Annual nitrogen leaching to subsurface drainage water from a clayey aquic soil cultivated with onions in Hokkaido, Japan

We studied the annual nitrogen leaching from an onion (Allium cepa L.) field with subsurface drainage in a clayey aquic soil in Hokkaido, Japan. Onion plants were transplanted early in May and harvested at the end of September in 1996, Throughout a year, the drainflow was measured automatically and the drainage water was sampled almost weekly. The nitrate concentration in the drainage water averaged about 10 mg N L-1. In the growing (May to September) and fallow (October to February) seasons, it increased with increasing drainflow, and in the snowmelt (March to April) season, it decreased with increasing drainflow. The amount of annual leached nitrogen was calculated by using the relationship between the drainflow and nitrogen concentration in the drainage water. We found that 179 kg N ha(-1) was leached, corresponding to 58% of applied nitrogen. The amount of nitrogen taken up by the plant was 133 kg ha(-1), corresponding to 43% of applied nitrogen. From harvest in 1996 to April 1997, 95 kg N ha(-1) was leached, and 21 kg N ha(-1) was leached in only 11 days during snowmelt. Thus excess nitrogen can be leached in the snowmelt season in snowy regions like Hokkaido, Nitrate concentrations in the drainage water were much higher than those in the soil solution at the depth of the drain pipe. This fact is probably due to the bypass how via macropores towards the subsurface drain. When the drainflow is large, the rainwater or snowmelt water is likely to mix with the soil solution in the topsoil and drain directly through the macropores in the subsoil.