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The effect of sulphur (S) application on the efficiency of nitrogen (N) use was investigated using cut plot experiments on two contrasting soil types. Nitrogen was applied at 200 and 450 kg N\ha per year, with and without 38 kg SO $ \ha (15n2 kg S\ha) per cut. Over three conventionally timed silage cuts for 2 years, measurements were made of herbage dry matter, the yield of N and S in herbage and losses of N and S by leaching, and N by denitrification.Herbage dry matter and N yields were significantly increased by the application of S at the high N level at the sandy loam site (Halse). At the clay loam site (Great Close) the application of S had no significant effect on herbage dry matter or N yields. At Halse, the pattern of response through the year was not the same in the 2 years studied, although in both, the effect of S was significant at third cut at high N. Deficiency was suggested by the N : S ratio of herbage on the plots without S, especially at first cut, and at later cuts at Halse. Nitrate leaching was reduced by S at Halse by 72 % and 58 % with high N in 1997 and 1998, respectively, and by 10 % and 5 % on the low N treatments in 1997 and 1998, respectively. Application of S at high N at Halse reduced the peak concentration of nitrate-N in leachate from 27n3 mg N\l to 9n3 mg N\l. At Great Close, application of S had no significant effect on the amount or peak concentration of nitrate-N leached. The improvement in efficiency reported at Halse suggests that on permeable soils receiving high levels of N, the application of S could have a large effect on nitrate leaching and its associated environmental impact.

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The distribution and impacts of different nitrogen pollutants are inextricably linked. To understand the problem fully, the interactions between the different pollutants need to be taken into account. This is particularly important when it comes to abatement techniques, since measures to reduce emissions of one nitrogen pollutant can often lead to an increase in another. This project represents a step towards greater understanding of these issues by linking together new and existing nitrogen flux models into a larger framework. The modelling framework has been constructed and some of the nitrogen flows between fields, farms and the atmosphere have been modelled for a UK study area for typical farm management scenarios.

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