Impact of nitrogen cycling on stream water quality in a basin associated with forest, grassland, and animal husbandry, Hokkaido, Japan

The annual N load from the farm to the Kepau River was measured at 14.4 t N yr−1. Ninety percent of this load, however, occurred during rainfall and spring snowmelt. Within one 2-week snowmelt period, 5.0 t N was discharged, which corresponds to 35% of the annual load. Although the average N concentration of stream water below the farm was 2.8 mg N L−1, the maximum concentration recorded during the snowmelt season was 13.5 mg N L−1. The N concentration of the stream water increased and the silica (Si) concentration decreased as the stream flow rate increased. Consequently, the molar ratio of Si/N frequently dropped below 2.7, the critical level for the occurrence of eutrophication. The large N load during rainfall and snowmelt could be ascribed to open ditches, which collect tile drainage and surface runoff from the fields, discharging it directly to the river, bypassing the forested riparian zone.