NITRATE LEACHING FROM SHEEP-GRAZED UPLAND PASTURES IN WALES

Ceramic cup samplers were used to measure nitrate leaching from grass/ clover pasture in Wales to which no N fertilizer had been applied and from a predominantly grass pasture receiving 100 kg fertilizer-N/ha an nually. Annual leaching losses at individual sampling points, measured over a 3-year period between 1988 and 1991, ranged from the equivalen t of < 0.1 to 226 kg N/ha. All data sets were positively skewed and in four out of six cases conformed to a log-normal distribution. The mar ked spatial heterogeneity was attributed to the uneven deposition of N in the excreta of grazing stock but variations in soil depth and hydr ology may also have contributed. Particularly large losses occurred fr om those areas of the plots where sheep congregated. As a result of th is heterogeneity, there were large standard errors associated with est imates of mean losses from the pastures as a whole. Overall losses ran ged from 13 to 24 kg N/ha per year from grass/clover plots and from 10 to 13 kg/ha from fertilized grass plots. There was no consistent rela tionship between relative losses from the two types of pasture. The qu antity of nitrate leached appeared to be independent of stocking rate, although there was a direct correspondence between the loss from gras s/clover plots and the proportion of clover in the sward. Estimates of nitrate concentrations in drainage never exceeded 5.6 mg N/l for eith er sward.