Studies on the nitrogen changes under set-aside have been carried out by Froment and Grylls (1992), Sinclair et al.(1992), Farr et al. (1992), Vinten and Smith (1993), Burt and Haycock (1993), Chalmers et al. (1994).
The leaching of nitrates into rivers and drinking water supplies from farmland has become a major environmental concern. Fertiliser application is only one source of nitrogen that crops utilise. Many agricultural soils depend heavily on the mineralisation of organic nitrogen. Parkinson (1993) discusses the effect of sward development on the nitrogen content of the soil and remarks that it may become a major source of significant nitrate losses once the sward has matured, and particularly if the sward is ploughed out. The practice of ploughing out grass and establishing a new crop, whether arable or grass, invariably leads to high losses of nitrate in the year immediately succeeding pasture inversion. This is especially true for grass/clover systems (Parkinson, 1993).
More than 90% of the nitrogen in most soils is in organic form. However, the inorganic component, mainly ammonium and nitrate is significantly greater in agricultural soils. Also, when nitrogen is returned in faeces and urine in grazed systems the leaching loss is generally much higher. The ploughing up of temporary grassland is also a major cause of nitrate leaching (Vinten and Smith, 1993). This is an important factor when considering the effect of set-aside on the local environment.
Burt and Haycock (1993) also argue that, although set-aside will cause a reduction in the amount of fertiliser applied to a farm, the inclusion of a rotational fallow option may mean that set-aside will have less impact on reducing nitrate leaching as periodic ploughing would release substantial amounts of nitrate.
Froment and Grylls (1992) studied soil mineral nitrogen changes under set-aside. This is important as one of the aims of the U.K. government is to reduce nitrate leaching from agricultural systems, therefore it is important that set-aside does not increase the potential for nitrate leaching. A comparison of perennial rye grass, perennial rye grass with white clover, natural regeneration and perennial rye grass receiving a low rate of applied nitrogen was carried out. The findings generally showed that set-aside reduced soil mineral nitrogen, and that soil mineral nitrogen levels were lower for perennial rye grass than natural regeneration or rye grass with clover. It is suggested that this is as a result of a more rapid establishment of a vegetative cover.
Sinclair et al. (1992) carried out a similar experiment, and concluded that there was a potential to reduce leaching of nitrate where a dense cover with a high content of rye grass is established. However, a sward with clover may result in an increase in nitrate leaching from agricultural land. The final conclusion was that “from the point of view of minimising loss of nitrate-N from set-aside land, grass is a better cover crop than clover or natural regeneration” (Sinclair et al., 1992).
Harris et al. (1992) also studied changes in soil mineral nitrogen. Soil mineral nitrogen showed reductions on all fields previously in winter cereals on a farm in Cambridgeshire.
Hewitt et al. (1992) studied soil mineral nitrogen on a sandy loam at Woburn, Bedfordshire, and found that “leaching under autumn sown rye grass was almost as great as under winter wheat” (Hewitt et al., 1992). Natural regeneration was surprisingly effective at decreasing leaching compared to winter wheat. This was after a single year of set-aside.
Farr et al. (1992) examined the effect of cutting management on naturally regenerated set-aside and its affect on the mineral nitrogen content on a farm in Elgin, Scotland. The data collected suggested that timing and frequency of cutting naturally regenerated plant cover had no effect on the level of mineral nitrogen in the soil. Whether the cuttings were left or removed also had no effect on soil mineral nitrogen. However, ammonium levels were found to be greater on set-aside generally.
Chalmers et al. (1994) studied the effects of various cover managements on the soil mineral nitrogen levels, and the levels of soil mineral nitrogen in the following crops. On one year set-aside there appeared to be a similar or smaller risk of nitrate leaching from autumn established set-aside compared with arable rotations. Ploughing increased soil mineral nitrogen on set-aside to levels greater than those on arable fields. During the third year of set-aside the risk of nitrate leaching was less than that of arable fields. Perennial rye grass had the smallest effect on soil mineral nitrogen, whereas perennial rye grass with white clover had the largest effect. Plant cover, nitrogen uptake and recycling via mowings, and other nitrogen supply from fixation by white clover, would have contributed to these differences. Ploughing of set-aside increased the soil mineral nitrogen during the following season compared to the arable rotations for all locations except the perennial rye grass plot at Boxworth. Perennial rye grass with white clover gave the highest increases of soil mineral nitrogen.
Jones et al. (1994) determined that set-aside covers of perennial rye-grass, perennial rye grass with white clover, and natural regeneration all increased the soil mineral nitrogen supply at sites throughout the UK. Melander and Jacobsen (1994) studied nitrogen uptake on set-aside in Denmark, and suggest that a sown cover is needed to reduce N-leaching.
Rose and Harris (1994) examined changes in the soil water quality on five year non-rotational set-aside on clay soils in Cambridgeshire. It was found that under set-aside lack of cultivations and the deterioration of the secondary drainage systems increased surface compaction and surface runoff which led to rapid reductions in the soil mineral nitrogen on fields previously in winter cereals. Nitrate-N concentrations in the drainage water fell to below the EC Directive (11.3 mg l-1) limit within one year of
set-aside. The ploughing up of set-aside encouraged the mineralisation of organic matter, and nitrate produced after ploughing was leached out into the drainage water in concentrations of up to 25 mg l-1 in the autumn, and in excess of 30 mg l-1 in the spring following application of fertilisers. However, it was still concluded that set-aside had beneficial effects on the environment.