Nitrate leaching from sheep-grazed grass/clover and fertilized grass pastures

Abstract

SUMMARYNitrate leaching from sheep-grazed pastures was measured over three winters between 1987 and 1990 at a lowland site in Wales. Losses from replicated plots of ryegrass/white clover (Lolium perenne/Trifoliwn repens) pasture that received no N fertilizer and from pure ryegrass pasture receiving 150–200 kg fertilizer-N/ha per year were determined using suction cup samplers. In Year 1, the average clover content of the grass/clover swards (April-September) was equivalent to 35% of total dry matter but declined during the course of the experiment and was only 4% in Year 3. Concentrations of nitrate and quantities of N leached from separate sampling positions exhibited a wide range of values and conformed to a log-normal distribution. In the first two winters, mean nitrate concentrations in soil water and quantities leached from grass/clover plots were greater than those from fertilized grass plots. In Year 3 the loss from the N-fertilized plots was higher and exceeded that from the grass/clover treatment. Total quantities of N leached during the three winters ranged from 6 to 33 kg N/ha from grass/clover plots and from 2 to 24 kg N/ha from fertilized grass plots. At no time did mean nitrate concentrations from either treatment exceed 11 mg N/litre. These values probably underestimate the true loss as the distribution of the main sets of water samplers did not provide adequate coverage of those areas of the plots where sheep congregated and which received high returns of excreta. These positions were characterized by particularly high leaching losses. Differences between losses from the two treatments and between years could only be partly explained by the decline in clover content and differences in stocking rates.