Report No FR0146

Dec 1990

I OBJECTIVES

To establish the environmental, edaphic, geographical, geological and agricultural factors which determine the extent of nitrate leaching to potable surface and groundwater supplies.

To assess the potential for nitrogen loss by nitrate leaching from sewage sludge applied to agricultural soils.

To determine the impact of measures imposed by NSAs, which aim to control nitrate losses by leaching, on agricultural utilisation of sewage sludge.

To establish a research strategy to provide a basis for the long-term security of sludge recycling to agriculture whilst minimising potential pollution risks from nitrate leaching.

II REASONS

Increasing concentration of nitrate in potable water supplies has raised concern over possible health and environmental risks associated with nitrate leaching pollution primarily from diffuse agricultural sources. The introduction of Nitrate Sensitive Areas and the associated Pilot Nitrate Scheme operated by MAFF, and the proposed EC Directive aim to control nitrate emissions to surface and ground waters within vulnerable zones. These measures will significantly constrain the agricultural utilisation of sewage sludge.

III RESUME OF CONTENTS

Accumulation of large concentrations of nitrate in soils under arable cultivation during autumn, which is leached by winter rainfall, can contribute significantly to groundwater nitrate levels. Application of certain organic manures to soil in autumn can further increase the quantity of nitrate lost by this route. In particular, large losses may potentially occur from applied liquid sewage sludges which contain appreciable levels of mineral nitrogen. However, there is little short-term risk of nitrate leaching from dewatered sludges due to low levels of application to the soil of mineral and readily mineralisable-nitrogen in these materials. Mechanisms of nitrate leaching are described in relation to environmental factors and land use.

These show that the nitrate concentration in winter drainage from soils is largest in Eastern and Midland areas of low rainfall and which are mainly arable compared with the West which receives more rain and is Predominantly grassland. The application of sewage sludge to stubble in the autumn is the most important window for agricultural recycling in regions with large areas of arable crop production, but with little available grassland. The current voluntary Pilot Nitrate Scheme introduced by MAFF will provide information on methods which aim to reduce nitrate emissions from agriculture in designated Nitrate Sensitive Areas (NSAs). However, the proposed EC Directive on nitrate in water may impose restrictions on agricultural practices across wider areas of England. Nitrate Sensitive Area restrictions on the timing and rate of application of sewage sludges are likely to significantly constrain agricultural utilisation of sewage sludge. In particular, the autumn window for sludge application will probably be closed, which will thus have a major impact in arable regions. However, soil and crop management methods are available which would limit nitrate leaching losses from sludge treated soils in the autumn and these are reviewed. Methods include: early sowing of winter cereals; establishment of cover crops; nitrification inhibitors; and straw incorporation, although further research is necessary to evaluate the efficiency and cost effectiveness of these approaches. In addition, development of technology to apply liquid sludges to growing arable crops in the spring would avoid the closed autumn period thus maintaining flexibility in sludge recycling to arable land. As there is currently no closed period for cake sludges, another possibility is to convert existing liquid sludge treatment processes to cake production although proposed EC legislation could restrict all autumn applications of organic manures. It is unlikely that NSA restrictions would have a significant impact in regions with a high proportion of available grassland as liquid sludge can generally be applied throughout the year in these areas.

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