Nitrogen accumulation in kaolin mining wastes in Cornwall. III. Nitrogenous fertilisers

In the establishment of a grass cover on china clay waste it is necessary, as in most land restoration, to supply fertiliser nitrogen in substantial amounts. However the objective of nitrogen accumulation is restricted in south west England by severe leaching. A series of experiments were devised to investigate the nitrogen retention from a range of fertiliser sources. When fertiliser-N was applied to established swards, three times as much nitrogen (about 90 per soil and vegetation at the end of the first season of establishment. Sodium nitrate was especially susceptible to leaching, in contrast to sources containing ammonium. Slow-release formulations and organic materials produced a lower sward growth throughout the first growing season but tended to give higher levels of residual-N in the sand waste. When fertiliser-N was applied at seeding less than 30 per cent of the nitrogen was retained in the cent of that applied) was retained. Most of this nitrogen was retained in the sand waste. There was little difference between different types of fertiliser: the main effect was to alter the amount of nitrogen in herbage tissues. Plant growth and a healthy grass sward could only be sustained in the absence of a legume component by repeated applications of fertiliser-N. When applications were discontinued sward degradation was shown by decreases in biomass. Losses in nitrogen from the sward biomass were more than accounted for by an increase in the nitrogen content of the sand waste. These results indicate that once fertiliser-N is retained by the sward ecosystem on sand waste, there are few further losses, presumably because of rapid microbial immobilisation of nitrogen in the sand waste. In view of the expense of land restoration using numerous maintenance applications of nitrogen, it is clear that legumes must be included in the seeds mixture for the reclamation of kaolin mining wastes in Cornwall. However, legumes are difficult to establish and more susceptible to drought than grass species. Results from these trials suggest that slow-release nitrogen fertilisers could enable a controlled level of grass growth, thereby avoiding competition between the legume and grass components. Alternatively grass swards could be rapidly established to stabilise the sand waste using soluble ammonium salts, and in the second season oversown with legumes.