Influences of topography on nitrate export from forested watersheds on Yakushima Island, a Natural World Heritage site.

In East Asia, high levels of atmospheric nitrogen are deposited onto land. This could elevate the nitrate levels in coastal waters river runoff, even from areas where anthropogenic sources are minimal. It is important to identify NO sources in river water and the mechanisms involved in NO runoff. Yakushima Island, Japan, is a Natural World Heritage site featuring numerous watersheds with diverse topography and rivers. The area receives significant precipitation, with up to 10 000 mm in mountainous regions. Its proximity to coastal urban areas in China (∼800 km) leads to substantial atmospheric nitrogen wet and dry deposition in the island's forests. The study aimed to clarify regional water quality characteristics by conducting long-term monitoring of dissolved ion components (Na, K, Mg, Ca, F, Cl , NO , and SO ) in river waters, and to determine the effects of NO sources and watershed topography on NO behavior. Dissolved ion concentrations were obtained from a long-term monitoring (2011-2014) dataset. Cluster analysis classified runoff water from the central mountainous region into three groups: western region, other regions, and groundwater. The average NO concentration in the western region was 10.2 μmol L, which was higher than the 6.24 μmol L observed in the other regions. Stable isotope analysis in December 2018 showed that river water NO (1.39 μmol L) in the western region had a high proportion of atmospheric NO . Topographic analysis indicated that NO and atmospheric NO increased in smaller watersheds and steeper terrain. This study conclude that NO output is controlled by topography.