatments, 0-280 kg N ha- ', were applied in spring. Soils were sampled at 0-15, 15-30,30-60 and 60-90 cm depths in late autumn and early spring each year except 1987-88 when 90-120 cm layers were also sampled to observe any effect of the excessive rainfall in October 1987. Soil mineral nitrogen was determined by standard methods.

Results of grain yield and % N in grain in the 1985-86 and 198687 crops, soil mineral N in the profile of untreated soils during four successive autumns and mineral N in soils receiving different levels of fertiliser N are shown.

Tentative interpretation of results indicates that a basic level of soil mineral N is achieved in the first autumn following break crops whereas the effect of residual N from different levels of fertiliser N continues to the second or later post-break crops. In comparison with results from other sites the uniformity of response is due to the homogeneity of the soils, and the level of basic mineral nitrogen is related to the % soil organic matter.

Managing Nitrate during Animal Production

Stephen C Jarvis

AFRC Institute for Grassland and Animal Production, Hurley, Maidenhead, Berkshire SL6 5LR, UK

Inorganic N fertilisers have played a key role in the success of UK grassland farmers in achieving self-sufficiency in animal products. Over the last 3 W O years there has been a substantial increase in the use of fertiliser N as farmers have capitalised on responses of production to up to 400 kg N ha-'. Little of the N input to grassland, especially in intensive systems, is harvested as milk, meat or wool: there is therefore a considerable flux of N through the animal. Where accumulations of mobile N occur in the soil profile as the result of excretal returns, the possibility of release of N compounds to the wider environment exists. Therefore, although NO; leaching from cut grassland is often lower than from arable cropping at comparable rates of fertiliser addition, movement of NO; into surface and subterranean waters from grazing systems may be substantial (Garwood and Ryden 1986).

As part of a wider series of investigations on N transformations in grazed


grassland, NO; leaching has recently been studied in a number of environments and with a range of N managements which include grass swards with fertiliser N additions as well as clover-based swards with no fertiliser N. Investigations have been carried out on a freely draining loam as well as other soils with a very wide range of fertiliser N addition, and the extent of leaching losses has been examined in relation to N input and management. For example, whilst grass/clover swards with no fertiliser N do not produce large leaching losses when compared with highly fertilised grassland (Macduff et a1 1989), preliminary information indicates that white clover monocultures may also provide environmentally unacceptable NO; inputs to drainage waters (Macduff J H pers comm). Nitrate in the soil profile in autumn can be related to subsequent losses (Jarvis et a1 1989a) and, apparently, greater proportions of fertiliser addition are available for loss above a critical level of fertiliser input (Jarvis et al 1989b). Although alternative fertiliser and management strategies have been devised which reduce the risk of NO; leaching (Titchen et a1 1989), more quantitative data are required on the partitioning of the total leaching load among the direct and indirect sources of NO; in a wide range of environmental conditions and management systems.

References Garwood E A, Ryden J C 1986 Nitrate loss through leaching and surface runoff from

grassland: effects of water supply, soil type and environment. In: Nitrogen Fluxes in Intensive Grassland Systems, eds Ryden J C & Ennik G C. Martinus Nijhoff, The Hague,

Jarvis S C, Macduff J H, Williams J R, Hatch D J 1989a Balances of forms of mineral N in grazed grassland soils-impact on N losses. Proc X VIth International Grassland Congress,

Jarvis S C, Barraclough D, Unwin R J, Royle S M 1989b Nitrate leaching from grazed grassland and after straw incorporation in arable soils. In: Management Systems to Reduce Impact of Nitrates, ed Germon J C. Elsevier Applied Science Publishers, London, pp 110-125.

Macduff J H, Jarvis S C, Roberts D H 1989 Nitrate leaching under grazed grassland: measurements using ceramic cup soil solution samplers. In: Proc Int Symp Fertilization and the Environment, Leuven, 1989 (in press).

Titchen N M, Wilkins R J, Philipps L, Scholefield D 1989 Strategies of fertilizer nitrogen applications to grassland for beef effects on production and soil mineral nitrogen. Proc Int Grass Congr pp 183-184.

pp 99-1 13.

pp 151-152.

Managing Land to Stop Nitrate Leakage-Forestry and Low Intensity Grassland

M Hornung

NERC Institute of Terrestrial Ecology, Merlewood Research Station, Grangeaver-Sands, Cumbria LA11 6JU, UK

Semi-natural grasslands and forests have efficient cycling and retention mechanisms for nitrogen. In these systems plant uptake +immobilisation by soil biota>


atmospheric inputs of NH, and NO3 + mineralisation of organic N. In some acid upland systems little nitrogen cycles in the nitrate form, most plants utilising NH,.

Some leaching of nitrate does, however, take place from these systems although concentrations and fluxes are very low compared with more intensively managed systems. Nitrate concentrations in leachates show distinct annual cycles, peaking in winter and with summer minima during which nitrate may be undetectable. Nitrate concentrations in streams draining afforested catchments are often slightly higher than in those draining grassland; this probably results from greater deposition of atmospheric N to the forest than the grassland canopy.

Significant increases in nitrate leaching occur from these systems following disturbance, in response to high pollution inputs and where N uptake is limited by deficiencies of other nutrients. Clearfelling of forests can produce large increases in concentrations and fluxes of nitrate in leachates; the magnitude and duration of the increase varies with site fertility and felling practice. Similarly, ploughing of grassland can result in nitrate leaching, with site fertility, method of tillage and vegetation management influencing the amount leached. Forests downwind of intensive stock rearing units can receive large inputs of NH, which are subsequently nitrified in the soil; in The Netherlands inputs of > 100 kg N ha-’ have been measured and have produced intense soil acidification, tree death and nitrate leaching. Increased nitrate leaching has also been reported from areas in West Germany where forest dieback has occurred. Significant nitrate leaching recorded from a plantation forest in North Wales seems to be linked to phosphate deficiency plus large atmospheric inputs of N.

Maintenance of vegetation cover is important in limiting nitrate leaching in these systems. Clearfelling will almost always result in an increase in nitrate leaching but the amount leached can be influenced by site management. Pollution inputs are now a major source of N and there is evidence that some forest systems are becoming ‘N saturated’ with consequent nitrate leaching. Planting of forests downwind of large- scale stock rearing operations may be inadvisable.

Managing Nitrate to Protect Water Resources and the Water Environment

Andrew C Skinner

National Rivers Authority, (Severn Trent Region), 550 Streetsbrook Road, Solihull, West Midlands B91 lQT, UK

The National Rivers Authority is the organisation established under the Water Act 1989 to undertake the regulatory functions associated with the water cycle, and to be, using the slogan adopted at the launch of the organisation at the beginning of September, ‘Guardians of the Water Environment’.

The paper describes the role of the National Rivers Authority in relation to the protection of water resources and its specific duties under the new Water Act in


respect of nitrate. The issues are dealt with under the same three categories used in the present draft directive from the European Commission, namely (i) protection of surface water supplies; (ii) protection of groundwater supplies; and (iii) prevention of eutrophication in surface waters.

Of these three aspects of the problem it is the protection of groundwaters which presents the greatest challenge and which poses the biggest problems in respect of impact upon agriculture. It has to be recognised that there are some areas of the country where intensive arable farming and the protection of water resources to the standards set by legislation are incompatible activities. Protection of water resources by agricultural change must be part of the solution to the problem. Large areas are vulnerable to nitrate leaching to potable water resources, and in all these areas it is necessary that ‘good nitrate practice’ be achieved. In addition certain critical water supply sources will need to be targeted for more stringent controls in the manner in which sites have recently been identified as potential ‘Nitrate Sensitive Areas’.

It is also the case that such approaches cannot provide the whole solution, at least in the short to medium term. Water protection measures take time to establish and even longer to take effect, during which time nitrate levels in water supplies, especially groundwaters, are expected to continue to rise and exceed the legal limits for water supply. In those situations other solutions, involving source abandonment, relocation, blending or treatment, will have to be adopted by the water supply companies.

Nitrate Leaching and Denitrification Losses from Grazed Grassland

C Jordan, R V Smith, R W J Steen, P J Taggart and C J Watson

Department of Agriculture, Agriculture & Food Science Centre, Newforge Lane, Belfast BT9 5PX, UK

Nitrogen fertiliser usage and grassland production have increased markedly in recent years. On average, little more than half of the nitrogen applied as fertiliser to grassland in the UK is recovered as harvested herbage and liveweight gain in grazing animals. The remainder enters one of the storage compartments or loss processes of the nitrogen cycle and represents a financial loss to the farmer.

In order to better understand the grassland nitrogen cycle under Northern Ireland conditions, the Department of Agriculture (NI) has recently established a study site at the Agricultural Research Institute, Hillsborough (about 15 km south- west of Belfast). Here, an integrated approach to research on the grassland nitrogen cycle has been applied. The work is focused on quantifying the individual processes involved (including leaching, denitrification, nitrification, ammonia volatilkation, changes in soil organic N, N fixation and rainfall deposition) so that management


practices can be developed which will make most efficient use of nitrogen, with consequent economic and environmental benefits.

The poster presents details of the treatments applied at this study site in 1989. Nitrate leaching and denitrification results from the first season April-September 1989 are presented and discussed.

Low-nitrate Water and Arable Farming-Can They Coexist?

Eunice Lord and Mark Shepherd

Soil Science Department, Agricultural Development and Advisory Service, Woodthorne, Wolverhampton WV6 STQ, UK

Winter rye was very effective in reducing loss of nitrate by overwinter leaching, compared with bare fallow on sandy soil. A catch crop of rape was less effective. The experiment, on a loamy sand over a sandstone aquifer, tests a range of options for reducing leaching in an intensive arable rotation. Cumulative effects over a number of years will be monitored.

In the first year, soil solution nitrate-N concentrations in March, at 90 cm, were less than 1 mg litre-' under winter rye, 8 to 9 mg litre-' under the catchcrop, and 24 mg litre-' under bare fallow. The increased mineralisation due to autumn cultivation for crop establishment was more than compensated for by crop uptake of nitrogen over winter.

Establishment of the catch crop by shallow tillage (to minimise mineralisation) was not satisfactory on this light, sandy soil in the presence of straw. The crop emerged but severe nitrogen deficiency appeared to limit early growth.

Soil mineral nitrogen concentrations in spring were greater after ploughing in the catch crop than after ploughing the bare fallow, suggesting that at least part of the nitrogen taken up by the catch crop was rapidly released.

Halving the fertiliser nitrogen applied reduced yields of rye by 15 % and nitrogen offtake in grain by 32 %. At the lower rate, the removal of nitrogen in grain exceeded input in fertiliser by 16 kg ha-'; while at the higher rate input exceeded offtake by 11 kg ha-'. The effects of these changes in nitrogen balance, on a soil with low organic matter, will be monitored over winter and in successive crops.

Nitrate Leaching from Organic Farming Systems

C Stopes and L Philipps

Elm Farm Research Centre, Newbury, Berkshire RG15 OHR, UK

Organic farming, based on rotations and the strategic use of manures, has potential to reduce nitrate pollution whilst allowing farmers to produce food of a quality which is in demand.

Keeping nitrate in i ts place 141

The rotation should achieve a balance of nitrogen supply and demand through the use of legumes, commonly as clover in leys. On such farms, the majority of the organic farmland is under a ley of low leaching potential and it is this characteristic which may explain why organic farming systems may be able to limit nitrate leaching.

Nitrate leaching from organic rotations has been estimated to be as low as 20 kg ha-' over the course of a rotation. However, the precise level depends upon the amounts leached in the first year after breaking the ley.

Cultivation of a ley can lead to high levels of nitrate leaching over the first winter. Delaying cultivation until early winter or spring for a spring cereal, where possible, will reduce the risk of leaching over the first winter.

Cover crops sown early can retain significant amounts of nitrogen. However, integrating their use within an organic ley-arable system poses some challenges.

agriculture group symposium keeping nitrate in its place

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