Transportation and Consumption Processes of Atmospheric Nitrate in Forested Watershed in Shiga Prefecture, Central Japan.

The purpose of this study is to clarify transportation and consumption processes of atmospheric nitrate deposited into forested ecosystems, and the influences on downstream aquatic ecosystem. We collected rainfall, throughfall, surface water, soilwater (10cm, 30cm), groundwater, spring water and streamwater at a forested watershed planted with Japanese cypress in Shiga prefecture, central Japan at about biweekly. Samples were analyzed for dissolved nitrogen, nitrate, ammonium, nitrite, and oxygen isotope of nitrate. Isotope analysis was conducted at ICRE in University of Yamanashi. We also collected soil at several month intervals, and measured nitrate, ammonium in soil and net mineralization and nitrification rate.

Precipitation in winter (from December 2012 to April 2013) was 602mm. Atmospheric nitrate deposition by throughfall during this period was 4.13 kgN/ha. Atmospheric nitrate flux by surface flow and soil water leaching from 10cm and 30cm were 0.005, 0.58 and 0.99 kgN/ha, respectively. Nitrified nitrate flux in this period by surface flow and soil water leaching from 10cm and 30cm were 0.0005, 0.27 and 0.32 kgN/ha, respectively.

On the other hand, precipitation in summer (from May 2013 to October) was 1199mm. Atmospheric nitrate deposition by throughfall during the period was 4.10 kg/ha. Atmospheric nitrate flux in surface flow and soil water leaching from 10cm and 30cm were 0.038, 0.51 and 0.99 kgN/ha, respectively at this period. Nitrified nitrate flux by surface flow and soil water leaching from 10cm and 30cm 0.020, 1.31, 1.57 kgN/ha, respectively at this period.

These results suggested that the amount of atmospheric nitrate leaching in soil water was influenced by not the amount of precipitation but atmospheric nitrate deposition by throughfall. On the other hand, nitrified nitrate leaching by soil water had seasonality that it was larger in summer than winter.