POTENTIAL CONTRIBUTION OF PLOWED GRASSLAND TO NITRATE LEACHING

An exponential decline was fitted to measured losses of organic nitrogen from grass soils during the 25 years after ploughing. The experiments were carried out on soils of the Romney and Blacktoft series from the Holland district of Lincolnshire. The fitted line estimated that half of the decline took place in the first 5.5 years and 90% in 18 years. Almost 4 t N ha-1 from soil organic matter were lost from the top 0-25 cm of soil. Losses calculated from the fitted equation were then imposed upon known rates of ploughing of grass throughout England and Wales so that the potential contribution of ploughed grass to nitrate leaching during the last 50 years could be estimated. By the end of the Second World War, ploughed grass could have increased the concentration of nitrate in draining water by more than 40 mg NO3-N 1(-1) in parts of Lincolnshire, the Midlands and East Anglia. In 1972, after another period of increased ploughing, there was a further increase of 15 mg N 1(-1). Water draining from a field of ploughed grass could contain up to 450 mg N 1(-1) in the first season after ploughing. A calculation was made of the area of land that may safely be ploughed out of pasture each year if the EC limit of 11.3 mg N 1(-1) is not to be exceeded in water draining from each of the counties of England and Wales. In parts of the Midlands and East Anglia, the nitrate from ploughed grass (representing less than 0.1 % of the land area) could currently be responsible for more than half of the loading towards the EC limit.