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Heterogeneous Atmospheric Nitrogen Deposition Effects Upon the Nitrate Concentration of Stream Waters in a Forested Mountain Area |
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| Authors: | Yu Tabayashi Keisuke Koba |
| Institution: | 1. Department of Natural Environmental Studies, Institute of Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8563, Japan 2. Geological Survey of Japan (GSJ), AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan 3. Faculty of Agriculture, Tokyo University of Agriculture and Technology, Saiwai-cho, 3-5-8, Fuchu, Tokyo, 183-8509, Japan |
| Abstract: | Nitrogen compounds generated by anthropogenic combustion deposits in forest watersheds and induce nitrogen saturation of the area. Because excess nitrogen is derived from atmospheric deposition, this action is expected to uniformly affect a wide area of forest soils. Geographically, heterogeneous nitrate concentration of stream water within a small area has been attributed to the tree type, geological setting and tree cut. In this article, we hypothesized that the effect of the atmospheric nitrogen deposition in the forest watershed may vary within a small area, and that such variation is induced by the degree of air mass containing a high concentration of nitrogen deposition of combustion origin. We measured major ion concentrations, including nitrate, nitrite oxygen and nitrogen stable isotope of nitrate sampled at 24 water streams in the Chichibu region, which is 50?C100 km from the Tokyo metropolitan area. The nitrate concentration showed a wide range (25.6?C237 ??mol L?1) within 300 km2, which was explained sufficiently by the air mass advection path and its contact with the mountain??s surface. The nitrate concentration showed a significant positive correlation with chloride (r?=?0.73; p?<?0.001). As chloride originates outside of the Chichibu region, the positive correlation between two ions showed that the nitrate concentration of the stream water was affected by the nitrogen compound from the Tokyo Metropolitan area as a form of atmospheric deposition. Between the nitrate concentration and the stable isotope ratio of oxygen of nitrate, there was a positive correlation until nitrate concentration of 100 ??mol L?1. When the nitrate is over 100 ??mol L?1, ??18O shows a stable value of ca. 5.7??. This indicates that the nitrification proceeds when the nitrate concentration was low to middle, but the reaction slowed when the nitrate concentration became high. Oxygen stable isotope of nitrate along with a set of nitrate concentrations can be used as a good indicator of nitrogen saturation. |
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