Organic nitrogen in drainage water from grassland in northern Ireland

Loss of organic N in land drainage may contribute to water pollution. This study was conducted to quantify the loss of organic nitrogen in drainage wa ter from grazed grassland (Lolium perenne L. cv. Talbot) swards receiving a range of fertilizer N inputs for 9 yr. Plots (each 0.2 ha) received 100, 2 00, 300, 400, or 500 kg N ha(-1) yr(-1) as calcium ammonium nitrate and wer e hydrologically isolated and drained to V-notch weirs with automatic sampl ing of drainage water. For comparison, organic N loss was quantified in riv ers draining six predominantly grassland catchments. The European Community maximum admissible concentration of total Kjeldahl N for drinking water of 1.0 mg N L-1 was exceeded in both land drainage and river water. Total org anic N accounted for up to 54.7 and 20.2% of the annual N drainage losses f rom the plots receiving 100 and 500 kg N ha(-1) yr(-1), respectively. Loads of soluble organic N were greater than particulate N but there was no sign ificant relationship with fertilizer N input for either fraction. The avera ge annual loss of total organic N in drainage water ranged from 3.5 to 9.1 kg N ha(-1). However, this loss cannot explain the large amount of unaccoun ted for N in calculations of nutrient balances where inputs invariably exce ed outputs. In comparison, the average annual loss of organic N from the si x river catchments ranged from 4.7 to 8.1 kg N ha(-1), which suggested that a large proportion of the organic N in river water was originating from la nd drainage.