Effects of the 1995-1997 drought on nitrate leaching in lowland England

The effect of drought between summer 1995 and 1997 on stream and river nitr ate concentrations was investigated using sites close to the long-running m eteorological station in Oxford, UK. Nitrate concentrations in the River Wi ndrush were relatively low during the drought, but after it had ended reach ed the highest level since records began in 1973. The low concentrations du ring the drought probably reflect a reduced contribution from agricultural runoff. High nitrate concentrations were found in a field drain at Wytham E nvironmental Change Network site during and after the drought, but discharg e was greatly reduced. A woodland stream at Wytham had much lower nitrate c oncentrations than the field drain but these similarly increased during and after the drought. There was evidence that both a concentrating effect of low water volumes and enhanced soil nitrogen mineralization and nitrificati on rates were causing concentrations to rise. The effects of mineralization and nitrification were more important in woodland than agricultural land. Nitrate load over the course of a year was determined largely by discharge, but steeper gradients for the relationship between cumulative load and cum ulative discharge were seen during and after the drought than before, refle cting the higher concentrations.