Nitrate leaching from dairy shed effluent and ammonium fertiliser applied to a free-draining pasture soil under spray or flood irrigation

The potential for contamination of groundwater by dairy shed effluent (DSE) and ammonium N fertiliser (NH4Cl) was assessed by measuring nitrate leaching from 16 large soil lysimeters (80 cm diam. x 120 cm depth). Each lysimeter contained an undisturbed monolith of Templeton fine sandy loam (Udic Ustochrept) located in Canterbury, New Zealand, with established perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture plants. DSE and NH4Cl were applied at 400 kg N ha(-1) yr(-1), in mio split applications of 200 kg N ha(-1). During the summer, each lysimeter received either spray (50 mm mo(-1)) or flood (100 mm mo(-1)) irrigation. During the winter, the natural rainfall received was supplemented with simulated rainfall in order to reach the 75th percentile of local rainfall records for the winter period. The concentration of nitrate in the leachate reached 5 mg N l(-1) under both flood and spray irrigation following the first N fertiliser application (Dec 1995), but did not increase above control values (<1 mg N l(-1)) following the first DSE application. After the second DSE/fertiliser application (May 1996), the nitrate concentration in the leachate increased significantly during the winter/spring period, and appeared to be influenced by the antecedent summer irrigation conditions. Under spray irrigation, the nitrate concentration peaked at 40 and 17 mg N l(-1) for the N fertiliser and DSE, respectively. Under flood irrigation, nitrate reached 25 and 10 mg N l(-1) for the N fertiliser and DSE, respectively. The lower nitrate concentrations in the leachate under flood irrigation were attributed to the greater loss of nitrogen by denitrification and the greater dilution of soil solution nitrate by the larger volume of irrigation water applied. Enhanced denitrification from the higher moisture content in the flood/DSE treatment resulted in a smaller N leaching loss compared with the spray/DSE treatment. The amount of total mineral N (nitrate, ammonium, and nitrite) leached was smaller from the DSE than from the N fertiliser.