Nitrate losses from application of sewage sludge to farmland
Report No FR/D0012
M A Shepherd
The UK Government is progressing the implementation of the Nitrate Directive (EC/91/676), which aims to reduce and prevent water pollution caused by nitrate from agricultural sources. Measures to reduce nitrate loss include the adoption of appropriate strategies for organic manure applications. Given this background, the Department of the Environment funded work to investigate the factors which influenced nitrate leaching from land dressed with sewage sludge (currently less than one per cent of agricultural land receives sludge). Experiments were carried out over two winters (1991/92 and 1992/93) to determine how nitrate leaching from sludge-treated land was influenced by method of application, type of sewage sludge, cropping, amount and time of application.
Each year there was an arable site in the midlands and two grassland sites in the south-west, all on light soils. Nitrate loss occurred during each autumn and winter. Leaching, measured by porous ceramic cups, was greater from the arable site than the grass, even though there was more drainage from the grassland sites; the established root system of the grass sward was able to utilise more of the applied nitrogen, whereas winter barley was less able to do so in the autumn.
Digested liquid sludge, with a large proportion of N in the ammoniacal form, was a greater leaching risk than raw sludge. Injecting deeply into the soil exacerbated loss, compared with surface applications. Generally, the earlier the application in the autumn, the greater the N loss, particularly when applied to bare soil or winter cereal. A simple approach to minimise nitrate loss would be to avoid autumn applications of liquid digested sludge (a high risk); there need be no timing restrictions on raw sludge or dewatered cake. This approach is used for animal wastes; slurries are restricted but FYM is not, under the NSA scheme. A problem with this simple categorisation of materials is their variability. An alternative, but more cumbersome, strategy might be to use NH4-N content as a guide; less than 25 per cent (of total N) a low risk and more than 25 per cent a high risk.
Limits for the total annual application of nitrogen should be set, based on a sludge's total, rather than available, N content (currently also the same approach with animal manures). Although it could be argued that a limit based on NH4-N content could still avoid overr-fertilising a crop, this approach migh lad to large applications of sludges with a small NH4-n content (liquid raw sludge and dewatered cake). Such regular, large, applications might contribute to a build-up of soil organic matter, with a concomitant increase in leaching risk. Further work on the long term effects of sludge applications is needed before guidelines could be revised upwards.
Sludges should be injected as shallowly as possible (15 cm or less), ensuring even application across the full injection width. Both of these are particularly important for winter applications, but will also aid efficient crop utilisation of sludge N irrespective of application time. Thus, the subsoiling effect of sludge application, favoured by many farmers, is unacceptable because it encourages leaching.
In a separate experiment, a nitrification inhibitor was mixed with liquid digested sludge before spreading. Then, more of the nitrogen was held as ammonium during the winter and, consequently, nitrate leaching decreased. More experimental work would be required before this could be recommended as a fail-safe technique for controlling nitrate loss from sludge.
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